[Haskell-cafe] Existentials and SYB [Was: GADTs and Scrap your Boilerplate]
Oscar Finnsson wrote:
I got the GADT
data DataBox where
DataBox :: (Show d, Eq d, Data d) = d - DataBox
and I'm trying to get this to compile
instance Data DataBox where
gfoldl k z (DataBox d) = z DataBox `k` d
gunfold k z c = k (z DataBox) -- not OK
As has been pointed out, DataBox is an ordinary existential data type,
only written using the new-style notation. For clarity, let us define
our DataBox with the conventional notation:
data DataBox = forall d. (Show d, Eq d, Data d) = DataBox d
So, the question becomes of using existentials with Scrap your
Boilerplate.
On one hand, it is unnervingly trivial to incorporate existentials
into the SYB framework.
Let us consider the signature of gunfold, the problematic member of
the (Data a) type class.
-- | Unfolding constructor applications
gunfold :: (forall b r. Data b = c (b - r) - c r)
- (forall r. r - c r)
- Constr
- c a
We have to eventually produce a value of the type (c DataBox). We
observe that our work is done if we manage to produce one value of the
type DataBox. We then apply the second argument to gunfold (which
`lifts' from any type r to the type c r), and we are done. To produce
an existential value, we need one witness of the constraints Show, Eq
and Data. For example, the type Int is such witness. Thus, we are
done indeed. Here is the complete code:
{-# LANGUAGE ExistentialQuantification, Rank2Types #-}
module F where
import Data.Generics
data DataBox = forall d. (Show d, Eq d, Data d) = DataBox d
instance Typeable DataBox where
typeOf _ = mkTyConApp (mkTyCon DataBox) []
instance Data DataBox where
gfoldl k z (DataBox d) = z DataBox `k` d
gunfold k z c = z (DataBox (42::Int))
One may complain that we have fixed not only the type of the argument
to DataBox but also the value. We should let gunfold to `produce' the
value. Here is a sketch:
enDataI :: (Int - DataBox)
enDataI = DataBox
enDataB :: (Bool - DataBox)
enDataB = DataBox
instance Data DataBox where
gfoldl k z (DataBox d) = z DataBox `k` d
gunfold k z c = (if True then k (z enDataI) else k (z enDataB))
Now, the particular Int value to supply to DataBox will be produced by
the function k. The above code used the constant True to fix the
choice of the type to Int. Generally, one could use the third argument
of gunfold to help make the choice. That is, we could incorporate the
choice of the type in the value of Constr.
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Re: [Haskell-cafe] Intuitive function given type signature
On May 21, 2010, at 3:51 AM, Brent Yorgey wrote: On Thu, May 20, 2010 at 11:53:09AM +1200, Richard O'Keefe wrote: On May 20, 2010, at 3:18 AM, Brent Yorgey wrote: On Wed, May 19, 2010 at 04:27:14AM +, R J wrote: What are some simple functions that would naturally have the following type signatures: f :: (Integer - Integer) - Integer The key point is the 'that would NATURALLY have', which I take to mean as a result of type inference without any forcibly imposed type signatures. Given that this is an exercise in Chapter 1, I kind of doubt this is really what it is supposed to mean. Are people reading chapter 1 really expected to understand the intricacies of type inference and the Num class? And to know about 'toInteger' and the fact that numeric constants are polymorphic? I really doubt it. I read the question much more simply, with naturally having a much more informal meaning than you suggest. I interpret the question as simply getting the reader some practice with basic higher-order types. The other possibility, of course, is a setup where Integer is the default type, so the function should just be f g = g 0 + 0. But naturally has to mean *something*, and the questions are clearly about type inference. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Intuitive function given type signature
On 20/05/2010, at 9:53 AM, Richard O'Keefe wrote: The key point is the 'that would NATURALLY have', which I take to mean as a result of type inference without any forcibly imposed type signatures. In my second edition of Bird, the question just says: Give examples of functions with the following types: Tom___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal to solve Haskell's MPTC dilemma
Hi Evan EHC - Essential Haskell Compiler - is the 'family of compilers' that UHC - Utrecht Haskell Compiler - is instance one of. The EHC family starts with a simple Haskell subset and adds features building up to (almost) Haskell98 for UHC and extended Haskell for some of the EHC variations. This style of software development is sometime called 'feature oriented development' or 'software product lines', the ML compiler MLPolyR is another compiler built as a family of variants. UHC had a version 1.0 release last year. From the documentation it doesn't look like UHC supports type class directives: http://www.cs.uu.nl/wiki/bin/view/Ehc/UhcUserDocumentation From a bit of nosing around, I'm not sure that any of the EHC variants support type class directives either. It does look like Helium (Utrecht's simplified Haskell variant for teaching) supports them, and Helium has certainly been released. That said, I've no association with the Utrecht developers, so I'm not really qualified to say for EHC, though I have studied its source a bit and grepping hasn't turned up an answer. Incidentally, studying the source of EHC is probably the best way to learn idioms and techniques for UUAG. From the 'Type Class Directives' paper here are some example directives: never Eq (a - b): functions cannot be tested for equality never Num Bool: arithmetic on booleans is forbidden disjoint Integral Fractional: something which is fractional can never be integral close Similar: the instances of Similar are @in...@. (Similar being an Eq like class that is available only for integers) Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal to solve Haskell's MPTC dilemma
On 21 May 2010 01:58, Carlos Camarao carlos.cama...@gmail.com wrote: But this type-correct program would become not typeable if instances such as the ones referred to before (by Daniel Fischer) I was thinking this through, and the situation is more complex than I had thought. It seems that single param type classes enjoy a nice property: * Adding an instance to the module defining the class cannot conflict with any non-orphan instance defined elsewhere * Adding an instance for a type for a class *to the module defining that type* cannot conflict with any non-orphan instance defined elsewhere As long as you complicate the definition of orphan, this seems to still hold for multi-param ones. The change you need is that an instance defined in a module other than that defining the class must be non-orphan *in each individual type variable*. So if a module defined the D and E data types you could make a C D E instance, but a C D Bool or C Bool E one is considered orphan by this definition. Functional dependencies refine this slightly: if a variable (a) functionally determines another (b), an instance can be declared non-orphan as long as the a variable is being instantiated with a data type which is defined in the same module. So if C a b has FD a |- b you can declare an instance for C D Bool but not C Bool D. With this definition of orphan I don't think it is possible to get the library fragility issue as long as you stick to non-orphan instances by that definition. I haven't tried to prove this, though. Where this gets more interesting is that GHC's -fwarn-orphans check does *not* flag a C D Bool instance in a module defining D but not C as an orphan, whether C has a functional dependency or not. It will only flag an instance as orphan if *all* of the class type variables are being instantiated to a datatype defined in another module. This seems like a bug? So in summary I think I agree with you that your proposed mechanism does have fragility characteristics similar to FDs as they stand. One benefit (that I can see) to using explicitly declared FDs is that the compiler could potentially use those FDs to implement a correct orphan instance check, such that code that passed the check was guaranteed not to cause the library fragility issue in those modules that import it. However, it appears that GHC doesn't currently do this, which is upsetting. (Incidentally, the link to your paper is broken, so I haven't actually been able to read it, sorry!) Cheers, Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: String rewriting
... to rewrite strings according to simple composable rules like ... more often than not, regexp/replace is a hack to avoid the real thing (parse - AST - transform - AST' - print). such hacks tend to grow into something that is definitely not composable. but, you can be a hero ... http://xkcd.com/208/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Недвижимость во Франции, ар енда и продажа вилл
Офис нашей компании находится в Ницце - в самом сердце Французской Ривьеры, что дает нам непосредственную возможность предложить виллы для аренды и продажи во всем их многообразии, а также сориентировать наших клиентов на самые интересные и значимые мероприятия в богатой культурной и светской жизни Лазурного берега. Мы предоставляем нашим клиентам широкий выбор вилл класса люкс по всей протяжённости Лазурного берега Франции - от Сан Тропе до Монако, оказываем визовую поддержку, а также организуем визиты на приглянувшиеся Вам объекты при заключении контрактов по аренде или покупке вилл и другой недвижимости http://www.property-serviceazur.com/ средиземноморского побережья. Уважаемые партнеры, мы предлагаем Вам сотрудничество в приобретении Вашими клиентами *недвижимости во Франции, на Лазурном берегу *, который всегда ценился поклонниками искусства, знатоками истории, любителями изысканной кухни и живописных пейзажей. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Недвижимость во Ф ранции, аренда и продажа вилл
Google translation: The office of our company is located in Nice - the heart of the Riviera, that gives us an immediate opportunity to offer villas for rent and sale in all their diversity, as well as guidance to our clients on the most interesting and important events in the rich cultural and social life Cote d'Azur. We offer our customers a wide choice of luxury villas on the entire length of the Côte d'Azur of France - from St Tropez to Monaco, visa support and arrange visits to your vending facility at the conclusion of contracts for lease or purchase of houses and other real estate Mediterranean coast. Dear partners, we offer you our cooperation in the acquisition of your clients property in France, Cote d'Azur, which has always valued art lovers, connoisseurs of history, lovers of fine cuisine and beautiful scenery. So spam for luxury villa's? I guess Haskell really failed in avoiding success... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] making the GHC Api not write to stderr
You could try changing the log_action[1] member of the DynFlags. A
while ago I turned most printed errors into some form of error
message, but I wouldn't be surprised if I missed some places. All
output should go through log_action, though, so try changing that to
intercept any output.
[1]:
http://haskell.org/ghc/docs/6.12-latest/html/libraries/ghc-6.12.2/DynFlags.html#v%3Alog_action
On 20 May 2010 19:05, Phyx loneti...@gmail.com wrote:
I was wondering how to forcibly quiet down the API. I have a custom handler
in place, but when I call the function on failure both my handler gets
called and somewhere somehow errors get printed to the stderr, which I
really need to avoid.
My current code looks like
getModInfo :: Bool - String - String - IO (ApiResults ModuleInfo)
getModInfo qual file path = handleSourceError processErrors $
runGhc (Just libdir) $ do
dflags - getSessionDynFlags
setSessionDynFlags $ configureDynFlags dflags
target - guessTarget file Nothing
addTarget target
setSessionDynFlags $ dflags { importPaths = [path] }
load LoadAllTargets
graph - depanal [] False
let modifier = moduleName . ms_mod
modName = modifier $ head graph
includes = includePaths dflags
imports = importPaths dflags
dflags' - Debug.trace (moduleNameString modName) getSessionDynFlags
setSessionDynFlags $ dflags' { includePaths = path:includes
, importPaths = path:imports
}
parsed - parse modName
checked - typecheckModule parsed
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Re: [Haskell-cafe] Type families vs. functional dependencies -- how to express something
Hello, On Tue, May 18, 2010 at 04:47:50PM -0700, Dan Weston wrote: Unifying those two types by hand, I get: P (A t - B a) ~ P (B a) Maybe the problem is that type families (and associated types, their class cousins) are not injective: P x ~ P y does not imply that x ~ y. Maybe you need a data type (with appropriate wrapping and unwrapping) to ensure injectivity. Cf: http://www.haskell.org/haskellwiki/GHC/Type_families#Injectivity.2C_type_inference.2C_and_ambiguity http://www.mail-archive.com/haskell-cafe@haskell.org/msg63359.html That's probably it, thanks. I think I'll just stay with functional dependencies this time, a data type with wrapping and unwrapping can't possibly make the code more readable :-). Regards, -- Tomáš Janoušek, a.k.a. Liskni_si, http://work.lisk.in/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] making the GHC Api not write to stderr
Hi,
I tried that, setting it to (\_ _ _ _ - return ()) and it still did the
same, also tried setting it to undefined to see whether the code that's
printing the error is using it, and it didn't crash
So I assume it's not.
---
*VsxParser getModInfo True
C:\\Users\\Phyx\\AppData\\Local\\Temp\\tmp5600.hs
return ()
C:\Users\Phyx\AppData\Local\Temp\tmp5600.hs:11:13:
parse error on input `='
Printf
-
I think parseModule might still have a hardcoded print statement in it.
-Original Message-
From: Thomas Schilling [mailto:nomin...@googlemail.com]
Sent: Friday, May 21, 2010 12:53
To: Phyx
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] making the GHC Api not write to stderr
You could try changing the log_action[1] member of the DynFlags. A while
ago I turned most printed errors into some form of error message, but I
wouldn't be surprised if I missed some places. All output should go through
log_action, though, so try changing that to intercept any output.
[1]:
http://haskell.org/ghc/docs/6.12-latest/html/libraries/ghc-6.12.2/DynFlags.h
tml#v%3Alog_action
On 20 May 2010 19:05, Phyx loneti...@gmail.com wrote:
I was wondering how to forcibly quiet down the API. I have a custom
handler in place, but when I call the function on failure both my
handler gets called and somewhere somehow errors get printed to the
stderr, which I really need to avoid.
My current code looks like
getModInfo :: Bool - String - String - IO (ApiResults ModuleInfo)
getModInfo qual file path = handleSourceError processErrors $
runGhc (Just libdir) $ do
dflags - getSessionDynFlags
setSessionDynFlags $ configureDynFlags dflags
target - guessTarget file Nothing
addTarget target
setSessionDynFlags $ dflags { importPaths = [path] }
load LoadAllTargets
graph - depanal [] False
let modifier = moduleName . ms_mod
modName = modifier $ head graph
includes = includePaths dflags
imports = importPaths dflags
dflags' - Debug.trace (moduleNameString modName)
getSessionDynFlags
setSessionDynFlags $ dflags' { includePaths = path:includes
, importPaths = path:imports
}
parsed - parse modName
checked - typecheckModule parsed
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Re: [Haskell-cafe] Proposal to solve Haskell's MPTC dilemma
Hi Max. Excerpts from Max Bolingbroke's message of Sex Mai 21 04:56:51 -0300 2010: (...) (Incidentally, the link to your paper is broken, so I haven't actually been able to read it, sorry!) It was easy to find it on google. http://www.dcc.ufmg.br/~camarao/CT/solution-to-mptc-dilemma.pdf Greetings. (...) -- marcot http://wiki.debian.org/MarcoSilva signature.asc Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Register now for: Haskell in Leipzig (Germany), June 4.
Schedule and registration for the Haskell in Leipzig meeting on June 4 are now on-line: http://www.iba-cg.de/hal5.html It's our fifth meeting, and here are five good reasons to go: * learn Haskell: attend tutorials for beginners * get first-hand information: authors of Leksah, HXT, Hawk give tutorials/talks * share your expert knowledge: at lambda camp * see beyond Haskell: learn dependently typed programming in Agda * relax at BBQ party and listen to strange sounds: unsafePerformance The meeting is presented by http://www.fit-leipzig.de/ , in co-operation with http://www.imn.htwk-leipzig.de/ , http://www.leipziger-medienstiftung.de/, http://www.iba-cg.de/ Best regards, Johannes Waldmann (for the Hal5 program committee) signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Tools to make tidy HTML
Hello! Please advise haskell libraries similar to convert real-world HTML to well-formed XML. I need something similat to HTML Tidy library: http://tidy.sourceforge.net/ Thanks! Dmitri ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ambiguous type variable problem when using forall, multiparameter type classes, and constraints on polymorphic values (and syb-with-class)
Hello,
I am trying to understand why I am getting an ambigious type variable error,
and what I can do to work around it. The problem is occurring while trying
to use syb-with-class, but I have stripped it down to it's bare essentials,
so the following code is self-contained, and does not require an
understanding of syb-with-class.
{-# LANGUAGE RankNTypes, KindSignatures, MultiParamTypeClasses,
FlexibleInstances, ScopedTypeVariables, FlexibleContexts #-}
module Main where
A Proxy data type used to pass some extra type info around in gmapQ.
data Proxy (a :: * - *) = Proxy
A simple multiple parameter type class.
class Data (ctx :: * - *) a
gmapQ takes a function, forall a. (Data ctx a) = a - r, applies the
function to all the immediate sub-types of 'a' and collects the
results as a list. This is possible because the 'a' is forall'd, and
(in the real implementation) the Data class provides the mechanisms
for accessing all the immediate sub-types of 'a' and doing useful
things with them.
gmapQ :: Proxy ctx - (forall a. (Data ctx a) = a - r) - (forall a.
(Data ctx a) = a - [r])
gmapQ = undefined
Now we create a context.
data Foo m a = Foo { unFoo :: m a }
And create a Data instance for it.
instance Data (Foo m) a
Here is an example that attempts to recursively apply itself to all
the subtrees.
(I believe this function will ultimately return 0, since it tries to
sum the results of the children. When you reach types with no more
sub-types, I think you will get an empty list, and sum [] == 0. Though
it's not really important to the issue at hand).
bar1 :: forall a m r. (Data (Foo m) a, Num r) = a - r
bar1 x = sum $ gmapQ (undefined :: Proxy (Foo m)) bar1 x
The problem I have is when I try to add an additional constraint on 'm',
such as (Monad m) =
-- bar2 :: forall a m r. (Monad m, Data (Foo m) a, Num r) = a - r
-- bar2 x = sum $ gmapQ (undefined :: Proxy (Foo m)) (bar2 :: forall b.
(Monad m, Data (Foo m) b, Num r) = b - r) x
I get the error:
/tmp/a.lhs:48:54:
Ambiguous type variable `m' in the constraint:
`Monad m' arising from a use of `bar2' at /tmp/a.lhs:48:54-57
Probable fix: add a type signature that fixes these type variable(s)
Failed, modules loaded: none.
Adding all the scoped type variable stuff does not seem to help. Alas,
I can not figure out if this is a limitation of the type-checker, or
something that is fundamentally impossible. Nor can I figure out how
to work around the issue.
In my real code I need to define the data instance like:
-- instance (Monad m) = Data (Foo m) a
Which, by itself is fine. But that results in my needing to add (Monad
m) to the 'bar' function. And that is what I can't figure out how to
do..
Thanks!
- jeremy
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[Haskell-cafe] FW: Why does this Ord-class instance crash?
Why does the following, trivial code snippet below hang GHCi when I typeScalene Failure, and what's the fix? data Triangle = Failure | Equilateral | Isosceles | Scalene deriving (Eq, Show) instance Ord Triangle whereFailure Failure = FalseFailure _= True Equilateral Failure = FalseEquilateral Equilateral = False Equilateral _= True IsoscelesScalene = TrueIsosceles_= False Scalene _= False (I tried submitting this to beginn...@haskell.org, but even though I've signed up for that mailing list, I got a bounce-back saying that I needed admin approval to submit anything to that list, and I haven't heard from an admin, so I'm posting it here.) _ The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multiaccountocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_4___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal to solve Haskell's MPTC dilemma
On Fri, May 21, 2010 at 3:56 AM, Max Bolingbroke batterseapo...@hotmail.com wrote: On 21 May 2010 01:58, Carlos Camarao carlos.cama...@gmail.com wrote: But this type-correct program would become not typeable if instances such as the ones referred to before (by Daniel Fischer) I was thinking this through, and the situation is more complex than I had thought. It seems that single param type classes enjoy a nice property: * Adding an instance to the module defining the class cannot conflict with any non-orphan instance defined elsewhere * Adding an instance for a type for a class *to the module defining that type* cannot conflict with any non-orphan instance defined elsewhere This is only true in the absence of recursive imports. Otherwise, those points imply that I can put one instance in the module defining the type and another in the module defining the class without conflict. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] FW: Why does this Ord-class instance crash?
2010/5/21 R J rj248...@hotmail.com:
Why does the following, trivial code snippet below hang GHCi when I type
Scalene Failure, and what's the fix?
An instance of Ord must declare compare or (=). You only defined (),
so () is using the default definition. Here are the defaults:
compare x y = if x == y then EQ
else if x = y then LT
else GT
x y = case compare x y of { LT - True; _ - False }
x = y = case compare x y of { GT - False; _ - True }
x y = case compare x y of { GT - True; _ - False }
x = y = case compare x y of { LT - False; _ - True }
max x y = if x = y then y else x
min x y = if x = y then x else y
Note that the default definitions of (=) and compare call each other,
leading to an infinite loop when both are used.
Simple fix: define (=) instead of ().
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] FW: Why does this Ord-class instance crash?
From Prelude.hs:
class (Eq a) = Ord a where
compare :: a - a - Ordering
(), (=), (), (=) :: a - a - Bool
max, min :: a - a - a
compare x y = if x == y then EQ
-- NB: must be '=' not '' to validate the
-- above claim about the minimal things that
-- can be defined for an instance of Ord:
else if x = y then LT
else GT
x y = case compare x y of { LT - True; _ - False }
x = y = case compare x y of { GT - False; _ - True }
x y = case compare x y of { GT - True; _ - False }
x = y = case compare x y of { LT - False; _ - True }
-- These two default methods use '=' rather than 'compare'
-- because the latter is often more expensive
max x y = if x = y then y else x
min x y = if x = y then x else y
So, in your case:
Scalene Failure
- {Gee, there's no definition of in this Ord instance; I'll
have to take the default one}
compare Scalene Failure
- {There's no definition of compare either; again, the default
one}
Scalene = Failure
- {Again, there is no definition of =}
compare Scalene Failure
- loop
On 21 May 2010, at 21:06, R J wrote:
Why does the following, trivial code snippet below hang GHCi when I
type
Scalene Failure, and what's the fix?
data Triangle = Failure
| Equilateral
| Isosceles
| Scalene
deriving (Eq, Show)
instance Ord Triangle where
Failure Failure = False
Failure _= True
Equilateral Failure = False
Equilateral Equilateral = False
Equilateral _= True
IsoscelesScalene = True
Isosceles_= False
Scalene _= False
(I tried submitting this to beginn...@haskell.org, but even though
I've signed up for that mailing list, I got a bounce-back saying
that I needed admin approval to submit anything to that list, and I
haven't heard from an admin, so I'm posting it here.)
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Re: [Haskell-cafe] FW: Why does this Ord-class instance crash?
On Friday 21 May 2010 19:06:51, R J wrote: Why does the following, trivial code snippet below hang GHCi when I typeScalene Failure, and what's the fix? For an Ord instance, you need to define at least one of compare and (=) or the other functions from the class won't work. All methods have default implementations in terms of compare, compare has a default implementation in terms of (=). If you only implement () and then query (), it's going in circles. Fixes: a) implement compare or (=) b) choose the correct order for the constructors and add Ord to the deriving clause. data Triangle = Failure | Equilateral | Isosceles | Scalene deriving (Eq, Show) instance Ord Triangle whereFailure Failure = False Failure _= True Equilateral Failure = False Equilateral Equilateral = FalseEquilateral _= True IsoscelesScalene = TrueIsosceles_= False Scalene _= False Your newlines never make it to my mail programme, is your mail programme configured to send only '\r' and not '\n' ? (I tried submitting this to beginn...@haskell.org, but even though I've signed up for that mailing list, I got a bounce-back saying that I needed admin approval to submit anything to that list, and I haven't heard from an admin, so I'm posting it here.) If you sent very shortly after subscribing, your subscription may not have been registered by the watchdog. If you continue to have problems you should contact the list manager. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ambiguous type variable problem when using forall, multiparameter type classes, and constraints on polymorphic values (and syb-with-class)
On Fri, May 21, 2010 at 12:30 PM, Jeremy Shaw jer...@n-heptane.com wrote: Adding all the scoped type variable stuff does not seem to help. Alas, I can not figure out if this is a limitation of the type-checker, or something that is fundamentally impossible. Nor can I figure out how to work around the issue. It's either one, depending on how you look at. To explain: The problem I have is when I try to add an additional constraint on 'm', such as (Monad m) = -- bar2 :: forall a m r. (Monad m, Data (Foo m) a, Num r) = a - r -- bar2 x = sum $ gmapQ (undefined :: Proxy (Foo m)) (bar2 :: forall b. (Monad m, Data (Foo m) b, Num r) = b - r) x How might the compiler decide what specific m is meant when this function is called, so that it can make sure that it's always a Monad? All it has to work with are a and r. The only connection to m is via the Data instance, but the second parameter to Data alone is not really sufficient to find a specific instance--in fact, there could well be multiple such instances. bar1 :: forall a m r. (Data (Foo m) a, Num r) = a - r bar1 x = sum $ gmapQ (undefined :: Proxy (Foo m)) bar1 x Note that m is actually ambiguous here as well, but GHC won't complain until it needs to care about the specific type. If everything looks fully polymorphic GHC will just shrug, but by adding a class constraint to the definition's context, you force the issue. A classic, minimalist example of this problem--class constraints on types that don't appear in the function signature--is the function show . read. The type is just String - String, but the behavior depends on an unknown intermediate type. In my real code I need to define the data instance like: -- instance (Monad m) = Data (Foo m) a Which, by itself is fine. But that results in my needing to add (Monad m) to the 'bar' function. And that is what I can't figure out how to do.. The question you should answer first is: How do you expect the bar function to know which monad to use--or, if it doesn't matter which monad it picks, why do you care that it's given a monad at all? - C. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Exception: : changeWorkingDirectory: does not exist (No such file or directory)
anyone else seeing this behavior? anato...@anatolyy-linux ~ $ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. 1/2 0.5 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) 1/2 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Exception: : changeWorkingDirectory: does not exist (No such file or directory)
On Friday 21 May 2010 20:50:39, Anatoly Yakovenko wrote: anyone else seeing this behavior? anato...@anatolyy-linux ~ $ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. 1/2 0.5 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) 1/2 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) Never seen that. Just to make sure, there's nothing in any of your .ghci files that might cause it? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] double2Float is faster than (fromRational . toRational)
Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float The program works on windows but it did not on OSX - it was too slow. Now, after big headaches and much frustration, I replaced the code above with this line (why didn't I come up with this earlier?): xf = double2Float xd and now everything works just fine. I am not really surprised by the speed-up (and no-one should be), but I am still surprised how often such kinds of unobvious problems occur while programming in Haskell. So I write this email just to remind me and you to look out for such pitfalls. Best, Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] double2Float is faster than (fromRational . toRational)
On Fri, 21 May 2010, Daniel van den Eijkel wrote: Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float I think realToFrac is the function to use here, and this might be replaced by double2Float by an optimizer rule. I think double2Float is from a GHC package and thus one should avoid to call double2Float directly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] double2Float is faster than (fromRational . toRational)
I see. And I changed the code, it works well. Thanks for that! Daniel Henning Thielemann schrieb: On Fri, 21 May 2010, Daniel van den Eijkel wrote: Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float I think realToFrac is the function to use here, and this might be replaced by double2Float by an optimizer rule. I think double2Float is from a GHC package and thus one should avoid to call double2Float directly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] double2Float is faster than (fromRational . toRational)
On Friday 21 May 2010 22:06:43, Henning Thielemann wrote:
On Fri, 21 May 2010, Daniel van den Eijkel wrote:
Dear Haskellers,
I just want to share an observation. I had to convert a Double to a
Float value in an inner loop of an application, and I used somethin
like this:
xf = (fromRational $ toRational xd) :: Float
I think realToFrac is the function to use here, and this might be
replaced by double2Float by an optimizer rule. I think double2Float is
from a GHC package and thus one should avoid to call double2Float
directly.
In GHC.Real:
-- | general coercion to fractional types
realToFrac :: (Real a, Fractional b) = a - b
realToFrac = fromRational . toRational
{-# RULES
realToFrac/Int-Int realToFrac = id :: Int - Int
#-}
There are more rules elsewhere. If you compile with optimisations, GHC
turns your realToFrac into double2Float# nicely, so it's okay to use
realToFrac.
However, without optimisations, no rules fire, so you'll get
(fromRational . toRational).
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Re: [Haskell-cafe] double2Float is faster than (fromRational . toRational)
dvde: Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float The program works on windows but it did not on OSX - it was too slow. Now, after big headaches and much frustration, I replaced the code above with this line (why didn't I come up with this earlier?): xf = double2Float xd and now everything works just fine. I am not really surprised by the speed-up (and no-one should be), but I am still surprised how often such kinds of unobvious problems occur while programming in Haskell. So I write this email just to remind me and you to look out for such pitfalls. There's no rewrite rule for this optimization. There's a ticket though with some of the solutions. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] double2Float is faster than (fromRational . toRational)
By the way, speaking of floating-point precision, is there a real reason why haskell forces us to write : foreign import ccall unsafe math.h frexp c_frexp::CDouble-(Ptr CInt)-IO () foreign import ccall unsafe math.h ldexp c_ldexp::CDouble-CInt-IO CDouble ulp::Double-Double ulp x=unsafePerformIO $ do expon-alloca (\e-do c_frexp (realToFrac x) e peek e) (c_ldexp 0.5 $ expon-52) = return.realToFrac To allow us to change the IEEE-754 rounding mode ? Shouldn't it be rather roundDown $ x+3*y/z or roundUp $ (cos x)**y ? Moreover, due to laziness it appears quite difficult (at least to me !) to implement these roundDown and roundUp functions with C calls to select a hardware rounding mode, then unsafePerformIO. It would be way faster than an allocation (on the stack, I agree), then C calls with a memory access (probably cached, but...) in between ! Does anyone know how this translates to LLVM (maybe in the forthcoming GHC backend) ? If anyone has got an answer or a solution... Cheers, PE El 21/05/2010, a las 17:10, Don Stewart escribió: dvde: Dear Haskellers, I just want to share an observation. I had to convert a Double to a Float value in an inner loop of an application, and I used somethin like this: xf = (fromRational $ toRational xd) :: Float The program works on windows but it did not on OSX - it was too slow. Now, after big headaches and much frustration, I replaced the code above with this line (why didn't I come up with this earlier?): xf = double2Float xd and now everything works just fine. I am not really surprised by the speed-up (and no-one should be), but I am still surprised how often such kinds of unobvious problems occur while programming in Haskell. So I write this email just to remind me and you to look out for such pitfalls. There's no rewrite rule for this optimization. There's a ticket though with some of the solutions. -- Don ___ 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] Proof question -- (==) over Bool
I'm trying to prove that (==) is reflexive, symmetric, and transitive over the
Bools, given this definition:
(==) :: Bool - Bool - Boolx == y =
(x y) || (not x not y)
My question is: are the proofs below for reflexivity and symmetricity
rigorous, and what is the proof of transitivity, which eludes me? Thanks.
Theorem (reflexivity): For all x `elem` Bool, x == x.
Proof:
x == x ={definition of ==} (x x) || (not x not x) =
{logic (law of the excluded middle)} True
Theorem (symmetricity): For all x, y `elem` Bool, if x == y, then y == x.
Proof:
x == y ={definition of ==} (x y) || (not x not y) =
{lemma: () is commutative} (y x) || (not x not y) ={lemma:
() is commutative} (y x) || (not y not x) ={definition of
==} y == x
Theorem (transitivity): For all x, y, z `elem` Bool, if x == y, and y ==
z,then x == z.
Proof: ?
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Re: [Haskell-cafe] making the GHC Api not write to stderr
Have you tried freopen on stderr?
On Fri, May 21, 2010 at 8:43 AM, Phyx loneti...@gmail.com wrote:
Hi,
I tried that, setting it to (\_ _ _ _ - return ()) and it still did the
same, also tried setting it to undefined to see whether the code that's
printing the error is using it, and it didn't crash
So I assume it's not.
---
*VsxParser getModInfo True
C:\\Users\\Phyx\\AppData\\Local\\Temp\\tmp5600.hs
return ()
C:\Users\Phyx\AppData\Local\Temp\tmp5600.hs:11:13:
parse error on input `='
Printf
-
I think parseModule might still have a hardcoded print statement in it.
-Original Message-
From: Thomas Schilling [mailto:nomin...@googlemail.com]
Sent: Friday, May 21, 2010 12:53
To: Phyx
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] making the GHC Api not write to stderr
You could try changing the log_action[1] member of the DynFlags. A while
ago I turned most printed errors into some form of error message, but I
wouldn't be surprised if I missed some places. All output should go
through
log_action, though, so try changing that to intercept any output.
[1]:
http://haskell.org/ghc/docs/6.12-latest/html/libraries/ghc-6.12.2/DynFlags.h
tml#v%3Alog_action
On 20 May 2010 19:05, Phyx loneti...@gmail.com wrote:
I was wondering how to forcibly quiet down the API. I have a custom
handler in place, but when I call the function on failure both my
handler gets called and somewhere somehow errors get printed to the
stderr, which I really need to avoid.
My current code looks like
getModInfo :: Bool - String - String - IO (ApiResults ModuleInfo)
getModInfo qual file path = handleSourceError processErrors $
runGhc (Just libdir) $ do
dflags - getSessionDynFlags
setSessionDynFlags $ configureDynFlags dflags
target - guessTarget file Nothing
addTarget target
setSessionDynFlags $ dflags { importPaths = [path] }
load LoadAllTargets
graph - depanal [] False
let modifier = moduleName . ms_mod
modName = modifier $ head graph
includes = includePaths dflags
imports = importPaths dflags
dflags' - Debug.trace (moduleNameString modName)
getSessionDynFlags
setSessionDynFlags $ dflags' { includePaths = path:includes
, importPaths = path:imports
}
parsed - parse modName
checked - typecheckModule parsed
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[Haskell-cafe] Enum instantiation
I'd like to make Day an instance of class Enum, but the definition of toEnum below seems to be completely wrong, because integers seem not permit pattern matching. How is toEnum defined? Thanks. data Day = Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday deriving (Eq, Ord, Show) instance Enum Day wherefromEnum Sunday= 0fromEnum Monday = 1fromEnum Tuesday = 2fromEnum Wednesday = 3fromEnum Thursday = 4fromEnum Friday= 5fromEnum Saturday = 6toEnum 0 = SundaytoEnum 1 = Monday toEnum 2 = TuesdaytoEnum 3 = Wednesday toEnum 4 = ThursdaytoEnum 5 = Friday toEnum 6 = Saturday _ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_2___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Enum instantiation
2010/5/21 R J rj248...@hotmail.com: I'd like to make Day an instance of class Enum, but the definition of toEnum below seems to be completely wrong, because integers seem not permit pattern matching. How is toEnum defined? Thanks. Hi, What error are you getting when you try your class instance? Thanks, Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Enum instantiation
R J rj248...@hotmail.com writes: I'd like to make Day an instance of class Enum, but the definition of toEnum below seems to be completely wrong, because integers seem not permit pattern matching. How is toEnum defined? Thanks. You could try using guards: toEnum x | x == 0= Sunday | x == 1= Monday | x == 2= Tuesday | x == 3= Wednesday | x == 4= Thursday | x == 5= Friday | x == 6= Saturday | otherwise = error bad enum BTW if none of your constructors have fields you can just add Enum to the deriving (...) list for your type, and the compiler will write basically the same instance for you. G. -- Gregory Collins g...@gregorycollins.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proof question -- (==) over Bool
2010/5/21 R J rj248...@hotmail.com:
I'm trying to prove that (==) is reflexive, symmetric, and transitive over
the Bools, given this definition:
(==) :: Bool - Bool - Bool
x == y = (x y) || (not x not y)
My question is: are the proofs below for reflexivity and symmetricity
rigorous, and what is the proof of transitivity, which eludes me? Thanks.
Theorem (reflexivity): For all x `elem` Bool, x == x.
Proof:
x == x
= {definition of ==}
(x x) || (not x not x)
= {logic (law of the excluded middle)}
True
This one depends on what you mean by rigorous. But you would have to
have lemmas showing that and || correspond to the predicate logic
notions. I would do this by cases:
x = True:
(True True) || (not True not True)
...
True || False
True
x = False
(False False) || (not False not False)
...
False || True
True
Theorem (symmetricity): For all x, y `elem` Bool, if x == y, then y == x.
Proof:
x == y
= {definition of ==}
(x y) || (not x not y)
= {lemma: () is commutative}
(y x) || (not x not y)
= {lemma: () is commutative}
(y x) || (not y not x)
= {definition of ==}
y == x
Yes, given the lemmas about and ||, this is rigorous. You can
prove those lemmas by case analysis.
Theorem (transitivity): For all x, y, z `elem` Bool, if x == y, and y == z,
then x == z.
Proof: ?
My first hunch here is to try the following lemma:
Lemma: if (x == y) = True if and only if x = y.
where == is the version you defined, and = is regular equality from
logic, if you are allowed to rely on that. I would prove this by
cases.
At this point, you can convert transitivity of == to transitivity of
=, which is assumed by the axioms. You could do the same for the
other proofs you asked about instead of brute-forcing them.
If you aren't allowed such magic, then I guess you could do all 8
cases of x, y, and z (i.e. proof by truth table). Somebody else might
have a cleverer idea.
Luke
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Re: [Haskell-cafe] Enum instantiation
On Fri, May 21, 2010 at 8:59 PM, R J rj248...@hotmail.com wrote: If I type toEnum 5, the error I get is: interactive:1:0: Ambiguous type variable `a' in the constraint: `Enum a' arising from a use of `toEnum' at interactive:1:0-7 Probable fix: add a type signature that fixes these type variable(s) *Main Here the problem is not in your instance of Enum, but that GHCi doesn't know which instance of enum to use. In most code this isn't a problem, because the surrounding context often gives enough information for the type-checker to work with. But when there isn't enough context, you'll need to provide a type signature. Does toEnum 5 :: Day work? Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [ANNOUNCE] First Public Release of the Snap Framework
Hello all, To coincide with Hac Phi 2010 (http://www.haskell.org/haskellwiki/Hac_%CF%86), the Snap team is happy to announce the first public release of the Snap Framework, a simple and fast Haskell web programming server and library for unix systems. For installation instructions, documentation, and more information, see our website at http://snapframework.com/. Snap is well-documented and has a test suite with a high level of code coverage, but it is early-stage software with still-evolving interfaces. Snap is therefore most likely to be of interest to early adopters and potential contributors. Snap is BSD-licensed and currently only runs on Unix platforms; it has been developed and tested on Linux and Mac OSX Snow Leopard. Snap Features: * A simple and clean monad for web programming, similar to happstack's but simpler. * A *fast* HTTP server library with an optional high-concurrency backend (using libev). * An XML-based templating system for generating xhtml that allows you to bind Haskell functionality to XML tags in your templates. * Some useful utilities for web handlers, including gzip compression and fileServe. * Iteratee-based I/O, allowing composable streaming in O(1) space without any of the unpredictable consequences of lazy I/O. If you have questions or comments, please contact us on our mailing list (http://mailman-mail5.webfaction.com/listinfo/snap) or in the #snapframework channel on the freenode IRC network. Cheers, G -- Gregory Collins g...@gregorycollins.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
