RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
| From: Ross Paterson [mailto:[EMAIL PROTECTED] | | On Thu, Mar 04, 2004 at 09:21:23AM -, Simon Peyton-Jones wrote: | My personal view is this: we should have adopted the ML view of records. | It solves the immediate problem, and no more elaborate scheme seems | sufficiently right to be declared the winner.Alas, like all other | proposals, it's not backward compatible, and hence not likely to fly. | | About a year ago, you were toying with a simple polymorphic system with | just has predicates. If these were automatically derived, it seems | you'd get something quite close to backward compatible, except for the | pesky extra lifting in record types, and not being able to omit fields | when constructing the record. (And update might not statically check | that all fields belong to the same constructor, for the simple version | of the type system.) Is that just too clunky? It's possible, but I don't know. Speaking for myself, I've rather lost motivation on the records front, because the design space does not seem to have a clear optimum point. Adding has predicates in full glory seems overkill to get re-use of labels. Ah, but they also give you polymorphic record operations! Yes, but so do other versions, with different and rather subtle technical tradeoffs. That's why I've backed up to something extremely simple: either (a) disambiguate record labels by qualifying with the type constructor or (b) go the ML route. None of this is to discourage people from experimenting with variants -- quite the reverse. That's how we may find out what a good system is. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Conor McBride wrote: [...] There's a catch, of course. When you write r | e you give the typechecker no clue as to the type of r: it just has to infer the type of r and hope it's a datatype. This is reminiscent of an issue I encountered a year or so ago, when designing a language. The main example I used was a function for calculating the magnitude of a two dimensional vector. It would be nice to simplify this magnitude v = sqrt ((v | x)*(v | x) + (v | y)*(v | y)) to this magnitude v = v | sqrt (x*x + y*y) but how was the compiler to know that x and y are fields of v, but sqrt and (+) and (*) aren't? What I ended up doing was making a rule that if the record expression (the part to the left of the |) could not be statically tracked to something which revealed the field names, then the right hand side (of the |) must contain exactly one free variable. This static tracking took place before type inference, so it was in line with Simon PJ's preference for keeping scope and type inference separate. I also used some syntactic sugar for explicit record narrowing, so the final version of the magnitude function was magnitude v = v(.x, .y) | sqrt (x*x + y*y) which was quite similar to Cayenne's open ... use ... in ... feature. Regards, Tom Pledger ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
| detail, of course ;). What I'm secretly hoping is that the
| GHC/hugs/HBC people will see what I'm trying to achieve, tell me I'm
| totally nuts, and then suggest an alternative, much simpler approach
| which gives us exactly the same goal ...
As I implied earlier, I am thus far unconvinced that the complexity cost
justifies the benefit. Let me suggest two simpler alternatives.
Alternative 1
Given
import FiniteMap( FM, add {- :: FM a b - a - b - FM a b -} )
and fm :: FM Int Bool,
you want
fm.add x y
to mean the same as
add fm x y
(but in addition allow lots of unrelated 'add' functions)
My objection is that the type of 'fm' is a matter of inference, whereas
in Java it'd be a matter of explicit declaration. But you could adapt
your story to be more explicit, thus
FM.add fm x y
The qualifier here is the *type* not the *module*. (Let's assume they
share a name space for now.) This would work for record selectors too:
data T = T { x::Int, y::Int }
data S = S { x::Int, y::Int }
Now
f p q = T.x p + S.y q
I guess the rule is that you can qualify a function by the name of the
outermost type constructor of its first argument.
I would restrict this only to functions with explicitly-declared types,
so that there's no interaction with inference.
Alternative 2
If the big bug-bear is record selectors, let's focus on them
exclusively. I now think that ML got it right. ML records are simply
labelled tuples. So just as (Bool,Int) is an anonymous type, so is
{x::Bool, y::Int}. Indeed (Bool,Int) is just shorthand for {#1::Bool,
#2::Int}.
In Haskell, a function with a tuple argument takes exactly that tuple:
f :: (Bool,Int) - Bool
f (x,y) = x
Here, f cannot take a 3-tuple or a 89-tuple. In ML it's the same with
records (I'm not getting the ML syntax right, but you'll get the idea):
f :: {x::Bool, y::Int} - Bool
f r = r.x
Here f cannot take a record of shape other than {x::Bool,y::Int}. You
are allowed to write
f {x, ...} = x
but you must then explain f's argument type separately. For example you
can write
f :: {x::Bool, p::String, q::Bool} - Bool
f {x, ...} = x
So there is no sub-typing, no row polymorphism, no attempt to give
f r = r.x
a fancy type that makes f applicable to any record with an x field.
On the other hand, there is also no problem with many records having the
same field name either, which is the problem we started with. There are
no implicitly-defined record selectors either: you have to use pattern
matching for that.
My personal view is this: we should have adopted the ML view of records.
It solves the immediate problem, and no more elaborate scheme seems
sufficiently right to be declared the winner.Alas, like all other
proposals, it's not backward compatible, and hence not likely to fly.
Simon
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RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
| So there is no sub-typing, no row polymorphism, no attempt to give
| f r = r.x
| a fancy type that makes f applicable to any record with an x field.
|
| On the other hand, there is also no problem with many records having
the
| same field name either, which is the problem we started with. There
are
| no implicitly-defined record selectors either: you have to use pattern
| matching for that.
Andreas says:
| Actually, #l is just syntactic sugar for (\{l=x,...}-x), which
implies
| that you might need type annotations.
Yes I was wrong to say that there are no implicitly-defined record
selectors; (#l r) is exactly that. Syntactically I'd prefer (r.l); but
regardless, it's a syntactic construct distinct from function
application, which must be monomorphic. Yes, that's a kind of
interaction between binding and type system (of the kind I objected to)
but a very weak and well-behaved kind of interaction.
Simon
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Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Simon Peyton-Jones wrote:
If the big bug-bear is record selectors, let's focus on them
exclusively. I now think that ML got it right. ML records are simply
labelled tuples.
Note that this is true only for SML, not for Caml.
So just as (Bool,Int) is an anonymous type, so is
{x::Bool, y::Int}. Indeed (Bool,Int) is just shorthand for {#1::Bool,
#2::Int}.
A bit of nitpicking: (Bool,Int) would be shorthand for {1::Bool,2::Int}.
In SML, labels may be numeric or alpha-numeric. OTOH, the hash is the
projection operator (ASCII art for \pi), which can be used for both
kinds of labels:
#2 (x,y,z)
#b {a=x, b=y, c=z}
Actually, #l is just syntactic sugar for (\{l=x,...}-x), which implies
that you might need type annotations.
There are
no implicitly-defined record selectors either: you have to use pattern
matching for that.
Or projection using #.
Cheers,
- Andreas
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Let's get rid of those possible thingies! -- TB
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Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Simon Peyton-Jones wrote:
| Actually, #l is just syntactic sugar for (\{l=x,...}-x), which
implies
| that you might need type annotations.
Yes I was wrong to say that there are no implicitly-defined record
selectors; (#l r) is exactly that. Syntactically I'd prefer (r.l); but
regardless, it's a syntactic construct distinct from function
application, which must be monomorphic.
I'm not sure I parsed your sentence correctly, but in SML, (#l r) indeed
*is* a function application, and #l is a perfectly normal function, as
its desugared form reveals. It just fails to have a principal type (due
to the lack of row polymorphism), so its type must be derivable from
context - which might involve a type annotation.
BTW, I'd prefer r.l as well. A section like (.l) could then give you the
equivalent of #l.
- Andreas
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Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On Thu, Mar 04, 2004 at 09:21:23AM -, Simon Peyton-Jones wrote: My personal view is this: we should have adopted the ML view of records. It solves the immediate problem, and no more elaborate scheme seems sufficiently right to be declared the winner.Alas, like all other proposals, it's not backward compatible, and hence not likely to fly. About a year ago, you were toying with a simple polymorphic system with just has predicates. If these were automatically derived, it seems you'd get something quite close to backward compatible, except for the pesky extra lifting in record types, and not being able to omit fields when constructing the record. (And update might not statically check that all fields belong to the same constructor, for the simple version of the type system.) Is that just too clunky? ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Hi
While we're talking about SML, I think there are a few other things
worth stealing...
1) I still miss multiline pattern matching in lambda
(fn p1 = e1 | p2 = e2 | ...)
not strictly necessary, but often less disruptive of the text than
either a let/where-introduced helper-function or \ x - case x of ...
especially when there's more than one argument.
Doubtless there's some deep reason why Haskell doesn't have these
that I'm too young to remember.
2) I miss local implementation-decls in interface-decls end
Is there some fabulous layout rule for `where' clauses that means
the same thing?
3) I miss `open' from the module system. This allows
let open Structure in expression end
local open Structure in declarations end
reducing your program's acne. I often wonder why the OO world has
no room for Pascal's lovely old `with ... do ...'. [Pascal even
had variant records with case expressions: it wasn't remotely
type-safe, but it looked like datatypes. What happened to them?
Programmers like sums, but software engineers like products...]
Haskell already has per-module namespaces; everything has a unique
long name; good start. Now what we need is more control over the
meaning of short names. Could we have local opening of (already
imported) modules, simply rebinding the relevant short names in
their scope? You'd get the localized namespace you might want
at the cost of one scoped declaration, rather than a zillion
projections.
And while we're at it...
Simon Peyton-Jones wrote:
If the big bug-bear is record selectors, let's focus on them
exclusively. I now think that ML got it right. ML records are simply
labelled tuples.
The pattern-matching was always dreadful, especially if you just
wanted to tweak one field, but I expect that could be dealt with.
Andreas Rossberg wrote:
Note that this is true only for SML, not for Caml.
So just as (Bool,Int) is an anonymous type, so is
{x::Bool, y::Int}. Indeed (Bool,Int) is just shorthand for {#1::Bool,
#2::Int}.
A bit of nitpicking: (Bool,Int) would be shorthand for {1::Bool,2::Int}.
In SML, labels may be numeric or alpha-numeric. OTOH, the hash is the
projection operator (ASCII art for \pi), which can be used for both
kinds of labels:
#2 (x,y,z)
#b {a=x, b=y, c=z}
Actually, #l is just syntactic sugar for (\{l=x,...}-x), which implies
that you might need type annotations.
That's a neat piece of syntax, but if you're willing to look at types,
you might be able to introduce an opening operator for records,
| say, at the cost of restricting field labels to being valid names.
ie if
r :: {a::A, b::B, c::C}
r | e means let {a=a,b=b,c=c} = r in e
Again, the usual projection is a special case.
No reason why you couldn't use this opening notation for the existing
field-labelled datatypes. OK, you'd be shadowing projection functions
with field names, but that's not so shocking.
This opens a further, if questionable, possibility, viz
data MyQuad = MyQuad {a::Int, b::Int, c::Int} |
f x = (a * x + b) * x + c
declaring a computed field f for that constructor. As usual, only your
conscience prevents you misusing this facility in a multi-constructor
type. For backward compatibility, you'd need to leave a, b and c as
global names, but you could choose to make f accessible only on opening.
That leaves open the possibility of
data MyQuad = MyQuad {globa::Int, globb::Int, globc::Int} |
a = globa
b = globb
c = globc
f :: Int - Int
f x = (a * x + b) * x + c
which I'm sure could be sugared to
data MyQuad = MyQuad {| a::Int, | b::Int, | c::Int} |
f :: Int - Int
f x = (a * x + b) * x + c
meaning that the fields should not have global names, only local
names on opening.
Or one could add `methods' to the whole type, being functions
on that type, defined by pattern matching, but with one argument
left of |
data Fred = F1 | F2 | F3
with cycle :: Fred
F1 | cycle = F2
F2 | cycle = F3
F3 | cycle = F1
Again, cycle wouldn't be globally declared.
Goodness me, it's a per-type namespace, but not for `ordinary'
application.
There's a catch, of course. When you write
r | e
you give the typechecker no clue as to the type of r: it just
has to infer the type of r and hope it's a datatype. I suggest
this is perfectly sustainable, given that
(1) your function already has a top-level type signature,
hasn't it?
(2) this sort of thing happens all the time with ad-hoc
polymorphism anyway; when you have
class Blah x where
blah :: x - x
instance Blah (Maybe Int) where
blah Nothing = Just 0
blah (Just x) = Just (x + 1)
what's
blah Nothing
?
Is this plausible?
Conor
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Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 28/02/2004, at 1:30 AM, Per Larsson wrote: In my humble opionon explicit module prefixes are a feature, which enhance code clarity, and not something you want get rid of using rather complex namespace extensions. However, as Alastair Reid's mail in this thread indicates there are weaknesses in haskell's export mechanism. But these would be far more easy to fix than introducing the suggested per type namespace extension. Sorry, I don't buy this. If there is effort made to improve the module system to cope with the problems Alastair mentioned, we still have to write 'import FiniteMap as FM' followed by 'FM.add fm', instead of simply 'fm.add'. We fix the problems Alastair mentioned, but are still left with an less-than-optimal solution. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Simon Peyton-Jones wrote:
In Haskell today, you can at least tell what value is bound to each
identifier in the program, *without* first doing type checking.
I'm afraid I'm confused. In the following code
data Z
data S a
class Card c where c2int:: c - Int
instance Card Z where c2int _ = 0
instance (Card c) = Card (S c) where c2int _ = 1 + c2int (undefined::c)
foo = c2int (undefined::(S (S (S (S Z)
how can one tell the value of foo without first doing the
typechecking? Without typechecking, we can't use c2int and can't even
construct any meaningful value of class Card. For c2int, the type is
the ``value.'' Overlapping instances, polymorphic recursion -- all
seem to make the value determination even more uncertain.
Andre Pang wrote:
1) now I have to manually declare a class definition for every single
function, and I have to declare it in advance before any module defines
that function (most serious problem; see below),
However, declaring the instance first requires declaring the type
class itself, and that _is_ a problem, because that's exactly what I'm
trying to work around. Without 20/20 hindsight, you cannot say with
certainty what type signatures a generic function (like 'phase' or
even 'add') can support, because it's not a generic function,
But in the solution posted previously, for each ad hoc overloadable
function, the corresponding class *always* has the same
signature:
class HasAdd a b | a-b where add:: a-b
Therefore, we don't need clairvoyance to define an overloadable
function that way. If I need an overloadable function add, I can go
ahead and define the above class, and then add an instance. I can do
that without knowing all possible overloadable instances of add,
present or future. What if somebody else did the same in some other
module? If that somebody else followed the conventions, he would
introduce exactly the same class declaration. If GHC or its developers
could somehow be persuaded to overlook _exact_ duplicate class
declarations, then that part of the problem can be solved. The class
declaration itself could perhaps be generated by Template Haskell.
The discussed solution is quite related to some of the Records
proposals (which have been discussed here half a year ago). There too
we have the inconvenience of choosing unique names for field labels.
data Person = Person {_pname:: String, _paddress:: String}
data Computer = Computer {_cname::[String], _caddress:: Int}
class HasName a b | a-b where name:: a-b
class HasAddress a b | a-b where address:: a-b
instance HasName Person String where name = _pname
instance HasAddress Person String where address = _paddress
instance HasName Computer [String] where name = _cname
instance (HasName n r) = HasName (Maybe n) (Maybe r) where
name = fmap name
-- Alas, the following will break the dependency...
--instance (Num a) = HasName a String where name = show
-- But the following works: overlapping instances at work
instance (HasAddress a b) = HasName a b where name = address
instance HasAddress Int (Int-String) where address x y = show (x+y)
newtype W a = W a
instance (Num a) = HasAddress (W a) String where address (W a) = show a
test2 = let p = Person Anonymous N/A
c = Computer [FQDN,localhost] 10
in person named ++ (name p) ++ at a computer ++ (head (name c))
++ and another ++ (show$ name p1)
where p1 = (Nothing::Maybe Person)
The first few lines of the code is boilerplate and could be
automatically generated. As you can see, we can even handle a limited
form of polymorphism, and even do a hand off. For example, if some
thing doesn't have a name but has an address, we can use the address
as its name. Alas this ``backtracking'' isn't as general as we might
wish. At some point we have to introduce wrappers (like W a above) to
hand over the dispatch to another class. It's possible to do the
dispatch on a class, but it's a bit too painful. OTH, the wrappers
such as 'W' may be considered as an 'alternative view' of an object.
By wrapping an object, we can switch its behavior from the main one to
an alternative without any run-time penalty.
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RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
| In Haskell today, you can at least tell what value is bound to each | identifier in the program, *without* first doing type checking. | | I'm afraid I'm confused. In the following code | | data Z | data S a | | class Card c where c2int:: c - Int | | instance Card Z where c2int _ = 0 | instance (Card c) = Card (S c) where c2int _ = 1 + c2int (undefined::c) | | foo = c2int (undefined::(S (S (S (S Z) | | how can one tell the value of foo without first doing the | typechecking? What I meant was that you can always tell what executable code a value is bound to, without type checking. 'foo' is bound to the code for 'c2int (undefined::(S (S (S (S Z)'. 'c2int' is bound to code that extracts a method from it's first argument (which is a dictionary for Card). In any higher-order language, a function might invoke one of its arguments f x g = g x but I still say that it's clear what code is executed when f is called! It's true that with type classes the value of one of the dictionary argument is dependent on type checking, which certainly muddies the waters. But at least the type of 'c2int' isn't affected by which method is chosen, which is the real difficulty I was pointing out Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. So, in the above example, instead of writing addToFM fm ..., we could instead associate an 'add' function with the FiniteMap type, so we could write fm.add ... instead. Provided that fm's type is monomorphic, it should be possible to call the 'correct' add function; if we defined another 'add' function that's associated with Remember, too, that in OO languages the type of 'fm' is usually declared, in advance, by the programmer. In Haskell it isn't. That makes it much harder to figure out which 'add' function is going to be used. Which 'add' function is chosen depends on type type of 'fm'. But the add function that is chosen in turn influences the type of the other arguments. For example, in the call (fm.add foo), the type of 'foo' is influenced by the choice of 'add'. But the type of 'foo' might (by the magic of type inference) affect the type of 'fm' In Haskell today, you can at least tell what value is bound to each identifier in the program, *without* first doing type checking. And the binding story, all by itself, is somewhat complicated. The typing story is also (very) complicated. Winding the two into a single indissoluble whole would make it far more complicated still. My nose tells me that this way lies madness. But I've been wrong before. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 27/02/2004, at 9:51 AM, David Bergman wrote: So at the moment, many Haskellers will append the type name to the function to indicate that it only works on that particular data type. In this respect, Haskell is at a disadvantage vs most object-oriented languages, because in them, you can write x.add, and the type system will perform object-oriented polymorphism for you and call the correct add method, no matter if x is a FiniteMap or a Set. Writing addToFM fm ... or addToSet set ... is surely a lot more inconvenient than writing fm.add or set.add, no? Yes. But, you are refering to overloading, no? And, not subtype polymorphism (which is what I denote with object-oriented polymorphism)? Just to make things clear in my mind. Yes, what I'm referring to is essentially overloading. I called it object-oriented polymorphism because that's typically what OO people call such a thing :). (I should know better to use OO terminology on a Haskell list; won't happen again ...). However, it's form of overloading that Haskell cannot currently handle well with type classes -- Oleg's post proves that you can do it, of course, but that's a (very good) hack rather than a long-term solution. So, you have thought of automatically, but implicitly, introduce a namespace for each data type, and then have Haskell employ Koenig Lookup, to decide which function an expression is refering to? It's a bit like Koenig lookup in that it has the same effect, although it's probably easier for the compiler to infer the namespace wanted, since we write expr.function ... rather than function expression Writing function expression ... would work too, but then it looks like a standard function call rather than a function call associated with a particular type, and I think that causes more confusion. Long-time Haskell users understand that writing foo.f means use f in namespace foo; changing around the language semantics to mean that f foo now means use f in namespace foo would make lots of people rather unhappy :). You realize, of course, that mere intranamespacial parameter type lookup (regular overloading) would achieve the same effect, without the (implicit) namespaces? I'm not sure what you mean by intranamespcial parameter type lookup -- can you explain? There are a number of means by which the x in x.add can be communicated to the actual function: it's similar to the hidden 'self' or 'this' variable that's present when you invoke a method on an object in OO. Perhaps x is passed to the function as its first parameter, or maybe it could be its last parameter, or even an arbitrary parameter (where the parameter it's passed as could be defined in the type signature of the function). Perhaps 'self' or 'this' could be an implicit parameter. Any one of them will work just fine, I think. Again, I think you are confusing the runtime dispatching subtype polymorpism from overloading. Overloading would do what you want, while the subtype polymorphism could (still) be handled by class, and instances of classes, the Generic Programming way. I (think I) understand the difference between dynamic binding vs overloading: here, all I'm after is trying to use the type system to give us a very simple form of overloading (e.g. based on the first argument to a function), that gives us the same effect as a per-type name space. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 27/02/2004, at 4:48 PM, Brandon Michael Moore wrote: On Fri, 27 Feb 2004 [EMAIL PROTECTED] wrote: On 27/02/2004, at 1:13 PM, [EMAIL PROTECTED] wrote: 1) now I have to manually declare a class definition for every single function, and I have to declare it in advance before any module defines that function (most serious problem; see below), 2) I then have to declare instances of that type class for every function I define, 3) the type signature for phase reveals no clues about how to use that function. Declaring a type class instance is really no problem. I agree that declaring a type class instance per function is not a huge deal (if it can be automatically done by the compiler). However, declaring the instance first requires declaring the type class itself, and that _is_ a problem, because that's exactly what I'm trying to work around. Without 20/20 hindsight, you cannot say with certainty what type signatures a generic function (like 'phase' or even 'add') can support, because it's not a generic function, it's a function which is When you declare a type class, you are making a trade-off: you are saying that the interface for this function is forever set in stone, and it cannot be changed by any instances under any circumstances. In return for saying that interface is immutable, you get two major benefits: (1) an immutable interface, i.e. so you can guarantee that whenever you use ==, you _know_ the type signature is :: Eq a = a - a - Bool, and no instance can try to subvert that (unless your name is Oleg ;), and (2) you get very powerful overloading capabilities. However, the disadvantage of this tradeoff is that because the type signature is now set, you just used up another function name in the namespace. So type classes are the wrong approach to solve this problem, because what I'm after is being able to clutter up a namespace as much as I like with whatever names I like, but I don't want a polymorphic function--I want a function which only operates on one specific, primary data type. With the per-type namespace separation I'm advocating, you do not need to know and declare in advance that each function will be overloaded, you simply write a FiniteMap.add function and a Set.add function, and you get a simpler form of namespace separation (or overloading) based on the first parameter that's passed to it. It is a solution which is more _flexible_ than requiring type class definitions, and it is better than having hungarian notation for functions. In fact, I think that, right now, if we replaced the current namespace separation offered by the hierarchical module system, and instead only had this sort of per-type namespace separation, things would still be better! How much of the structure of the first paramater would you look at? Could you an implementation for pairs that depended on the actual types in the pair? I think you should try to take advantage of the existing type class machinery as much as possible here, even if what you want are not exactly (standard) type classes. The idea is if you write fm.add, you look at the type of fm as much as possible. If you see that fm is polymorphic, all bets are off, and the compiler raises an error and quits with prejudice. If fm is monomorphic, you should be able to infer its type (which includes pairs/tuples) and thus know which namespace to select to find the correct add function. So the main requirement for this to work is whether it's possible to infer the type of fm; since I'm not a type theorist, I have no idea if that is in fact possible at all. I realise my idea isn't very general in that it only allows this namespace lookup/overloading based on the type of a single argument parameter, and I think it would be possible with a bit more thinking to generalise it to work based on multiple arguments (e.g. via argument-dependent lookup, or whatnot). But even in its current form, I honestly think it offers far more flexibility and would lead to cleaner APIs than is currently possible. Read the paper and see if you think something like that might be useful. In any case, I think there's a decent chance that something useful for this would also be useful for building interfaces to object-oriented libraries, and vicea versa. I think there's probably something that covers both cases nicely and uniformly. I've had a read of both the SPJ/Shields paper on OO-style overloading in Haskell, and I've also had a skim over another paper called A Second Look at Overloading which describes another overloading calculus called System O. I don't think either paper directly addresses the problem I'm trying to solve, although some elements in the paper (e.g. closed classes) may provide a framework which is capable of addressing the problem, if something like fm.add can be translated to such a framework via major syntactic sugar :). -- % Andre Pang : trust.in.love.to.save
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
At 09:28 27/02/04 +, Simon Peyton-Jones wrote: Which 'add' function is chosen depends on type type of 'fm'. But the add function that is chosen in turn influences the type of the other arguments. For example, in the call (fm.add foo), the type of 'foo' is influenced by the choice of 'add'. But the type of 'foo' might (by the magic of type inference) affect the type of 'fm' In Haskell today, you can at least tell what value is bound to each identifier in the program, *without* first doing type checking. [...] Nicely explained. Until this, I had a feeling that the proposed type-directed value-binding resolution was potentially problematic for Haskell, but couldn't say why. #g Graham Klyne For email: http://www.ninebynine.org/#Contact ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On Fri, 27 Feb 2004, Simon Peyton-Jones wrote: The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. So, in the above example, instead of writing addToFM fm ..., we could instead associate an 'add' function with the FiniteMap type, so we could write fm.add ... instead. Provided that fm's type is monomorphic, it should be possible to call the 'correct' add function; if we defined another 'add' function that's associated with Remember, too, that in OO languages the type of 'fm' is usually declared, in advance, by the programmer. In Haskell it isn't. That makes it much harder to figure out which 'add' function is going to be used. Which 'add' function is chosen depends on type type of 'fm'. But the add function that is chosen in turn influences the type of the other arguments. For example, in the call (fm.add foo), the type of 'foo' is influenced by the choice of 'add'. But the type of 'foo' might (by the magic of type inference) affect the type of 'fm' In Haskell today, you can at least tell what value is bound to each identifier in the program, *without* first doing type checking. And the binding story, all by itself, is somewhat complicated. The typing story is also (very) complicated. Winding the two into a single indissoluble whole would make it far more complicated still. I thought this wasn't the case if there are type classes invovled. What value is + bound to in 1 + 1? All I can think is to say that the appropriate value of + is selected based on the types, or to say that the value here is the class member (subsuming several instances). Either way I don't see a method for overloading individual function names having a greatly different story either way. Actually, picking a version of a function (from the versions in scope) based on which type actually works might be useful. It seems to extend the handling of overlapping names in a useful direction again, resolving ambiguity by assuming you meant to write a typeable program. We would probably want some special syntax with the imports to request/flag this behaviour, like import A; import B; import C; resolve foo. One heuristic would be typechecking with no information on the name(s) and checking that there is a unique way to resolve the ambiguity at each point. My nose tells me that this way lies madness. I think the general principle of using types to capture and infer intent is still sound. It would be nice to have ad-hoc overloading also in cases where we don't see a common intent between several functions to capture with a typeclass (intents that we can't capture are arguments for improving the class system). A lot of haskell already looks like madness already anyway :) We just need to find things that look like good madness ;) But I've been wrong before. Simon Brandon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
In my humble opionon explicit module prefixes are a feature, which enhance code clarity, and not something you want get rid of using rather complex namespace extensions. However, as Alastair Reid's mail in this thread indicates there are weaknesses in haskell's export mechanism. But these would be far more easy to fix than introducing the suggested per type namespace extension. The present export list is also rather ugly from an aesthetic point of view, clottering the module header. I would love to see an, repeatable, explicit 'export' directive with similar keywords as the import directive. Allowing for example: 'export [all] hiding (...)' with obvious semantics. Per Larsson ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Andre Ozone wrote: On 27/02/2004, at 9:51 AM, David Bergman wrote: So at the moment, many Haskellers will append the type name to the function to indicate that it only works on that particular data type. In this respect, Haskell is at a disadvantage vs most object-oriented languages, because in them, you can write x.add, and the type system will perform object-oriented polymorphism for you and call the correct add method, no matter if x is a FiniteMap or a Set. Writing addToFM fm ... or addToSet set ... is surely a lot more inconvenient than writing fm.add or set.add, no? Yes. But, you are refering to overloading, no? And, not subtype polymorphism (which is what I denote with object-oriented polymorphism)? Just to make things clear in my mind. Yes, what I'm referring to is essentially overloading. I called it object-oriented polymorphism because that's typically what OO people call such a thing :). No, they do not. What they call polymorphism, we call subype polymorphism or, if we are really hard-core and/or old school, even ad-hoc polymorphism. They do not even realize that overloading falls in the category of polymorphism at all... (I should know better to use OO terminology on a Haskell list; won't happen again ...). I think it is good that you do. We need that touch of engineering realism sometimes ;-) However, it's form of overloading that Haskell cannot currently handle well with type classes -- Oleg's post proves that you can do it, of course, but that's a (very good) hack rather than a long-term solution. I personally use (Haskell) classes for that overloading purpose, but in a sense that Generic Programmers would call concept modelling. So, you have thought of automatically, but implicitly, introduce a namespace for each data type, and then have Haskell employ Koenig Lookup, to decide which function an expression is refering to? It's a bit like Koenig lookup in that it has the same effect, although it's probably easier for the compiler to infer the namespace wanted, since we write expr.function ... rather than function expression Whether it is prefix or postfix should not alter the complexity of the lookup considerably. Writing function expression ... would work too, but then it looks like a standard function call rather than a function call associated with a particular type, and I think that causes more confusion. Long-time Haskell users understand that writing foo.f means use f in namespace foo; changing around the language semantics to mean that f foo now means use f in namespace foo would make lots of people rather unhappy :). I would want it to look as an ordinary function. The single biggest problem with Haskell, in my extremely humble opinion, is the shared namespace for all data type accessors, with which you probably agree. It is what irritated me the most with Entity-Relationship Diagram, that all fields need to have unique name globally. This in contrast to instance variables, methods and general overloading, as often found in OO languages. You realize, of course, that mere intranamespacial parameter type lookup (regular overloading) would achieve the same effect, without the (implicit) namespaces? I'm not sure what you mean by intranamespcial parameter type lookup -- can you explain? ah, I meant regular overloading, i.e., have a function be identified not by its name, but by its whole signature, including the arity and parameter type(s) [yes, curry, curry...] There are a number of means by which the x in x.add can be communicated to the actual function: it's similar to the hidden 'self' or 'this' variable that's present when you invoke a method on an object in OO. Perhaps x is passed to the function as its first parameter, or maybe it could be its last parameter, or even an arbitrary parameter (where the parameter it's passed as could be defined in the type signature of the function). Perhaps 'self' or 'this' could be an implicit parameter. Any one of them will work just fine, I think. Again, I think you are confusing the runtime dispatching subtype polymorpism from overloading. Overloading would do what you want, while the subtype polymorphism could (still) be handled by class, and instances of classes, the Generic Programming way. I (think I) understand the difference between dynamic binding vs overloading: here, all I'm after is trying to use the type system to give us a very simple form of overloading (e.g. based on the first argument to a function), that gives us the same effect as a per-type name space. When I read my own response, I know realize that it sounds harsh. Sorry about that. That was not the intention. I also think you understand and appreciate the difference between the two forms of polymorphisms. You touch at one of the core problems of Haskell. Thanks, David
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
David Bergman [EMAIL PROTECTED] writes: | The idea that I've been throwing around is to be able to define a | separate namespace for each type; a function can either belong in a | global (default) namespace, or belong in a particular type's | namespace. So, in the above example, instead of writing addToFM fm | ..., we could instead associate an 'add' function with the FiniteMap | type, so we could write fm.add ... instead. Provided that fm's type | is monomorphic, it should be possible to call the 'correct' add | function; if we defined another 'add' function that's associated with | the Set type, that will only get called if the 'x' in x.add is of | type :: Set. So, like OO languages which inherently give separate | namespaces to their different objects, here we give separate | namespaces to different | (monomorphic) types. In this case, if one simply writes add instead | of x.add, the compiler throws an error, because there is no 'add' | function defined in the default namespace; add is only defined when a | programmer writes x.add where x :: FiniteMap or x :: | Set[1]. | | This overloading by namespace is usually called either ADL | (Argument-Dependent Lookup) or Koenig Lookup (especially in C++.) Actually in C++, it is called argument dependent name lookup, and that is the way the C++ definition text calls it. As Andy Koenig has himself pointed out, he did not invent that rule. He mentionned it when the C++ committee was solving a name look-up problem posed by namespaces to operator functions. That name look-up rule was later generalized to non-operator to cover the function-call syntax -- which is what is most known today and referred to above. This ends my C++ hour on Haskell list :-) -- Gaby ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
[Haskell] Per-type function namespaces (was: Data.Set whishes)
I've had an idea stewing in my head to do with per-type function namespaces, that the current module namespace discussion reminded me about. The problem is that there is a limited namespace for functions, so that if you define a new data type, it is unwise to call functions which work on that data type a very generic name such as 'add'. An example of this is Data.FiniteMap and Data.Set: both data types define a function to add things to their respective data types. addToFM :: Ord key = FiniteMap key elt - key - elt - FiniteMap key elt addToSet :: Ord a = Set a - a - Set a So at the moment, many Haskellers will append the type name to the function to indicate that it only works on that particular data type. In this respect, Haskell is at a disadvantage vs most object-oriented languages, because in them, you can write x.add, and the type system will perform object-oriented polymorphism for you and call the correct add method, no matter if x is a FiniteMap or a Set. Writing addToFM fm ... or addToSet set ... is surely a lot more inconvenient than writing fm.add or set.add, no? The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. So, in the above example, instead of writing addToFM fm ..., we could instead associate an 'add' function with the FiniteMap type, so we could write fm.add ... instead. Provided that fm's type is monomorphic, it should be possible to call the 'correct' add function; if we defined another 'add' function that's associated with the Set type, that will only get called if the 'x' in x.add is of type :: Set. So, like OO languages which inherently give separate namespaces to their different objects, here we give separate namespaces to different (monomorphic) types. In this case, if one simply writes add instead of x.add, the compiler throws an error, because there is no 'add' function defined in the default namespace; add is only defined when a programmer writes x.add where x :: FiniteMap or x :: Set[1]. There are a number of means by which the x in x.add can be communicated to the actual function: it's similar to the hidden 'self' or 'this' variable that's present when you invoke a method on an object in OO. Perhaps x is passed to the function as its first parameter, or maybe it could be its last parameter, or even an arbitrary parameter (where the parameter it's passed as could be defined in the type signature of the function). Perhaps 'self' or 'this' could be an implicit parameter. Any one of them will work just fine, I think. However, this scheme is only for functions which have such a 'primary' data type to be associated with, such as FiniteMap or Set. For functions which are truly polymorphic (such as ==), you still leave them in the default namespace. Perhaps it's sensible to even make it a requirement that functions in the default namespace must be polymorphic: if they are monomorphic, they are associated with operating on a specific data type, so they should belong in a type-specific namespace. You then still guarantee that such commonly-used polymorphic functions cannot be 'hijacked' to have stupid type signatures; i.e. == is always guaranteed to be :: Eq a - a - Bool. Anyhow, feedback is more than welcome; I would certainly welcome this addition if it's feasible. It feels inferior to be typing in 'addToFM foo' all the time when our OO brethren type the simpler and more succinct 'foo.add', especially given that Haskell's type system is far more powerful! 1. I haven't thought hard enough about whether it would be possible to have the same function name in both the 'default' namespace as well as in per-type namespaces, but my gut feeling says it should be OK. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
I've had an idea stewing in my head to do with per-type function namespaces, that the current module namespace discussion reminded me about. The problem is that there is a limited namespace for functions, so that if you define a new data type, it is unwise to call functions which work on that data type a very generic name such as 'add'. [..] The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. This feature would seem to be in competition with type classes; could you elaborate on the relative advantages and disadvantages? The type class story has the advantage of being well understood and quite effective, but there are certainly some limitations too. --KW 8-) ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 27/02/2004, at 3:47 AM, Keith Wansbrough wrote: I've had an idea stewing in my head to do with per-type function namespaces, that the current module namespace discussion reminded me about. The problem is that there is a limited namespace for functions, so that if you define a new data type, it is unwise to call functions which work on that data type a very generic name such as 'add'. [..] The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. This feature would seem to be in competition with type classes; could you elaborate on the relative advantages and disadvantages? The type class story has the advantage of being well understood and quite effective, but there are certainly some limitations too. I don't think type classes can solve the problem I'm trying to tackle. As an example of why, check out the types of FiniteMap and Set's 'add' functions: addToFM :: Ord key = FiniteMap key elt - key - elt - FiniteMap key elt addToSet :: Ord a = Set a - a - Set a Note that the type of addToFM takes in two parameters (besides the FiniteMap itself): a key and an element, whereas the type of addToSet only takes in one parameter, which is the thing to add. So, how can you come up with a type class which provides a polymorphic 'add' function, considering you don't even know how many parameters each data type's individual add function uses? Even if you could define such a type class (which I don't think is possible), you then have one less function in the namespace to use, which is another problem. For example, say I'm writing the Data.Complex module; there's a function in that module phase :: RealFloat a = Complex a - a. So, how do you put this phase function into a type class? Perhaps you could abstract away from the RealFloat and Complex bits, so you have a phase function which is generalised to work over a Num and an arbitrary data type instead; e.g. class Phase c where phase :: Num a = c a - a. But what happens if, say, somebody adds a Moon data type, and they want to write a phase function which returns the phase of such a moon? Phases of the moon certainly aren't Nums, nevermind the fact that you probably want to supply your moon phase's function with some sort of date as an extra parameter, which means the Phase type class isn't flexible enough. Type classes are designed to provide a type-consistent interface to functions which perform different behaviour, unifying them as one function like + or == -- but it's designed to work for arbitrary types. What I'm after is an interface for a function which may change depending on a primary type it's working with, which is almost the opposite to type classes. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
I think that this is a problem that can be solved with a simple convention change, rather than a language extension - instead of appending type names, I think it would be much better if modules simply used the short, convenient, common names and expected the user to import them qualified where overlap is a problem - in short, do exactly what DData does. It's slightly more verbose than OO-style: Map.add map key value instead of map.add(key, value); but I don't think that what OO does is a good language design target. Another random thought: what you describe sounds awfully similar to typeclasses, just with a single function in each typeclass. Abe [EMAIL PROTECTED] writes: I've had an idea stewing in my head to do with per-type function namespaces, that the current module namespace discussion reminded me about. The problem is that there is a limited namespace for functions, so that if you define a new data type, it is unwise to call functions which work on that data type a very generic name such as 'add'. An example of this is Data.FiniteMap and Data.Set: both data types define a function to add things to their respective data types. addToFM :: Ord key = FiniteMap key elt - key - elt - FiniteMap key elt addToSet :: Ord a = Set a - a - Set a So at the moment, many Haskellers will append the type name to the function to indicate that it only works on that particular data type. In this respect, Haskell is at a disadvantage vs most object-oriented languages, because in them, you can write x.add, and the type system will perform object-oriented polymorphism for you and call the correct add method, no matter if x is a FiniteMap or a Set. Writing addToFM fm ... or addToSet set ... is surely a lot more inconvenient than writing fm.add or set.add, no? The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. So, in the above example, instead of writing addToFM fm ..., we could instead associate an 'add' function with the FiniteMap type, so we could write fm.add ... instead. Provided that fm's type is monomorphic, it should be possible to call the 'correct' add function; if we defined another 'add' function that's associated with the Set type, that will only get called if the 'x' in x.add is of type :: Set. So, like OO languages which inherently give separate namespaces to their different objects, here we give separate namespaces to different (monomorphic) types. In this case, if one simply writes add instead of x.add, the compiler throws an error, because there is no 'add' function defined in the default namespace; add is only defined when a programmer writes x.add where x :: FiniteMap or x :: Set[1]. There are a number of means by which the x in x.add can be communicated to the actual function: it's similar to the hidden 'self' or 'this' variable that's present when you invoke a method on an object in OO. Perhaps x is passed to the function as its first parameter, or maybe it could be its last parameter, or even an arbitrary parameter (where the parameter it's passed as could be defined in the type signature of the function). Perhaps 'self' or 'this' could be an implicit parameter. Any one of them will work just fine, I think. However, this scheme is only for functions which have such a 'primary' data type to be associated with, such as FiniteMap or Set. For functions which are truly polymorphic (such as ==), you still leave them in the default namespace. Perhaps it's sensible to even make it a requirement that functions in the default namespace must be polymorphic: if they are monomorphic, they are associated with operating on a specific data type, so they should belong in a type-specific namespace. You then still guarantee that such commonly-used polymorphic functions cannot be 'hijacked' to have stupid type signatures; i.e. == is always guaranteed to be :: Eq a - a - Bool. Anyhow, feedback is more than welcome; I would certainly welcome this addition if it's feasible. It feels inferior to be typing in 'addToFM foo' all the time when our OO brethren type the simpler and more succinct 'foo.add', especially given that Haskell's type system is far more powerful! 1. I haven't thought hard enough about whether it would be possible to have the same function name in both the 'default' namespace as well as in per-type namespaces, but my gut feeling says it should be OK. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 27/02/2004, at 8:28 AM, Abraham Egnor wrote: I think that this is a problem that can be solved with a simple convention change, rather than a language extension - instead of appending type names, I think it would be much better if modules simply used the short, convenient, common names and expected the user to import them qualified where overlap is a problem - in short, do exactly what DData does. It's slightly more verbose than OO-style: Map.add map key value instead of map.add(key, value); but I don't think that what OO does is a good language design target. This is exactly what was discussed in the thread before I barged in with per-type function namespaces, and it's not a good solution because of what Alastair has mentioned. It's also not a good solution because I still have to type Map.add map instead of map.add: the type system already knows that map of type Map, so why should I have to qualify it even more by sticking a module name in front, and also encode the type name into my function because the module/namespace system isn't good enough to deal with this issue? I also agree that what OO does is not a good language design target, but I do think that leverage type system to make programming nicer for you is a good design target :). We're using a form of hungarian notation for function names, which is necessary because of a global namespace; OO people abolished this a long time ago. Another random thought: what you describe sounds awfully similar to typeclasses, just with a single function in each typeclass. It's not the same as a single-function type class, for the reasons that I pointed out to Keith Wansbrough in an earlier email. -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Mr. Ozone wrote: [snip] So at the moment, many Haskellers will append the type name to the function to indicate that it only works on that particular data type. In this respect, Haskell is at a disadvantage vs most object-oriented languages, because in them, you can write x.add, and the type system will perform object-oriented polymorphism for you and call the correct add method, no matter if x is a FiniteMap or a Set. Writing addToFM fm ... or addToSet set ... is surely a lot more inconvenient than writing fm.add or set.add, no? Yes. But, you are refering to overloading, no? And, not subtype polymorphism (which is what I denote with object-oriented polymorphism)? Just to make things clear in my mind. The idea that I've been throwing around is to be able to define a separate namespace for each type; a function can either belong in a global (default) namespace, or belong in a particular type's namespace. So, in the above example, instead of writing addToFM fm ..., we could instead associate an 'add' function with the FiniteMap type, so we could write fm.add ... instead. Provided that fm's type is monomorphic, it should be possible to call the 'correct' add function; if we defined another 'add' function that's associated with the Set type, that will only get called if the 'x' in x.add is of type :: Set. So, like OO languages which inherently give separate namespaces to their different objects, here we give separate namespaces to different (monomorphic) types. In this case, if one simply writes add instead of x.add, the compiler throws an error, because there is no 'add' function defined in the default namespace; add is only defined when a programmer writes x.add where x :: FiniteMap or x :: Set[1]. This overloading by namespace is usually called either ADL (Argument-Dependent Lookup) or Koenig Lookup (especially in C++.) So, you have thought of automatically, but implicitly, introduce a namespace for each data type, and then have Haskell employ Koenig Lookup, to decide which function an expression is refering to? You realize, of course, that mere intranamespacial parameter type lookup (regular overloading) would achieve the same effect, without the (implicit) namespaces? The core problem in Haskell is to bypass the generics, i.e., make sure that a certain definition is used for a certain type, or combination of types. This can only be done by class instances, as of now, but there have been discussions of non-class overloading. There are a number of means by which the x in x.add can be communicated to the actual function: it's similar to the hidden 'self' or 'this' variable that's present when you invoke a method on an object in OO. Perhaps x is passed to the function as its first parameter, or maybe it could be its last parameter, or even an arbitrary parameter (where the parameter it's passed as could be defined in the type signature of the function). Perhaps 'self' or 'this' could be an implicit parameter. Any one of them will work just fine, I think. Again, I think you are confusing the runtime dispatching subtype polymorpism from overloading. Overloading would do what you want, while the subtype polymorphism could (still) be handled by class, and instances of classes, the Generic Programming way. /David ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Gabriel wrote: | This overloading by namespace is usually called either ADL | (Argument-Dependent Lookup) or Koenig Lookup (especially in C++.) Actually in C++, it is called argument dependent name lookup, and that is the way the C++ definition text calls it. As Andy Koenig has himself pointed out, he did not invent that rule. He mentionned it when the C++ committee was solving a name look-up problem posed by namespaces to operator functions. That name look-up rule was later generalized to non-operator to cover the function-call syntax -- which is what is most known today and referred to above. This ends my C++ hour on Haskell list :-) Yeah! Get back to that dark corner where people solve real problems! ;-) /David ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On Fri, 27 Feb 2004 [EMAIL PROTECTED] wrote: On 27/02/2004, at 1:13 PM, [EMAIL PROTECTED] wrote: 1) now I have to manually declare a class definition for every single function, and I have to declare it in advance before any module defines that function (most serious problem; see below), 2) I then have to declare instances of that type class for every function I define, 3) the type signature for phase reveals no clues about how to use that function. Declaring a type class instance is really no problem. You just need to write an instance Class (Type) instead of function :: Type on the line before the function declaration. The type on phase itself wouldn't provide much information, but the list of instances in each module defines would be informative. Something like :info wouldn't be much help without modification. So unfortunately, this is hardly a scalable solution. The entire reason I came up with the idea is because if we use type classes to implement this sort of overloading, we have to know every single possible function that any module author will ever create, and declare classes for those functions in advance. This is fine if you're declaring truly polymorphic functions which are designed from the start to be totally general, but it is not designed for functions which may do vastly different things and may contain totally different type signatures, but share the same name because that would be a sensible thing to do. (e.g. the phase function mentioned above.) In the paper Object-Oriented Style Overloading for Haskell, Mark Shields and Simon Peyton-Jones. One of the things they propose is adding method constraints to the type system which (as far as I can tell) basically amounts to generating a type class for each funtion name, and letting you write constraints like (foo :: Int - Int) on your function. They would set up the type classes like class Has_foo a where foo :: a, which can causes problems if your argument and return value are polymorphic under a class constraint rather than concrete types. Making the method classes implicitly closed would probably help here. (closed classes are another suggestion). While making that closed world assumption it would probably be nice if it only selected between the versions of the function that were actually in scope at the moment (so these would act kind of like methods that overload if you import several of them, rather than conflicting like normal). As long as we are integrating these special type classes into the language we can make sure things like error messages and ghci give decent information, maybe listing all the different types the function is imported at, and where each version is defined. With the per-type namespace separation I'm advocating, you do not need to know and declare in advance that each function will be overloaded, you simply write a FiniteMap.add function and a Set.add function, and you get a simpler form of namespace separation (or overloading) based on the first parameter that's passed to it. It is a solution which is more _flexible_ than requiring type class definitions, and it is better than having hungarian notation for functions. In fact, I think that, right now, if we replaced the current namespace separation offered by the hierarchical module system, and instead only had this sort of per-type namespace separation, things would still be better! How much of the structure of the first paramater would you look at? Could you an implementation for pairs that depended on the actual types in the pair? I think you should try to take advantage of the existing type class machinery as much as possible here, even if what you want are not exactly (standard) type classes. I realise my idea isn't very general in that it only allows this namespace lookup/overloading based on the type of a single argument parameter, and I think it would be possible with a bit more thinking to generalise it to work based on multiple arguments (e.g. via argument-dependent lookup, or whatnot). But even in its current form, I honestly think it offers far more flexibility and would lead to cleaner APIs than is currently possible. Read the paper and see if you think something like that might be useful. In any case, I think there's a decent chance that something useful for this would also be useful for building interfaces to object-oriented libraries, and vicea versa. I think there's probably something that covers both cases nicely and uniformly. Brandon -- % Andre Pang : trust.in.love.to.save ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
On 27/02/2004, at 1:13 PM, [EMAIL PROTECTED] wrote:
For example, say I'm writing the Data.Complex module; there's a
function in that module phase :: RealFloat a = Complex a - a. So,
how do you put this phase function into a type class? Perhaps you
could abstract away from the RealFloat and Complex bits, so you have a
phase function which is generalised to work over a Num and an
arbitrary data type instead; e.g. class Phase c where phase :: Num a
= c a - a. But what happens if, say, somebody adds a Moon data
type, and they want to write a phase function which returns the phase
of such a moon? Phases of the moon certainly aren't Nums, nevermind
the fact that you probably want to supply your moon phase's function
with some sort of date as an extra parameter, which means the Phase
type class isn't flexible enough.
Here's the code that does exactly as you wish:
{-# OPTIONS -fglasgow-exts #-}
import qualified Complex
class Phase a b | a - b where
phase:: a - b
instance (RealFloat a) = Phase (Complex.Complex a) a where
phase = Complex.phase
data MoonPhase = P1 | P2 | P3 | P4 deriving Show
instance Phase Int MoonPhase where
phase x = if x `mod` 4 == 0 then P1 else P4
instance Phase MoonPhase (Int-Int) where
phase P1 x = x
phase P2 x = x+1
main = do
putStrLn $ show $ phase ( (1.0::Float) Complex.:+
(1.0::Float))
putStrLn $ show $ phase (0::Int)
putStrLn $ show $ phase P1 (2::Int)
Very, very nice Oleg :). I'm glad to know that we can achieve such
things using the existing type class mechanisms already. However, this
still doesn't solve the problem, because:
1) now I have to manually declare a class definition for every single
function, and I have to declare it in advance before any module defines
that function (most serious problem; see below),
2) I then have to declare instances of that type class for every
function I define,
3) the type signature for phase reveals no clues about how to use that
function.
So unfortunately, this is hardly a scalable solution. The entire
reason I came up with the idea is because if we use type classes to
implement this sort of overloading, we have to know every single
possible function that any module author will ever create, and declare
classes for those functions in advance. This is fine if you're
declaring truly polymorphic functions which are designed from the start
to be totally general, but it is not designed for functions which may
do vastly different things and may contain totally different type
signatures, but share the same name because that would be a sensible
thing to do. (e.g. the phase function mentioned above.)
With the per-type namespace separation I'm advocating, you do not need
to know and declare in advance that each function will be overloaded,
you simply write a FiniteMap.add function and a Set.add function, and
you get a simpler form of namespace separation (or overloading) based
on the first parameter that's passed to it. It is a solution which is
more _flexible_ than requiring type class definitions, and it is better
than having hungarian notation for functions. In fact, I think that,
right now, if we replaced the current namespace separation offered by
the hierarchical module system, and instead only had this sort of
per-type namespace separation, things would still be better!
I realise my idea isn't very general in that it only allows this
namespace lookup/overloading based on the type of a single argument
parameter, and I think it would be possible with a bit more thinking to
generalise it to work based on multiple arguments (e.g. via
argument-dependent lookup, or whatnot). But even in its current form,
I honestly think it offers far more flexibility and would lead to
cleaner APIs than is currently possible.
--
% Andre Pang : trust.in.love.to.save
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Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
Hello!
So, how can you come up with a type class which provides a polymorphic
'add' function, considering you don't even know how many parameters
each data type's individual add function uses?
Very easily: every Haskell function takes only one
argument. Always. Ever.
For example, say I'm writing the Data.Complex module; there's a
function in that module phase :: RealFloat a = Complex a - a. So,
how do you put this phase function into a type class? Perhaps you
could abstract away from the RealFloat and Complex bits, so you have a
phase function which is generalised to work over a Num and an
arbitrary data type instead; e.g. class Phase c where phase :: Num a
= c a - a. But what happens if, say, somebody adds a Moon data
type, and they want to write a phase function which returns the phase
of such a moon? Phases of the moon certainly aren't Nums, nevermind
the fact that you probably want to supply your moon phase's function
with some sort of date as an extra parameter, which means the Phase
type class isn't flexible enough.
Here's the code that does exactly as you wish:
{-# OPTIONS -fglasgow-exts #-}
import qualified Complex
class Phase a b | a - b where
phase:: a - b
instance (RealFloat a) = Phase (Complex.Complex a) a where
phase = Complex.phase
data MoonPhase = P1 | P2 | P3 | P4 deriving Show
instance Phase Int MoonPhase where
phase x = if x `mod` 4 == 0 then P1 else P4
instance Phase MoonPhase (Int-Int) where
phase P1 x = x
phase P2 x = x+1
main = do
putStrLn $ show $ phase ( (1.0::Float) Complex.:+ (1.0::Float))
putStrLn $ show $ phase (0::Int)
putStrLn $ show $ phase P1 (2::Int)
You can evaluate a phase of a complex number, get a phase of the moon
corresponding to some integer, and even convert a phase of the moon to
a time (given another integer as a reference time). Whereas the first
two functions take one argument, the latter phase takes two
arguments. The class Phase takes the classical first-argument
overloading. Other overloading schemes are possible (e.g., the ones
that overload based on the result -- something that C++ just can't do:
e.g., Read). If we need to evaluate phases of Saturn moons (and we
overload on the first argument), we can resolve the overloading using
newtype:
newtype SaturnTime a = ST a
instance Phase (SaturnTime Int) (Int - MoonPhase) where
phase x moon_index = P1
newtypes add no run-time overhead, and actually help in making the
code more explicit.
Regarding Koening lookup: as I read in DDJ, it's just a hack! First
the committee added the namespaces, and then realized that using
operators like became hugely inconvenient. So Koening came up with
a hack. Shouldn't a language be designed in a more systematic way?
Speaking of the language design, November 2003 issue of Dr.Dobbs
J. has an interesting article: C++ Compilers and ISO Conformance [by
Brian A. Malloy, James F. Power and Tanton H. Gibbs, pp. 54-60].
Here's a summary. C++ standard has been ratified by the ISO Committee
in September 1998. There is no conformance suite however. So, we
cannot tell how well a particular compiler complies with a
standard. The authors of an article decided to create an approximate
conformance suite -- from the examples given in the standard
itself. It's a hard job -- the examples aren't meant to be a compiled
code, so some declarations and other pieces have to be filled in. The
result cannot be considered a truly compliance suite because not all
features of the language are illustrated in examples, and the
distribution of the examples is uneven. Nevertheless, it's a start.
The authors of the article have tested several compilers. The bottom
line -- after five years, no single compiler fully complies with the
standard. The best compiler, from the Edison Group (a three-person
company) fails only 2 tests. Intel's compiler fails three. Visual C++
7.1 from Microsoft fails 12. Gcc 3.3 fails 26. The latter number shows
that a wide community participation and OpenSource do not necessarily
lead to a better product. Gcc 3.3 is also one of the slowest
compilers.
But there is worse news for C++. C++ Language Standard consists of 776
pages, describing C++ language and the C++ core library. At present,
411 points in the C++ language part and 402 points in the library
part have been identified as questionable or outright erroneous. 93
language issues have been already acknowledged as errors. That is,
EVERY page of the standard, on average, contains some issue! The
committee obviously didn't bother to check their examples. Well, even
now there isn't a compiler that complies with the standard -- whatever
the compliance may mean.
Not only programmers don't know what some C++
rules mean. Not only compiler writers are puzzled. Even the committee
itself obviously doesn't know how _many_ features are supposed to
work. Can you imagine more shoddy work?
Incidentally, here's one questionable example
Re: [Haskell] Per-type function namespaces (was: Data.Set whishes)
[EMAIL PROTECTED] writes: addToFM :: Ord key = FiniteMap key elt - key - elt - FiniteMap key elt addToSet :: Ord a = Set a - a - Set a So, how can you come up with a type class which provides a polymorphic 'add' function, considering you don't even know how many parameters each data type's individual add function uses? Why, by chea^H^H^Hurrying, of course: class Collection a b | a - b where add :: a - b - a instance Collection Set a where add = addToSet instance Collection FiniteMap k e where add fm (k,e) = addToFM fm k e But I take your point, this could be hard to do in the general case. E.g. 'delete' would probably only want a key. -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
