Re: [Haskell-cafe] Using _ on the RHS of an equation?
Hi Jason, I like the idea. I've seen some code from Oleg Kiselyov which uses __ (two underscores) in this way. The thing that stops me though, is when I get it wrong, and undefined fires somewhere, but I don't know where. Something like you propose, but with a line number, would be sweet. Paul The University of Glasgow, charity number SC004401 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Largest types in SYB
I was looking for a simple generic technique targeting and transforming the largest terms of a particular type. For example, with Expr and Val declared as: data Expr = Val Val | Add Expr Expr | Sub Expr Expr deriving (Show, Eq, Typeable, Data) data Val = Var String | Struct [Expr] deriving (Show, Eq, Typeable, Data) and a test Expr e, e = Sub (Val $ Var A) (Add (Val $ Var X1) (Val $ Struct [Add (Val $ Var Y1) (Val $ Var Y2)])) I wanted to replace only the inner Add expression Add (Val $ Var Y1) (Val $ Var Y2) because its parent is not of the same type as itself (it's a list), using a function such as chopAdd: chopAdd (Add _ _) = Val $ Var AddChop chopAdd e = e But using everywhere from Scrap Your BoilerPlate (SYB): everywhere (mkT repAdd) e I'd get: Sub (Val (Var A)) (Val (Var AddChop)) while I was hoping for: Sub (Val (Var A)) (Add (Val (Var X1)) (Val (Struct [Val (Var AddChop)]))) The Haskell.org SYB wiki presents listifyWholeLists. This is relevant, though it applies queries rather than transformations. It uses a function called synthesize, which I was ultimately unable to properly reference, or apply to the problem at hand. So here is my solution: everywhereBar :: GenericQ Bool - GenericT - GenericT everywhereBar q f x | q x = (gmapT (everywhereBar (typeEq x) f) x) | otherwise = f (gmapT (everywhereBar (typeEq x) f) x) where typeEq p c = typeOf p == typeOf c It's like SYB's everywhereBut, except 1. The consequence of q x being True is only to remove the application of f to the parent; not to stop traversal. 2. q is not constant. Instead it is the partial application of a local function, typeEq. An application looks like: everywhereBar (const False) (mkT repAdd) e with (const False) the user is choosing the outcome of the very first q x in everywhereBar. I'm enjoying using SYB, and had hoped to use only functions from the package, but couldn't find a way; and this does the job for now. I've also seen that there are many other approaches to generic programming than SYB (even for AST transformations in particular) but I wanted to understand SYB first. I'm interested to know if anyone has a more elegant SYB solution. And here's the monadic version: everywhereBarM :: Monad m = GenericQ Bool - GenericM m - GenericM m everywhereBarM q f x | q x = gmapM (everywhereBarM (typeEq x) f) x | otherwise = do x' - gmapM (everywhereBarM (typeEq x) f) x f x' where typeEq p c = typeOf p == typeOf c Cheers, Paul The University of Glasgow, charity number SC004401 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [: Where the bracket things are? :]
I'd like to add my own custom list delimiters to ghc; such as the [: and :] of Data Parallel Haskell. The purpose is mainly to learn a little about GHC's internals. Any suggestions on the GHC files I should look at first? Alternatively, maybe this is actually possible from outside the compiler. Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ghci and applicative
Hi, I'm finding that some data types which use Applicative to instantiate the Num class, give responses I wasn't expecting at the ghci prompt. A simple example is list: import Control.Applicative instance (Num a) = Num [a] where as + bs = (+) $ as * bs (*) = undefined;abs = undefined signum = undefined; fromInteger = undefined f1 = let op = (+) in [1,2,3] `op` [1,1,1] f2 = let op = (+) in op [1,2,3] [1,1,1] Functions f1 and f2 give no problems at the ghci prompt. However if I instead type the body of either interactively, I get an error: *Main let op = (+) *Main [1,2,3] `op` [1,1,1] interactive:1:0: Couldn't match expected type `Integer' against inferred type `[a]' In the first argument of `op', namely `[1, 2, 3]' In the expression: [1, 2, 3] `op` [1, 1, 1] In the definition of `it': it = [1, 2, 3] `op` [1, 1, 1] I get the same error message with op [1,2,3] [1,1,1]. Any thoughts? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] ghci and applicative
Thanks Ryan, I'm slowly becoming aware of the effects of Monomorphism. I'll look again at Neil Mitchell's blog post. I guess it's the same thing when I try: let a = 1 a + 1.0 I'm taking the mono as a clue that the type inferencing will complete after each ghci carriage return; once only. In this example when a is set, it is to an Integer. One might imagine ghci could wait until I use a somewhere, but that's not how things are. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Nested Lists
Hi all, If I have a list, and I'd like to convert it to a list of lists, each of length n, I can use a function like bunch: bunch _ [] = [] bunch n as = let (c,cs) = splitAt n as in c:bunch n cs bunch 8 [1..16] [[1,2,3,4,5,6,7,8],[9,10,11,12,13,14,15,16]] If I now want to do the same for the nested lists, I can compose an application involving both map and bunch: map (bunch 4) . bunch 8 $ [1..16] [[[1,2,3,4],[5,6,7,8]],[[9,10,11,12],[13,14,15,16]]] and I can bunch the new length 4 lists again: map (map (bunch 2)) . map (bunch 4) . bunch 8 $ [1..16] 1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16 Clearly there is a pattern here involving the bunch function and latterly, three Int parameters; 2, 4 and 8. My question is, can I create a function that will take such parameters as a list, and give the same result, for example: f [2,4,8] [1..16] 1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16 or perhaps: f [bunch 2, bunch 4, bunch 8] [1..16] 1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16 I think it may not be possible because the type signature of f would depend on the length of its list parameter; but I'm not sure. -Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] RE: Nested Lists
Emil, Felipe, Thanks. I don't know Type Families, but take the point that the input can be parameterised with something something other than a list. i.e. (8 :+: 4 :+: 2 :+: ()) presumably has the same type as (4 :+: 2 :+: ()). My intention was to use common list functions on the sublists, but always then a concat for each level, to return to a flat list. With that in mind I made the following oddity, which in any case doesn't compile due to its use of infinite types. app (f:fs) es = appUp (f:fs) es where len = genericLength (f:fs) appUp [] es = appDown es len appUp (f:fs) es = appUp (map map fs) (f es) appDown es len= appDown (concat es) (len - 1) appDown es 0 = es Henning, I agree with you, a tree would be much better for this. Thanks. -Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] iota
Hi all, I was looking for an APL-style iota function for array indices. I noticed range from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: let (a,b) = (2,3) index ((0,0),(a-1,b-1)) [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn't long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) = [a] - [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] = genericReplicate r lists = map buildRepList tups in transpose lists length $ iota [2,3,4] 24 Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] RE: iota
That is quite spectacular. I revised my knowledge of sequence with a little function, akin to sequence [xs1,xs2]: seq2 xs1 xs2 = do x1 - xs1 x2 - xs2 return [x1,x2] seq2 [0,1] [0,1,2] [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2]] I like your point-free style too; and that's a nice use of pred. Many thanks, Paul The iota function you're looking for can be a whole lot simpler if you know about monads (list monad in particular) and sequence. For lists, sequence has the following behaviour: sequence [xs1,xs2, ... xsn] = [[x1,x2, ... , xn] | x1 - xs1, x2 - xs2, ... , xn - xsn] Using this, you can reduce your iota function to a powerful one-liner: iota = sequence . map (enumFromTo 0 . pred) Kind regards, Raynor Vliegendhart From: Paul Keir Sent: 01 June 2009 10:01 To: haskell-cafe@haskell.org Subject: iota Hi all, I was looking for an APL-style iota function for array indices. I noticed range from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: let (a,b) = (2,3) index ((0,0),(a-1,b-1)) [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn't long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) = [a] - [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] = genericReplicate r lists = map buildRepList tups in transpose lists length $ iota [2,3,4] 24 Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [] == []
Hi all, GHC is not happy with this: f = [] == [] nor this: f' = ([]::(Eq a) = [a]) == ([]::(Eq a) = [a]) but this is OK: f'' = ([]::[Integer]) == ([]::[Integer]) GHCI is comfortable with [] == [], so why not GHC? 'Just curious. Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] [] == []
f''' = ([]::[()]) == ([]::[()]) (Very pretty.) So why doesn't ghc have 'default' instances? -Original Message- From: Eugene Kirpichov [mailto:ekirpic...@gmail.com] Sent: Fri 29/05/2009 10:51 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] [] == [][MESSAGE NOT SCANNED] 2009/5/29 Paul Keir pk...@dcs.gla.ac.uk: Hi all, GHC is not happy with this: f = [] == [] This fails because GHC doesn't know which 'a' you mean, and can't choose an Eq instance. nor this: f' = ([]::(Eq a) = [a]) == ([]::(Eq a) = [a]) This fails for the same reason. but this is OK: f'' = ([]::[Integer]) == ([]::[Integer]) GHCI is comfortable with [] == [], so why not GHC? 'Just curious. Because GHCI has some 'default' instances, whereas GHC doesn't. This time, it probably chooses a=(). Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Eugene Kirpichov Web IR developer, market.yandex.ru ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Floating instance and pi
Hi, I'd like to make my ADT an instance of the Floating class, but I'm not sure what to put for pi, and GHC gives a warning without it: Warning: No explicit method nor default method for `GHC.Float.pi' I tried setting it to undefined, but that gives an error: `pi' is not a (visible) method of class `Floating' Any idea? Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Floating instance and pi
Oops, I was hiding the Prelude's pi. My apologies. -Original Message- From: Deniz Dogan [mailto:deniz.a.m.do...@gmail.com] Sent: Fri 5/29/2009 5:01 PM To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Floating instance and pi[MESSAGE NOT SCANNED] 2009/5/29 Paul Keir pk...@dcs.gla.ac.uk: Hi, I'd like to make my ADT an instance of the Floating class, but I'm not sure what to put for pi, and GHC gives a warning without it: Warning: No explicit method nor default method for `GHC.Float.pi' I tried setting it to undefined, but that gives an error: `pi' is not a (visible) method of class `Floating' Any idea? Paul Are you sure that your ADT fits into the Floating class in the first place? I reckon if it did, defining pi for it wouldn't be a problem. Could you show us the code you have? -- Deniz Dogan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Type class context propagation investigation[MESSAGE NOT SCANNED]
Thanks. GHC at one stage suggested I add (Num a) =
to my Num instance (after I'd added (Eq a) = to my Eq
instance) and I didn't make the connection.
-Original Message-
From: Ryan Ingram [mailto:ryani.s...@gmail.com]
Sent: Thu 28/05/2009 01:18
To: Paul Keir
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Type class context propagation
investigation[MESSAGE NOT SCANNED]
Think of classes like data declarations; an instance with no context
is a constant, and one with context is a function. Here's a simple
translation of your code into data; this is very similar to the
implementation used by GHC for typeclasses:
data EqDict a = EqDict { eq :: a - a - Bool }
data ShowDict a = ShowDict { show :: a - String }
data NumDict a = NumDict { num_eq :: EqDict a, num_show :: ShowDict a, plus
:: a - a - a }
The goal of the compiler is to turn your instance declarations into
these structures automatically. Here's a translation of your original
instance:
eq_foo :: EqDict (Foo a)
eq_foo = EqDict { eq = undefined }
show_foo :: ShowDict (Foo a)
show_foo = ShowDict { show = undefined }
num_foo :: NumDict (Foo a)
num_foo = NumDict { num_eq = eq_foo, num_show = show_foo, plus = undefined }
Now if you add a constraint on the Eq instance, this means that eq
from eq_foo might refer to eq in the dictionary for a. How do we
get that dictionary? We just pass it as an argument!
eq_foo :: EqDict a - EqDict (Foo a)
eq_foo eq_a = EqDict { eq = undefined }
However, you don't have a similar constraint on the Num instance:
num_foo :: NumDict (Foo a)
num_foo = NumDict { num_eq = eq_foo something, num_show = show_foo, plus =
undefined }
The compiler wants to fill in something, but it can't; it doesn't
have a dictionary of the type EqDict a. So it tells you so, saying
that Eq a is missing!
Once you add the (Eq a) constraint to the Num instance, it works:
num_foo :: EqDict a - NumDict (Foo a)
num_foo eq_a = NumDict { num_eq = eq_foo eq_a, num_show = show_foo, plus =
undefined }
You can also add a (Num a) constraint instead, and the compiler can
use it to get the Eq instance out:
num_foo :: NumDict a - NumDict (Foo a)
num_foo num_a = NumDict { num_eq = eq_foo (num_eq num_a), num_show =
show_foo, plus = undefined }
Of course, I'm glossing over the interesting details of the search,
but the basic idea is to attempt to fill in the blanks in these
definitions.
-- ryan
On Wed, May 27, 2009 at 2:10 PM, Paul Keir pk...@dcs.gla.ac.uk wrote:
Hi,
How does the context of a class instance declaration affect its subclasses?
The Num class instance outlined below has its requirement for Eq and Show
satisfied on the preceding lines, and the code will compile. But if I, say,
add an (Eq a) constraint to the Eq instance, in preparation for a simple
(==) definition, I find that the Num instance declaration is left lacking.
If I add the same (Eq a) constraint now to Num, calm is restored.
data Foo a = F a
instance Eq (Foo a) where
(==) = undefined
instance Show (Foo a) where
show = undefined
instance Num (Foo a)
(+) = undefined
... etc.
The thing that confuses me with this is that it seems like Num knows that
an (Eq a) context has been applied, and so what it sees as a problem, is
somehow also the solution. Any advice/rules of thumb? Does this situation
occur elsewhere? How do these constraints propagate?
Thanks,
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Type class context propagation investigation
Thanks Wren, that makes sense.
Ryan Ingram wrote:
Think of classes like data declarations; an instance with no context
is a constant, and one with context is a function. Here's a simple
translation of your code into data; this is very similar to the
implementation used by GHC for typeclasses:
data EqDict a = EqDict { eq :: a - a - Bool }
data ShowDict a = ShowDict { show :: a - String }
data NumDict a = NumDict { num_eq :: EqDict a, num_show :: ShowDict a, plus
:: a - a - a }
The goal of the compiler is to turn your instance declarations into
these structures automatically.
Another way of explaining this, if you're a Curry--Howard fan, is that
the compiler is looking for a proof that the type belongs to the class,
where = is logical implication. This is very similar to how Prolog
searches for proofs, if you're familiar with logic programming.
Classes declare the existence of logical predicates, along with the form
of what a proof of the predicate looks like. Instances declare a
particular proof (or family of proofs if there are free type variables).
Thus, the Num class is declared as,
class (Eq a, Show a) = Num a where ...
which says: for any type |a|, we can prove |Num a| if (and only if) we
can prove |Eq a| and |Show a|, and can provide definitions for all the
functions in the class using only the assumptions that |Eq a|, |Show a|,
and |Num a|.
When you declare,
instance Eq b = Eq (Foo b) where ...
you're providing a proof of |Eq b = Eq (Foo b)|. That is, you can
provide a conditional proof of |Eq (Foo b)|, given the assumption that
you have a proof of |Eq b|.
Notice how the context for instances is subtly different from the
context for classes. For instances you're saying that this particular
proof happens to make certain assumptions; for classes you're saying
that all proofs require these assumptions are valid (that is, providing
the functions isn't enough to prove membership in the class).
Later on you declare,
instance Num (Foo b) where ...
but remember that this proof must have the same form as is declared by
the class definition. This means that you must have proofs of |Eq (Foo
b)| and |Show (Foo b)|. Unfortunately, you don't actually have a proof
of |Eq (Foo b)|, you only have a proof of |Eq b = Eq (Foo b)|. In order
to use that proof you must add the |Eq b| assumption to this proof as well:
instance Eq b = Num (Foo b) where ...
When the compiler is complaining about the original one, what it's
telling you is that the |Num (Foo b)| proof can never exist because you
can never provide it with a proof of |Eq (Foo b)| in order to fulfill
its requirements.
--
Live well,
~wren
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] The essence of my monad confusion
Thanks for all the help. The simplified example indeed threw away too much. There were no side effects. Brent, of course I couldn't create your function; though I gained through trying. I then found it useful to consider the type of: fmap (\x - putStrLn x) getLine which is IO (IO ()) and hence displays nothing to the screen. Felipe, your recursive example was also compelling and concise. Antoine, I see how the join capacity of a Monad can be useful in this issue. I'm also reminded of what * can bring to fmap/$. On reflection, I often trip up when learning by comparing IO to simpler monads such as [] and Maybe. But [] and Maybe never have effects, and so are poor foils. The ((-) t) monad is henceforth in my toolbox. Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type class context propagation investigation
Hi, How does the context of a class instance declaration affect its subclasses? The Num class instance outlined below has its requirement for Eq and Show satisfied on the preceding lines, and the code will compile. But if I, say, add an (Eq a) constraint to the Eq instance, in preparation for a simple (==) definition, I find that the Num instance declaration is left lacking. If I add the same (Eq a) constraint now to Num, calm is restored. data Foo a = F a instance Eq (Foo a) where (==) = undefined instance Show (Foo a) where show = undefined instance Num (Foo a) (+) = undefined ... etc. The thing that confuses me with this is that it seems like Num knows that an (Eq a) context has been applied, and so what it sees as a problem, is somehow also the solution. Any advice/rules of thumb? Does this situation occur elsewhere? How do these constraints propagate? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] The essence of my monad confusion
On the wiki page for Applicative Functors (http://www.haskell.org/haskellwiki/Applicative_functor) a familiar characteristic of monads is quoted; that they allow you to run actions depending on the outcomes of earlier actions. I feel comfortable with Functors and Applicative Functors, but I don't yet get that extra power that monads provide. An example immediately follows that quotation on the wiki: do text - getLine if null text then putStrLn You refuse to enter something? else putStrLn (You entered ++ text) For simplicity's sake, I modified it to avoid using the IO monad; the text binding is now provided by the first parameter, and (=) is used due to its similarity to fmap: bar :: Monad m = m String - m String bar as = (=) (\a - if null a then return nothing else return something) as This works fine, so bar [Blah] gives [something], and bar (Just ) gives [nothing]. But, I can get the same effect using a Functor (replacing (=) with fmap): bar2 :: Functor f = f String - f String bar2 as = fmap (\a - if null a then nothing else something) as Can anyone help me out of the swamp? Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] fromInteger for Lists
Thanks Andy, et al. I can stop hacking for now then. I'm using a simple fromList function already which seems like a reasonable, and at least semi-standard solution (http://www.haskell.org/hoogle/?hoogle=fromList) Paul -Original Message- From: sploin...@gmail.com on behalf of andy morris Sent: Sat 02/05/2009 00:13 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] fromInteger for Lists[MESSAGE NOT SCANNED] 2009/5/1 Paul Keir pk...@dcs.gla.ac.uk: There's nothing better than making a data type an instance of Num. In particular, fromInteger is a joy. But how about lists? For example, if I have data Foo a = F [a] I can create a fromInteger such as fromInteger i = F [fromInteger i] and then a 19::(Foo Int), could become F [19]. Is it possible to do something similar for lists? So could [1,2,3]::(Foo Int) become something slightly different, say, F [1,2,3] Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe If you mean what I think you're referring to, you can't. The only reason it works for integer literals is that the compiler replaces occurrences of, say, 19 with (fromInteger 19). There's no function that's automatically applied to list literals, so ([1,2,3] :: Foo Int) isn't able to do anything useful, unfortunately. However, there's an extension in GHC, OverloadedStrings, which lets you use the method fromString of class Data.String.IsString to overload literals. (That's not what you asked, though, I know. :) ) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] fromInteger for Lists
There's nothing better than making a data type an instance of Num. In particular, fromInteger is a joy. But how about lists? For example, if I have data Foo a = F [a] I can create a fromInteger such as fromInteger i = F [fromInteger i] and then a 19::(Foo Int), could become F [19]. Is it possible to do something similar for lists? So could [1,2,3]::(Foo Int) become something slightly different, say, F [1,2,3] Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Unary Minus
If I use :info (-) I get information on the binary minus. Is unary minus also a function? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Infix tuple comma query (,)
module Main where data (:%^) a b = a :%^ bderiving (Show) main = do print $ 18 :%^ (Just 99) print $ (,) 9 10 print $ 9 , 10 The last line in the code above causes a compile error. Why does infix use of the comma (tuple constructor?) function fail without brackets? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] GLUT (glutGet undefined reference)
Thanks Kazuya, that link is perfect. Basically, I have to explicitly add -lglut : ghc -package GLUT -lglut HelloWorld.hs -o HelloWorld Great. GLUT (and now FreeGLUT) remains the simplest and most reliable standard cross-platform OpenGL windowing API. (It seems from the link that the information used by ghc-pkg is wrong, and stems from the GLUT package's relationship to libraries known as Xmu and Xi.) Cheers, Paul -Original Message- From: Kazuya Sakakihara [mailto:kaz...@gmail.com] Sent: Thu 22/01/2009 02:14 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] GLUT (glutGet undefined reference)[MESSAGE NOT SCANNED] Check this thread: http://groups.google.com/group/fa.haskell/browse_thread/thread/1716fa5e5643541e/38373ec65e2537fd?lnk=gst Kazuya 2009/1/20 Paul Keir pk...@dcs.gla.ac.uk: Hi all, I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I followed the wiki OpenGLTutorial1 until: ghc -package GLUT HelloWorld.hs -o HelloWorld at which point my screen is filled with errors. The errors begin with: /home/paul/.cabal/lib/GLUT-2.1.1.2/ghc-6.8.2/libHSGLUT-2.1.1.2.a(Begin.o): In function `szEn_info': (.text+0x26c): undefined reference to `glutGet' This surprised me a little because I've already seen these same errors recently on two separate Windows boxes. Somehow I'd got the idea it was (on Windows) due to installing from a binary; though I guess the story is the same with apt-get. Should I look for an apt-get switch to reinstall GHC from source instead? Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GLUT (glutGet undefined reference)
Hi all, I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I followed the wiki OpenGLTutorial1 until: ghc -package GLUT HelloWorld.hs -o HelloWorld at which point my screen is filled with errors. The errors begin with: /home/paul/.cabal/lib/GLUT-2.1.1.2/ghc-6.8.2/libHSGLUT-2.1.1.2.a(Begin.o): In function `szEn_info': (.text+0x26c): undefined reference to `glutGet' This surprised me a little because I've already seen these same errors recently on two separate Windows boxes. Somehow I'd got the idea it was (on Windows) due to installing from a binary; though I guess the story is the same with apt-get. Should I look for an apt-get switch to reinstall GHC from source instead? Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] forkIO on multicore[MESSAGE NOT SCANNED]
Hi Duncan, I'm following the story regarding (parallel) GC in this example with interest, but forgive me if I ask a more minor question regarding your modification of an extra parameter, n, to heavytask. Does this really help (to ensure that each core does work independently)? Surely, with fibs now described in a where clause, the 0:1:etc. form would not be shared among the (8) instantiations of heavytask? heavytask m n = putMVar m $! (fibs !! 10) where fibs = n : (n+1) : zipWith (+) fibs (tail fibs) Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] forkIO on multicore
So this benchmark is primarily a stress test of the parallel garbage collector since it is GC that is taking 75-80% of the time. Note that the mutator elapsed time goes down slightly with 2 cores compared to 1 however the GC elapsed time goes up slightly. Thanks Duncan, Jake et al. I'm more familiar with MPI and OpenMP for parallelism; it seems I've got a lot more thinking to do when it comes to Haskell. I'll look at some more tutorials, and then most likely Data Parallel Haskell. Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] forkIO on multicore
Hi all, I'm seeing no performance increase with a simple coarse-grained 2-thread code using Control.Concurrent. I compile with: hc conc.hs -o conc --make -threaded and I run with time ./conc +RTS -N2 But using either -N1 or -N2, the program runs in about 1.8secs. (I'd prefer a longer running thread task, but my fib function currently runs out of memory). Anyway, my program is below, and I'm using GHC version 6.8.2 on a 2-core Pentium D. Can anyone help? module Main where import Control.Concurrent fibs = 0 : 1 : zipWith (+) fibs (tail fibs) heavytask m = putMVar m (fibs !! 10) main = do ms - sequence $ replicate 2 newEmptyMVar mapM_ (forkIO . heavytask) $ tail ms heavytask $ head ms ms' - mapM takeMVar ms mapM_ print ms' Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] forkIO on multicore[MESSAGE NOT SCANNED]
Thanks Luke, and everyone else. Ok, back to the drawing board. Paul From: Luke Palmer [mailto:lrpal...@gmail.com] Sent: 19 December 2008 16:44 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] forkIO on multicore[MESSAGE NOT SCANNED] On Fri, Dec 19, 2008 at 9:27 AM, Paul Keir pk...@dcs.gla.ac.uk wrote: module Main where import Control.Concurrent fibs = 0 : 1 : zipWith (+) fibs (tail fibs) heavytask m = putMVar m (fibs !! 10) Oh, also, heavytask is not very heavy at all. It just writes the thunk (fibs !! 10) into the MVar. Not a single number is added in this thread. You probably meant to have the thread evaluate its argument _before_ writing it to the variable: heavytask m = putMVar m $! (fibs !! 10) (Or more transparently) heavytask m = let answer = fibs !! 10 in answer `seq` putMVar m answer But as per my other comments, you will not see a speedup (in fact, you will probably see some slowdown as two threads compete to compute the same value). Luke main = do ms - sequence $ replicate 2 newEmptyMVar mapM_ (forkIO . heavytask) $ tail ms heavytask $ head ms ms' - mapM takeMVar ms mapM_ print ms' Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] forkIO on multicore
I did indeed intend for the threads to evaluate before writing to the two variables, thanks. heavytask m = putMVar m $! (fibs !! 10) I now see a time difference, but as you suggested, in the wrong direction (1.5s for one, and 3.6s for two threads). I was hoping for each thread to independently calculate a fib number (but only to easily give them something to do) . Are the threads really in competition though? I'm hoping for each thread to write its own result; so giving the same answer twice. With the -N2 and -threaded switches, can I not expect each thread to run on a separate core? Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Multi-parameter Type Class
Hi all, I've been trying to refactor my tree conversion code to make better use of type classes; and I've discovered multi-parameter type classes and functional dependencies. I have a class with a function a2b, and I'd like map to be used when it's a list of type a. I've created a simple failing example: data Foo = Foo Barderiving(Show) data Bar = Bar String deriving(Show) class ZOT a b | a - b where zot :: a - b instance ZOT Foo Integer where zot x = 17 instance ZOT Bar String where zot x = Eighteen instance ZOT [x] [y] where -- This bit zot xs = map zot xs-- fails main = do print $ zot $ Foo $ Bar Blah print $ zot $ Bar Blah print $ zot $ [Bar Blah, Bar Blah] -- No map here please I know this would work if the third instance of zot explicitly took [Bar] and [String]. Can I not instead generalise for all the ADTs in my tree in the way I've outlined? Must I instantiate for the type of each list pair? Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Multi-parameter Type Class[MESSAGE NOT SCANNED]
I took your suggestion and it worked exactly as I had hoped. Thankyou. GHCI (6.8.2) was though a little concerned, and told me I had an: Illegal instance declaration for `ZOT [x] [y]' and recommended I use -fallow-undecidable-instances. I did, and it worked. What have I done though? The word undecidable scares me a little :) ---SNIP--- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Multi-parameter Type Class[MESSAGE NOT SCANNED]
Thanks to you both, that also looks fantastic. I'll print it out;
put it under my pillow; let it brew overnight and then push in
tomorrow ;)
-Original Message-
From: Thomas DuBuisson [mailto:[EMAIL PROTECTED]
Sent: Thu 11/12/2008 15:30
To: Paul Keir
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Multi-parameter Type Class[MESSAGE NOT SCANNED]
I see Lennart answered your question. For more fun you could also do this
with TypeFamilies, which are the new hot thing in Haskell type level logic.
Since you are just getting into MPTC, FunDeps etc I figured you'd be
interested.
-- START CODE --
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
TypeSynonymInstances, FlexibleInstances, TypeFamilies #-}
data Foo = Foo Barderiving(Show)
data Bar = Bar String deriving(Show)
-- A family of types will evaluate from one type to another.
-- Here, I chose the word 'Eval', which you could make more meaningful.
-- It is basically a function over types.
type family Eval b
-- This is three definitions for the type function 'Eval'
type instance Eval Foo = Integer
type instance Eval Bar = String
type instance Eval [x] = [Eval x]
-- And instead of a functional dependency
-- you have a type level function (Eval) that operates on the type 'a'.
class ZOT a where
zot :: a - Eval a
instance ZOT Foo where
zot x = 17
instance ZOT Bar where
zot x = Eighteen
-- And don't forget that x must be an instance of ZOT to apply zot.
instance (ZOT x) = ZOT [x] where
zot xs = map zot xs
main = do print $ zot $ Foo $ Bar Blah
print $ zot $ Bar Blah
print $ zot $ [Bar Blah, Bar Blah] -- No map here please
2008/12/11 Paul Keir [EMAIL PROTECTED]
Hi all,
I've been trying to refactor my tree conversion code to make
better use of type classes; and I've discovered multi-parameter
type classes and functional dependencies. I have a class with a
function a2b, and I'd like map to be used when it's a list of
type a.
I've created a simple failing example:
data Foo = Foo Barderiving(Show)
data Bar = Bar String deriving(Show)
class ZOT a b | a - b where
zot :: a - b
instance ZOT Foo Integer where
zot x = 17
instance ZOT Bar String where
zot x = Eighteen
instance ZOT [x] [y] where -- This bit
zot xs = map zot xs-- fails
main = do print $ zot $ Foo $ Bar Blah
print $ zot $ Bar Blah
print $ zot $ [Bar Blah, Bar Blah] -- No map here please
I know this would work if the third instance of zot
explicitly took [Bar] and [String]. Can I not instead generalise
for all the ADTs in my tree in the way I've outlined? Must I
instantiate for the type of each list pair?
Cheers,
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] followedBy parser in Parsec
Hi,
Is there a way in Parsec to check what the next token is, and if it is what
you're hoping for, leave it there.
This is an example of something which doesn't work at all:
testpar = try $
do ae - array_element
option [] $ try $ satisfy (\c - c /= '(') unexpected
return ae
I'm finding this totally confusing by now %0 Can I invert notFollowedBy
somehow, or maybe there's a peek function I don't know about? Help!
Thanks,
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] followedBy parser in Parsec
'lookAhead' is exactly what I needed: try $ array_element = \ae - lookAhead (reservedOp () return ae Many thanks, Paul Maybe you're looking for 'lookAhead'? [...] //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Searching for ADT patterns with elem and find
Thanks Tom, That is indeed a very elegant solution; I too often forget about the wonders of list comprehension. I guess one drawback compared to Neil's suggested use of any (and staying with a separate isTypeB) is that your solution will iterate over the entire list, regardless of an early hit. But I don't think your second (as-pattern) solution for findBs is ugly; I quite like it actually. Cheers, Paul -Original Message- From: Tom Nielsen [mailto:[EMAIL PROTECTED] Sent: Wed 12/11/2008 12:39 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Searching for ADT patterns with elem and find somebody pointed out a few months back that list comprehensions do this nicely: containsTypeB ts = not $ null [x | (B x) - ts] no need for defining isTypeB. not quite sure how you would write findBs :: [T]-[T] succinctly; maybe findBs ts = [b | b@(B _) - ts] or findBs ts = [B x | (B x) - ts] both of them compile but the first is ugly and the second is inefficient (Tags a new T for every hit). Tom 2008/11/12 Paul Keir [EMAIL PROTECTED]: Hi All, If I have an ADT, say data T = A String Integer | B Double | C deriving(Eq) and I want to find if a list (ts) of type T contains an element of subtype B Double, must my containsTypeX function use a second isTypeX function as follows: isTypeB :: T - Bool isTypeB (B _) = True isTypeB _ = False containsTypeB :: [T] - Bool containsTypeB ts = maybe False (\x - True) (find isTypeB ts) I understand that while something like find C ts will work, find (isTypeB _) ts will not, but is there no such thing as a pattern combinator(?), or lambda that could help with this situation. I find I have many individual isTypeB functions now. Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Searching for ADT patterns with elem and find
Hi All, If I have an ADT, say data T = A String Integer | B Double | C deriving(Eq) and I want to find if a list (ts) of type T contains an element of subtype B Double, must my containsTypeX function use a second isTypeX function as follows: isTypeB :: T - Bool isTypeB (B _) = True isTypeB _ = False containsTypeB :: [T] - Bool containsTypeB ts = maybe False (\x - True) (find isTypeB ts) I understand that while something like find C ts will work, find (isTypeB _) ts will not, but is there no such thing as a pattern combinator(?), or lambda that could help with this situation. I find I have many individual isTypeB functions now. Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Searching for ADT patterns with elem and find
Thanks Neil, Great. I hadn't noticed isJust, and I'd forgotten any. Actually I was browsing Prelude just the other day and picked up zipWith f as bs as a replacement for map f $ zip as bs. Cheers, Paul -Original Message- From: Mitchell, Neil [mailto:[EMAIL PROTECTED] Sent: Wed 12/11/2008 10:23 To: Paul Keir; haskell-cafe@haskell.org Subject: RE: [Haskell-cafe] Searching for ADT patterns with elem and find Hi Paul, maybe False (\x - True) (find isTypeB ts) This can be more neatly expressed as: isJust (find isTypeB ts) But your entire thing can be expressed as: containsTypeB ts = any isTypeB ts I recommend reading through the Prelude interface and the List interface, it has many useful functions that will help. Thanks Neil From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir Sent: 12 November 2008 10:09 am To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Searching for ADT patterns with elem and find Hi All, If I have an ADT, say data T = A String Integer | B Double | C deriving(Eq) and I want to find if a list (ts) of type T contains an element of subtype B Double, must my containsTypeX function use a second isTypeX function as follows: isTypeB :: T - Bool isTypeB (B _) = True isTypeB _ = False containsTypeB :: [T] - Bool containsTypeB ts = maybe False (\x - True) (find isTypeB ts) I understand that while something like find C ts will work, find (isTypeB _) ts will not, but is there no such thing as a pattern combinator(?), or lambda that could help with this situation. I find I have many individual isTypeB functions now. Regards, Paul == Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html == ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Simple Table Update
Hi, I'd like to create a new list based on an original list, using information from a second (symbol) list. That second list should be updated as each element in the new list is added. I've been using map a lot, but that's not an option here, and I'm having trouble obtaining a good recursive structure for my function. The minimal fixpus function below shows the problem. I have sts on both sides of the : list construct, but I'd like the second sts to be a version modified by the where (singular) function, fixpu. fixpus :: [ProgUnit] - [ProgUnitSymbolTable] - [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = fixpu pu sts : fixpus pus sts where fixpu pu sts = pu (Below) I tried making fixpu return a tuple, and then use fst, snd and let, but I think it looks strange. I know it's quite basic, but I'd like a strong foundation for what's likely to become a medium-scale project. Can anyone offer advice? fixpus :: [ProgUnit] - [ProgUnitSymbolTable] - [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = let a = (fixpu pu sts) in fst a : fixpus pus (snd a) where fixpu pu sts = (pu,sts ++ [(,[])]) Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Simple Table Update
It brings tears to my eyes to see such a beautiful function ;) You've hit the nail on the head Ryan. The new version of my second previous code snippet now no longer requires fixpu*, and is: typeCheckAST (Program pus) = Program $ snd $ mapAccumL f [] pus where f st pu = (st ++ [(,[])],pu) Many thanks, Paul -Original Message- From: Ryan Ingram [mailto:[EMAIL PROTECTED] Sent: Wed 08/10/2008 18:03 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Simple Table Update Prelude :t Data.List.mapAccumL Data.List.mapAccumL :: (acc - x - (acc, y)) - acc - [x] - (acc, [y]) I'm sure you can fill in the details :) -- ryan 2008/10/8 Paul Keir [EMAIL PROTECTED]: Hi, I'd like to create a new list based on an original list, using information from a second (symbol) list. That second list should be updated as each element in the new list is added. I've been using map a lot, but that's not an option here, and I'm having trouble obtaining a good recursive structure for my function. The minimal fixpus function below shows the problem. I have sts on both sides of the : list construct, but I'd like the second sts to be a version modified by the where (singular) function, fixpu. fixpus :: [ProgUnit] - [ProgUnitSymbolTable] - [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = fixpu pu sts : fixpus pus sts where fixpu pu sts = pu (Below) I tried making fixpu return a tuple, and then use fst, snd and let, but I think it looks strange. I know it's quite basic, but I'd like a strong foundation for what's likely to become a medium-scale project. Can anyone offer advice? fixpus :: [ProgUnit] - [ProgUnitSymbolTable] - [ProgUnit] fixpus [] _ = [] fixpus (pu:pus) sts = let a = (fixpu pu sts) in fst a : fixpus pus (snd a) where fixpu pu sts = (pu,sts ++ [(,[])]) Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] One liner?
Hi all, There's a common little situation I keep bumping up against. I don't understand where I'm going wrong, so I've made a little example. It's to do with binding a result to a variable name using -. This code works fine: -- module Main where import System.Directory (getDirectoryContents) main = do dc - getDirectoryContents ./foo/ mapM_ putStrLn dc -- But if I try to avoid the use of the bind to dc, I fail: -- mapM_ putStrLn (getDirectoryContents ./foo/) -- I've tried using map instead of mapM_, and inserted returns here and there, but no luck. Can anyone tell me where and why I'm going wrong? The error message is below. Cheers, Paul Couldn't match expected type `[String]' against inferred type `IO [FilePath]' In the second argument of `mapM_', namely `(getDirectoryContents ./foo/)' In the expression: mapM_ putStrLn (getDirectoryContents ./foo/) In the definition of `main': main = mapM_ putStrLn (getDirectoryContents ./foo/) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] One liner?
Thanks, and to Ketil too. I did see past the missing ./foo/. That's certainly a solution I'm happy with, and I didn't know the term eta reduction, so thanks for that too. Paul -Original Message- From: Mitchell, Neil [mailto:[EMAIL PROTECTED] Sent: Thu 02/10/2008 16:26 To: Paul Keir; haskell-cafe@haskell.org Subject: RE: [Haskell-cafe] One liner? Hi You can translate this step by step. main = do dc - getDirectoryContents ./foo/ mapM_ putStrLn dc Translating out the do notation (http://www.haskell.org/haskellwiki/Keywords#do): main = getDirectoryContents = \dc - mapM_ putStrLn dc Then we can chop out the dc argument, as its \x - x, and can be removed (eta reduction): main = getDirectoryContents = mapM_ putStrLn And finally we just remove the newline: main = getDirectoryContents = mapM_ putStrLn Alternatively, we can flip the = for = and write: main = mapM_ putStrLn = getDirectoryContents This is now one line, and mirrors how you would write the function if it was pure using function composition. Thanks Neil This material is sales and trading commentary and does not constitute investment research. Please follow the attached hyperlink to an important disclaimer www.credit-suisse.com/emea/legal outbind://31/www.credit-suisse.com/emea/legal From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Paul Keir Sent: 02 October 2008 4:20 pm To: haskell-cafe@haskell.org Subject: [Haskell-cafe] One liner? Hi all, There's a common little situation I keep bumping up against. I don't understand where I'm going wrong, so I've made a little example. It's to do with binding a result to a variable name using -. This code works fine: -- module Main where import System.Directory (getDirectoryContents) main = do dc - getDirectoryContents ./foo/ mapM_ putStrLn dc -- But if I try to avoid the use of the bind to dc, I fail: -- mapM_ putStrLn (getDirectoryContents ./foo/) -- I've tried using map instead of mapM_, and inserted returns here and there, but no luck. Can anyone tell me where and why I'm going wrong? The error message is below. Cheers, Paul Couldn't match expected type `[String]' against inferred type `IO [FilePath]' In the second argument of `mapM_', namely `(getDirectoryContents ./foo/)' In the expression: mapM_ putStrLn (getDirectoryContents ./foo/) In the definition of `main': main = mapM_ putStrLn (getDirectoryContents ./foo/) == Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html == ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Pretty Print, text or ++?
Hi there, I'm writing a pretty printer using the Text.PrettyPrint library, and there's a pattern I'm coming across quite often. Does anyone know whether, text (a ++ b ++ c ++ d) or text a + text b + text c + text d runs quicker? Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Pretty Print, text or ++?
Thanks, So you're recommending: text (concat [a,b,c,d,e]) Might this not transform my pretty printing into ugly printing; when longer strings are used? Paul -Original Message- From: [EMAIL PROTECTED] on behalf of John Van Enk Sent: Fri 15/08/2008 14:31 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Pretty Print, text or ++? Paul, Something tells me you might want to look at `concat': concat :: [[a]] - [a] /jve 2008/8/15 Paul Keir [EMAIL PROTECTED] Hi there, I'm writing a pretty printer using the Text.PrettyPrint library, and there's a pattern I'm coming across quite often. Does anyone know whether, text (a ++ b ++ c ++ d) or text a + text b + text c + text d runs quicker? Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- /jve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] RE: Pretty Print, text or ++?
Awesome! Thanks to you all. I'll start with hsep[map a, b, c, d] and then I can try changing hsep for other things. Paul -Original Message- From: Benedikt Huber [mailto:[EMAIL PROTECTED] Sent: Fri 15/08/2008 14:53 To: Paul Keir Cc: haskell-cafe@haskell.org Subject: Re: Pretty Print, text or ++? Paul Keir schrieb: Hi there, I'm writing a pretty printer using the Text.PrettyPrint library, and there's a pattern I'm coming across quite often. Does anyone know whether, text (a ++ b ++ c ++ d) or text a + text b + text c + text d runs quicker? Hi Paul, text (a ++ b ++ c ++ d) is the same as hcat (map text [a,b,c,d]) (horizontal concatenation without seperating spaces) while text a + text b + text c + text d corresponds to hsep (map text [a,b,c,d]) or text (unwords [a,b,c,d]) With +, hsep or hcat, pretty printing won't choose the best layout - you tell the pretty printer to layout documents 'beside'. For autolayout, see sep,cat and the paragraph-fill variants fsep and fcat. Regarding performance: `unwords` will propably be a little faster (untested), but less flexible. There is no asymptotic overhead when using the pretty printer. cheers, benedikt Cheers, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec expressions with alphaNum operators
Thanks Chris, When I looked at the Fortran alphaNum operators (.and. .or. etc.) I had hoped that supplying Parsec's opStart with a dot would have been hitting the nail on the head. Oh well. I have noticed something interesting though. If I simply omit the a from opLetter, the problem is gone. In fact, leaving opStart, opLetter, and reservedOpNames all empty works fine too. Perhaps I should aim to leave aside the Language part of Parsec altogether. Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec Expected Type
Thanks. reservedOp is a better fit; :+ should only be :+. I also overcame my type issues in an ad-hoc manner, adding return () whenever I needed to. -Original Message- From: Tillmann Rendel [mailto:[EMAIL PROTECTED] Sent: 30 March 2008 12:30 To: Paul Keir; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Parsec Expected Type Paul Keir wrote: What I'd like is to parse either the string parameter, or the string :. I'm using 'reserved' and 'symbol' because they seem to correspond well to the concepts in the language I'm parsing. You may consider using reservedOp for :, depending on how :+ should be parsed: for :+ use reservedOp for : + use symbol If you use reserved, then :name will be parsed as :name not : name as you probably expect. generally, reserved is for identifier-like keywords, and reservedOp for operator-like keywords. Perhaps I'd express my confusion better if I ask: Why are 'reserved' and 'symbol' different types? I have no idea. They aren't in the Parsec manual on Daans site: http://legacy.cs.uu.nl/daan/download/parsec/parsec.html You can fix this by defining reserved name = ParsecToken.reserved tokenParser name return name instead of reserved = ParsecToken.reserved tokenParser to import the reserved component from the tokenParser to the toplevel. Now, reserved :: String - CharParser st String Another option is to fix it the other way, by defining symbol name = ParsecToken.symbol tokenParser name return () or to fix it in a ad-hoc manner, by defining ignored = ( return ()) and using it in the approbiate places, like parameterOrColon = reserved parameter | ignored (symbol :) Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Parsec expressions with alphaNum operators
Hi,
I'm using buildExpressionParser, and I'd like to use alphanumeric
operator characters. I get an (unexpected a) error though. With a test
string like -a if a is used in any of the reservedOpNames. I'm
aiming for the Fortran operators like .and..
The listing below may be helpful. It's taken from the Haskell wiki's
Parsing expressions and statements article (minus the statement
part).I've added an :a: operator. The article uses ~ as a unary
operator (I'm heading for +/-). It can be tested with:
$ parseTest exprparser ~a
-- code begins
module Main where
import Control.Monad(liftM)
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
import Text.ParserCombinators.Parsec.Token
import Text.ParserCombinators.Parsec.Language
data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr
deriving Show
data Unop = Not deriving Show
data Duop = And | Iff deriving Show
data Stmt = Nop | String := Expr | If Expr Stmt Stmt | While Expr Stmt
| Seq [Stmt]
deriving Show
def = emptyDef{ commentStart = {-
, commentEnd = -}
, identStart = letter
, identLetter = alphaNum
, opStart = oneOf ~=:
, opLetter = oneOf ~=:a
, reservedOpNames = [~, , =, :=, :a:]
, reservedNames = [true, false, nop,
if, then, else, fi,
while, do, od]
}
TokenParser{ parens = m_parens
, identifier = m_identifier
, reservedOp = m_reservedOp
, reserved = m_reserved
, semiSep1 = m_semiSep1
, whiteSpace = m_whiteSpace } = makeTokenParser def
exprparser :: Parser Expr
exprparser = buildExpressionParser table term ? expression
table = [ [Prefix (m_reservedOp ~ return (Uno Not))]
, [Infix (m_reservedOp return (Duo And)) AssocLeft]
, [Infix (m_reservedOp = return (Duo Iff)) AssocLeft]
, [Infix (m_reservedOp :a: return (Duo Iff)) AssocLeft]
]
term = m_parens exprparser
| liftM Var m_identifier
| (m_reserved true return (Con True))
| (m_reserved false return (Con False))
play :: String - IO ()
play inp = case parse exprparser inp of
{ Left err - print err
; Right ans - print ans
}
-- code ends
Cheers,
Paul Keir
Research Student
University of Glasgow
Department of Computing Science
[EMAIL PROTECTED]
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec Expected Type
Many thanks guys, you've really taught me how to catch a fish here!
Paul
-Original Message-
From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED]
Sent: Sat 3/29/2008 1:41 AM
To: haskell-cafe@haskell.org Cafe
Cc: Paul Keir
Subject: Re: [Haskell-cafe] Parsec Expected Type
On Mar 28, 2008, at 21:12 , Ryan Ingram wrote:
On 3/28/08, Paul Keir [EMAIL PROTECTED] wrote:
What I'd like is to parse either the string parameter, or the
string :.
I'm using 'reserved' and 'symbol' because they seem to correspond
well to
the concepts in the language I'm parsing. I could try,
tester3 = reserved parameter | do { symbol :; return () }
Or you could factor this behavior out into a new combinator:
or_ :: Parser a - Parser b - Parser ()
or_ x y = (x return ()) | (y return ())
tester3 = reserved parameter `or_` symbol :
Or if you'd like to be inscrutable:
import Data.Function
or_ = ( return ()) `on` (|)
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon universityKF8NH
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec Expected Type
Thanks, I'd thought it was something to do with incompatible types. I can now
create a simpler problem:
tester2 = reserved parameter | symbol :
Certainly I could use reserved : instead of symbol, but where is my thinking
with symbol here going wrong? Surely the above example isn't so odd?
Paul
-Original Message-
From: Luke Palmer [mailto:[EMAIL PROTECTED]
Sent: Fri 3/28/2008 12:26 AM
To: Paul Keir
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Parsec Expected Type
Hi Paul,
2008/3/27 Paul Keir [EMAIL PROTECTED]:
Hi,
Does anyone know why this reduced Parsec production stops compilation, and
how I can fix it?
tester = reserved parameter
| do { reserved dimension; symbol : }
Look at the types of reserved and symbol (from
http://www.haskell.org/ghc/docs/latest/html/libraries/parsec/Text-ParserCombinators-Parsec-Token.html):
symbol :: String - CharParser st String
reserved :: String - CharParser st ()
The type of a do block is the same as the type of its last statement.
But (reserved parameter) and (symbol ;) do not have the same type.
Luke
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec Expected Type
Could tester2 return some kind of base type, which the two inherit from? I
don't know. I'm really only presenting the ugly tester2 function because I'm
looking for a Parsec-concordant solution to what appears a simple problem.
What I'd like is to parse either the string parameter, or the string :. I'm
using 'reserved' and 'symbol' because they seem to correspond well to the
concepts in the language I'm parsing. I could try,
tester3 = reserved parameter | do { symbol :; return () }
but that's feels a bit contrived; or I could use 'reserved' twice.
Perhaps I'd express my confusion better if I ask: Why are 'reserved' and
'symbol' different types?
Paul (Haskell Novice)
-Original Message-
From: Jonathan Cast [mailto:[EMAIL PROTECTED]
Sent: Fri 3/28/2008 2:05 PM
To: Paul Keir
Cc: Luke Palmer; haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Parsec Expected Type
On 28 Mar 2008, at 2:02 AM, Paul Keir wrote:
Thanks, I'd thought it was something to do with incompatible types.
I can now create a simpler problem:
tester2 = reserved parameter | symbol :
Certainly I could use reserved : instead of symbol, but where is
my thinking with symbol here going wrong? Surely the above example
isn't so odd?
What type do you expect tester2 to return?
jcc
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] No newlines in the whitespace for Parsec lexeme parsers
Thankyou all. Once I get more familiar with Parsec I might do my own lexeme parser. For now I've changed its definition of simpleSpace to simpleSpace = skipMany1 (satisfy (\c - (c /= '\n') (isSpace c))) so that it skips whitespace, but not newlines. My parser then explicitly matches newlines whenever I need to. Cheers, Paul -Original Message- From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED] Sent: 26 March 2008 11:48 To: haskell-cafe@haskell.org Cafe; Paul Keir Subject: Re: [Haskell-cafe] No newlines in the whitespace for Parsec lexeme parsers On Mar 26, 2008, at 7:42 , Bulat Ziganshin wrote: Wednesday, March 26, 2008, 2:32:53 PM, you wrote: I'm looking to parse a Fortran dialect using Parsec, and was afair, some months ago BASIC parsing was discussed here. the first solution one can imagine is to add preprocessing stage replacing line ends with ';'-alike FWIW I just ignored the lexeme parser and did my own on top of the basic Parsec primitives. You may need to do that anyway if you want to support older variants of Fortran, which don't actually have keywords and ignore spaces outside of string constants (Hollerith constants) --- Parsec's lexeme stuff doesn't even pretend to support this. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Parsec Expected Type
Hi,
Does anyone know why this reduced Parsec production stops compilation, and how
I can fix it?
tester = reserved parameter
| do { reserved dimension; symbol : }
Thanks,
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] No newlines in the whitespace for Parsec lexeme parsers
Hi, I'm looking to parse a Fortran dialect using Parsec, and was hoping to use the ParsecToken module. Fortran though is unlike the Parsec examples, and prohibits default line continuation (requiring instead an explicit ampersand token ). The lexical parsers of the ParsecToken module skip whitespace after each symbol parsed (lexeme parsers); including the newline. I was thinking of making a minor change to Parsec: In Token.hs, lexeme, whiteSpace, and simpleSpace are defined. lexeme uses whiteSpace which uses simpleSpace. simpleSpace is defined: simpleSpace = skipMany1 (satisfy isSpace) I could replace this locally with: simpleSpace = skipMany1 (satisfy isSpacePlusAmpersandMinusNewline) This approach may have other problems though. Has anyone experience of using Parsec to parse such a language? For example assembly or a preprocessor? Thanks in advance, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Parsec (Zero or One of)
Hi,
I'm having some difficulty using the Parsec library, perhaps you could
help. I've reduced my problem as shown below. I would like the
'only_prod' parser to require the reserved string only, _optionally_
followed by an identifier. As part of 'mytest', this should then be
followed by the reserved string end.
mytest = do { only_prod; end }
only_prod = do { reserved only; try identifier }
end = reserved end
i.e. I'd like both
parseTest mytest only end
and
parseTest mytest only green end
to parse successfully. As it stands, only the second is successful. The
first fails with:
parse error at (line 1, column 9):
unexpected end of input
expecting letter or digit or end
Does anyone have an idea how I could repair the situation?
Thanks in advance,
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec (Zero or One of)
Thanks. I can't find optionMaybe in my version 2.1 of Parsec, but in any case,
defining my only_prod as
only_prod = do { reserved only; option [] identifier }
or
only_prod = do { reserved only; identifier | return [] }
gives the same error responses as before. I will anyway look closer at option.
You're right that I don't understand try, but it's not for lack of trying. My
examples' use of try though was just a stab at a readable failure. Maybe I
should refactor my example.
Paul
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec (Zero or One of)
Thankyou. Yes, I'd also noticed that only end could result in the end part
being taken as an identifier. The language I'm parsing actually doesn't have
reserved words though; so end and only are both possible valid identifiers.
I should then probably replace my use of say, reserved only, with string
only; whiteSpace; for clarity. Still stuck though...
P
-Original Message-
From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED]
Sent: Tue 25/03/2008 19:58
To: Paul Keir; haskell-cafe@haskell.org Cafe
Subject: Re: [Haskell-cafe] Parsec (Zero or One of)
On Mar 25, 2008, at 12:43 , Paul Keir wrote:
Thanks. I can't find optionMaybe in my version 2.1 of Parsec, but
in any case, defining my only_prod as
only_prod = do { reserved only; option [] identifier }
or
only_prod = do { reserved only; identifier | return [] }
gives the same error responses as before. I will anyway look closer
at option.
The other problem here is that just using a given string in
reserved doesn't prevent it from being parsed elsewhere by
identifier. (Note the character offset of the error was 9, i.e.
just past only end, and it was looking for end or more identifier
characters.)
Are you using the higher level parser facilities from
Text.ParserCombinators.Parsec.Token, or rolling your own? If the
latter, you will need to modify identifier to not accept keywords.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon universityKF8NH
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Parsec (Zero or One of)
Many thanks. -Original Message- From: Brandon S. Allbery KF8NH [mailto:[EMAIL PROTECTED] Sent: Tue 25/03/2008 20:29 To: Paul Keir; haskell-cafe@haskell.org Cafe Subject: Re: [Haskell-cafe] Parsec (Zero or One of) On Mar 25, 2008, at 16:26 , Paul Keir wrote: Thankyou. Yes, I'd also noticed that only end could result in the end part being taken as an identifier. The language I'm parsing actually doesn't have reserved words though; so end and only are both possible valid identifiers. I should then probably replace my use of say, reserved only, with string only; whiteSpace; for clarity. Still stuck though... But now you have an ambiguity in your language, which is exactly why the parse is failing: only end could be waiting for end, or for end of file / whatever tokens might follow this clause. In the worst case, the latter might lead to a situation where an unambiguous parse is impossible. You might want to provide a better description of the full language --- and think about how it would need to be implemented to avoid ambiguity. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
