[Haskell-cafe] Why this cannot be compiled?
Hi,
The following code failed to compiled, with error:
Attribute.hs:46:91:
Couldn't match expected type `[t]'
against inferred type `(a, String)'
In the expression: (color_, rest_)
In a case alternative: [(color_, rest_)] - (color_, rest_)
In the expression:
case reads rest1 of {
[(color_, rest_)] - (color_, rest_)
_ - (Color 0 0 0, rest1) }
43 instance Read Attribute where
44 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) - lex str
45 , (color, rest)
- case reads rest1 of
46
[(color_, rest_)] - (color_, rest_)
47
_ - (Color 0 0 0, rest1) ]
48 where mkAttr AttrFgColor color = AttrFgColor color
49 mkAttr AttrBgColor color = AttrBgColor color
50 mkAttr AttrInverse _ = AttrInverse
51 mkAttr AttrWeak _ = AttrWeak
52 mkAttr AttrUnderline _ = AttrUnderline
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Re: [Haskell-cafe] Why this cannot be compiled?
Magicloud Magiclouds magicloud.magiclo...@gmail.com writes: 43 instance Read Attribute where 44 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) - lex str 45 , (color, rest) - case reads rest1 of 46[(color_, rest_)] - (color_, rest_) 47_ - (Color 0 0 0, rest1) ] Doesn't the (color,rest) pair in the list comprehension need to be pulled from a list? The case expression only returns a pair so you're basically left with [something| something, (color,rest) - (somecolor,somerest) ] Since you're only producing a single (color,rest) for each rest1, you might consider lifting it out in the left side of the list comprehension. -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why this cannot be compiled?
Hum I must lost my mind Thank you. On Mon, Jul 13, 2009 at 3:33 PM, Ketil Maldeke...@malde.org wrote: Magicloud Magiclouds magicloud.magiclo...@gmail.com writes: 43 instance Read Attribute where 44 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) - lex str 45 , (color, rest) - case reads rest1 of 46 [(color_, rest_)] - (color_, rest_) 47 _ - (Color 0 0 0, rest1) ] Doesn't the (color,rest) pair in the list comprehension need to be pulled from a list? The case expression only returns a pair so you're basically left with [something| something, (color,rest) - (somecolor,somerest) ] Since you're only producing a single (color,rest) for each rest1, you might consider lifting it out in the left side of the list comprehension. -k -- If I haven't seen further, it is by standing in the footprints of giants -- 竹密岂妨流水过 山高哪阻野云飞 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Hylomorphisms (was: excercise - a completely lazy sorting algorithm)
Brent Yorgey wrote: Raynor Vliegendhart wrote: One of the examples I tried was: hylo (unfoldr (\a - Just (a,a))) head $ 42 This expression fails to determinate. Here are two examples copumpkin tried on IRC: copumpkin let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) length 5 lambdabot 5 copumpkin let hylo f g = g . fmap (hylo f g) . f in hylo (flip replicate 2) sum 5 lambdabot * Exception: stack overflow [] is a strange functor to use with hylo, since it is already recursive and its only base case (the empty list) doesn't contain any a's. Think about the intermediate structure that hylo (unfoldr (\a - Just (a,a))) head is building up: it is a list of lists of lists of lists of lists of lists of no wonder it doesn't terminate! =) Yep, not terminating is the correct behavior here. In particular, we have example = hylo repeat head = cata head . ana repeat and the intermediate data structure is Fix [] which is an infinite nested tower of infinite lists. Regards, apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Are GADTs what I need?
Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value - doSomething (VWrapper String s) = . Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell.org: what can be improved causing what efforts?
code snippet: no hello world please. That's not a way to judge a language! But: a random haskell one line snippet with explanation would be cool. Perhaps a solution to a problem like the ones you can find on Project Euler (http://projecteuler.net/index.php?section=problems). Of course you can't take an actual problem from Project Euler, because they do not like solutions to be posted in the wild. But you can get your inspiration from there. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type families and polymorphism
Dan Doel wrote: Hope that helps. It does, thanks! Jeremy -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Are GADTs what I need?
Hey Kev, The types are thrown away during compile time. Therefore, if you have a constructor VWrapper :: a - Value nothing is known about that a when you scrutinize it. What you could do, however, is something like this: data Value a where VInt :: Integer - Value Integer ... VWrapper :: a - Value a And then you can write a function doSomething: doSomething :: Value String - String doSomething (VWrapper s) = s HTH, -chris On 13 jul 2009, at 12:41, Kev Mahoney wrote: Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value - doSomething (VWrapper String s) = . Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin. ___ 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] haskell.org: what can be improved causing what efforts?
On Mon, 13 Jul 2009 12:43:07 +0200, Matthias Görgens matthias.goerg...@googlemail.com wrote: code snippet: no hello world please. That's not a way to judge a language! But: a random haskell one line snippet with explanation would be cool. Perhaps a solution to a problem like the ones you can find on Project Euler (http://projecteuler.net/index.php?section=problems). Of course you can't take an actual problem from Project Euler, because they do not like solutions to be posted in the wild. But you can get your inspiration from there. I like the quicksort example at http://www.haskell.org/haskellwiki/Introduction very much; it shows how much time you can save when you use Haskell. -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell.org: what can be improved causing what efforts?
I like the quicksort example at http://www.haskell.org/haskellwiki/Introduction very much; it shows how much time you can save when you use Haskell. Nice idea. Perhaps use a merge sort, because that is actually useful, because it does not degenerate for large lists. Matthias. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] haskell.org: what can be improved causing what efforts?
Hello Matthias, Monday, July 13, 2009, 6:05:06 PM, you wrote: I like the quicksort example at Nice idea. Perhaps use a merge sort, because that is actually useful, because it does not degenerate for large lists. Great idea if we want to keep Haskell community compact :))) -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] haskell.org: what can be improved causing what efforts?
Nice idea. Perhaps use a merge sort, because that is actually useful, because it does not degenerate for large lists. Great idea if we want to keep Haskell community compact :))) Or stay with quicksort --- which is treesort. :o) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Are GADTs what I need?
Then you could add a specific constructor for String. The main point is: the case construct only works for values, not for types. There is no typecase construct. If you want to have certain restrictions on the 'a', such as the Show class, you could also do something like this: data Value where VWrapper :: (Show a) = a - Value If you could elaborate a bit on what you're trying to accomplish (from a higher viewpoint) then maybe we can help you some more. -chris On 13 jul 2009, at 17:42, Kev Mahoney wrote: Thanks, that helps. I was hoping to not have to parametrize Value as there is a fair bit of code to change, and it cascades down through the data structures (maybe a forall a . Value a will help here?) I will have a go at this approach. In case anyone is interested the code is at http://github.com/KMahoney 2009/7/13 Chris Eidhof ch...@eidhof.nl: Hey Kev, The types are thrown away during compile time. Therefore, if you have a constructor VWrapper :: a - Value nothing is known about that a when you scrutinize it. What you could do, however, is something like this: data Value a where VInt :: Integer - Value Integer ... VWrapper :: a - Value a And then you can write a function doSomething: doSomething :: Value String - String doSomething (VWrapper s) = s HTH, -chris On 13 jul 2009, at 12:41, Kev Mahoney wrote: Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value - doSomething (VWrapper String s) = . Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin. ___ 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] ANN: AC-Vector, AC-Colour and AC-EasyRaster-GTK
It’s tempting to say, we should use the original English, which is British English. Some suggest the original English remained in Britain when the North American colonies were founded; others claim it was brought to the Americas by the British settlers, leaving a pale imitation back in Britain. The truth is much stranger: the original English was actually smuggled out of Britain to the West Indies in a wardrobe belonging to General Sir Ralph Abercromby, where it ended up on the island of Trinidad after Sir Ralph took possession of that territory in the name of the British Crown. It came to be used and modified freely by the various immigrants to Trinidad (and later Tobago) and their descendants (largely African, Indian, British, Portuguese, German, Spanish, and Chinese). Many of these peoples then emigrated, bringing the original English to North America and back to Britain. A copy of it has fallen into my hands, and so I can, without bias, make the following call: both color and colour shall be acceptable in Haskell programming. 'Kerb' and 'gaol' are right out, however. Cheers, Robert (who's grandfather is from London and grandmother from Trinidad; but is nevertheless American) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Are GADTs what I need?
Oops, wrong mail account for my last email. Apologies. What I'm trying to accomplish is being able to write haskell libraries for the interpreter that don't use the interpreter's predefined Value types, without having to edit the Value type itself and add a new constructor (i.e. it's abstracted away). I believe the concept is called 'userdata' in Lua, for example. In C interpreters it is usually accomplished by throwing away type information and casting to 'void*' and then casting back to the type to get it out again. That said, I think I may defer this until I understand the ins and outs of Haskell's type system a little better. I think a parametrized type will be the only way to do it. The only reason I thought GADTs may be able to do this is because I read some literature that suggested GADTs could be used as a kind of typecase construct (I think it was 'Fun with Phantom Types'?) but I could have very easily misunderstood it. 2009/7/13 Chris Eidhof ch...@eidhof.nl: Then you could add a specific constructor for String. The main point is: the case construct only works for values, not for types. There is no typecase construct. If you want to have certain restrictions on the 'a', such as the Show class, you could also do something like this: data Value where VWrapper :: (Show a) = a - Value If you could elaborate a bit on what you're trying to accomplish (from a higher viewpoint) then maybe we can help you some more. -chris On 13 jul 2009, at 17:42, Kev Mahoney wrote: Thanks, that helps. I was hoping to not have to parametrize Value as there is a fair bit of code to change, and it cascades down through the data structures (maybe a forall a . Value a will help here?) I will have a go at this approach. In case anyone is interested the code is at http://github.com/KMahoney 2009/7/13 Chris Eidhof ch...@eidhof.nl: Hey Kev, The types are thrown away during compile time. Therefore, if you have a constructor VWrapper :: a - Value nothing is known about that a when you scrutinize it. What you could do, however, is something like this: data Value a where VInt :: Integer - Value Integer ... VWrapper :: a - Value a And then you can write a function doSomething: doSomething :: Value String - String doSomething (VWrapper s) = s HTH, -chris On 13 jul 2009, at 12:41, Kev Mahoney wrote: Hi there, I'm currently writing an interpreter that I would like to be able to use with other haskell programs. I would like to be able to pass along arbitrary types though the interpreter. I've seen hints that GADTs can do this, but I am having trouble understanding them. So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. doSomething :: Value - doSomething (VWrapper String s) = . Also, anything that can help me out with GADTs in general will be much appreciated. Thanks, Kevin. ___ 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] Are GADTs what I need?
On Mon, Jul 13, 2009 at 12:41 PM, Kev Mahoneymaill...@kevinmahoney.co.uk wrote: So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. As such this type is pretty useless, since you don't know anything about a, you can't do anything with it... Which is why you add typeclass constraints, so you can use this value. Data.Dynamic adds a Typeable constraint, which allows you to do safe coercing, so you can have a typecase, if not like you tried to do it. -- Jedaï ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Questions about haskell CPP macros
Hello Cafe,
I am trying to improve the error reporting in my sendfile library, and I
know I can find out the current file name and line number with something
like this:
{-# LANGUAGE CPP #-}
main = putStrLn (__FILE__ ++ : ++ show __LINE__)
This outputs:
test.hs:2
Unfortunately, if your file is in a hierarchy of folders, this flat file
name doesn't give much context. Is there a macro to find out the current
module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be able
to output something like this on error:
Foo.Bar.Car.MyModule:2
Any help is appreciated!
Thanks,
Matt
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Re: [Haskell-cafe] Are GADTs what I need?
Thanks, I hadn't noticed Data.Dynamic. It never even occurred to me that something like this would be in the standard libraries. It looks like it's precisely what I was looking for, after a brief scan of the documentation. I will report back if I bump into any problems with it 2009/7/13 Chaddaï Fouché chaddai.fou...@gmail.com: On Mon, Jul 13, 2009 at 12:41 PM, Kev Mahoneymaill...@kevinmahoney.co.uk wrote: So far, I've learnt you can do this: data Value where VInt :: Integer - Value ... VWrapper :: a - Value which can let you encode arbitrary 'dynamic' types into Value. I was hoping to be able to pattern match to get the value out again e.g. As such this type is pretty useless, since you don't know anything about a, you can't do anything with it... Which is why you add typeclass constraints, so you can use this value. Data.Dynamic adds a Typeable constraint, which allows you to do safe coercing, so you can have a typecase, if not like you tried to do it. -- Jedaï ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Are GADTs what I need?
On Mon, Jul 13, 2009 at 9:18 AM, Kev Mahoneymaill...@kevinmahoney.co.uk wrote: That said, I think I may defer this until I understand the ins and outs of Haskell's type system a little better. I think a parametrized type will be the only way to do it. The only reason I thought GADTs may be able to do this is because I read some literature that suggested GADTs could be used as a kind of typecase construct (I think it was 'Fun with Phantom Types'?) but I could have very easily misunderstood it. The big problem is that you haven't parametrized your Value type at all, so there's no information about the value inside. Try this: data Value a where VPrim :: Type a - a - Value a VAbs :: Type a - (Value a - Value b) - Value (a - b) VApp :: Value (a - b) - Value a - Value b data Type a where TInt :: Type Int TBool :: Type Bool TChar :: Type Char TList :: Type a - Type [a] TFun :: Type a - Type b - Type (a - b) data SomeType = forall a. SomeType (Type a) data SomeValue = forall a. SomeValue (Value a) Now you can do: interpret :: Value a - a interpret (VPrim _ x) = x interpret (VAbs t f) = \x - f (VPrim t x) interpret (VApp f x) = interpret f $ interpret x And: typeOf :: Value a - Type a typeOf (VPrim t _) = t typeOf (VAbs t f) = typeOf (f $ VPrim t (representative t)) typeOf (VApp f _) = case typeOf f of (TFun _ b) - b representative :: Type a - a representative TInt = 0 representative TBool = False representative TChar = 'a' representative (TList _) = [] representative (TFun _ b) = \_ - representative b Your compiler will generally have type (String - Maybe SomeValue), or (String - Type a - Maybe (Value a)). -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Questions about haskell CPP macros
I am trying to improve the error reporting in my sendfile library, and I
know I can find out the current file name and line number with something
like this:
{-# LANGUAGE CPP #-}
main = putStrLn (__FILE__ ++ : ++ show __LINE__)
This outputs:
test.hs:2
Unfortunately, if your file is in a hierarchy of folders, this flat file
name doesn't give much context. Is there a macro to find out the current
module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be able
to output something like this on error:
Foo.Bar.Car.MyModule:2
Sounds like a job for cabal or ghc, to define appropriate macros for
package and module when compiling the source?
Any help is appreciated!
For actually making use of such information, see
http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack
http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack/StackTraceExperience
and also the recent thread on how to improve the quality of +RTS -xc
output via mapException (hmm, can't reach the archive at the moment,
one subject was Should exhaustiveness testing be on by default?, about
May; http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ).
If you really mean any help, you could also use Template Haskell:-)
{-# LANGUAGE TemplateHaskell #-}
module Oh.Hi where
import Language.Haskell.TH
main = print $( location = \(Loc f p m s e)-
stringE (f++:++p++:++m++:++show s++:++show e))
Claus
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Re: [Haskell-cafe] Questions about haskell CPP macros
Matthew Elder wrote:
{-# LANGUAGE CPP #-}
main = putStrLn (__FILE__ ++ : ++ show __LINE__)
This outputs:
test.hs:2
Unfortunately, if your file is in a hierarchy of folders, this flat file
name doesn't give much context. Is there a macro to find out the current
module? IE if I had a module Foo.Bar.Car.MyModule, I would like to be
able to output something like this on error:
Foo.Bar.Car.MyModule:2
As mentioned by Claus, template-haskell offers a solution. But in some
cases, this is an overkill; consider using Control.Exception.assert, it
will provide module and line information without having to use CPP:
myHead :: [a] - a
myHead (x:_) = x
myHead []= assert False undefined
[...]
//Stephan
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Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.
- Dieter Nuhr
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Re: [Haskell-cafe] Questions about haskell CPP macros
{-# LANGUAGE CPP #-}
main = putStrLn (__FILE__ ++ : ++ show __LINE__)
This outputs:
test.hs:2
if I had a module Foo.Bar.Car.MyModule, I would like to be able to
output something like this on error:
Foo.Bar.Car.MyModule:2
It works for me. If you place that text in Try/Me.hs and call
ghc -E Try/Me.hs
you get
Try/Me.hs:2
If you just want to turn slashes into dots, and remove the suffix,
that is a simple exercise in Haskell itself
main = putStrLn (mangle __FILE__)
where mangle ('/':cs) = '.': mangle cs
mangle .
Regards,
Malcolm
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Re: [Haskell-cafe] Are GADTs what I need?
On Mon, Jul 13, 2009 at 6:09 AM, Chris Eidhof ch...@eidhof.nl wrote: Hey Kev, The types are thrown away during compile time. Therefore, if you have a constructor VWrapper :: a - Value nothing is known about that a when you scrutinize it. What you could do, however, is something like this: data Value a where VInt :: Integer - Value Integer ... VWrapper :: a - Value a And then you can write a function doSomething: doSomething :: Value String - String doSomething (VWrapper s) = s I would like to put in a thumbs up on this approach. I'm currently experimenting with interpreters, and have found that parameterizing over the value type works quite smoothly. Specifically, this is my usual starting point for the values: data Value a = VFun (Value a - Value a) | VPrim a Then I use a typeclass to endow the primitives with the structure they need: class ValueType a where apply :: a - a - a Here's a simple interpreter for terms in De Bruijn notation: data Term a = TLit a | TApp (Term a) (Term a) | TLam (Term a) | TVar Int eval :: (ValueType a) = Term a - [Value a] - Value a eval (TLit x) = const (VPrim x) eval (TApp x y) = let x' = eval x y' = eval y in \env - x' env % y' env eval (TLam body) = let body' = eval body in \env - VFun (\x - body (x:env)) eval (TVar z) = \env - env !! z (%) :: (ValueType a) = Value a - Value a - Value a VFun f % x = f x VPrim x % VFun _ = error Apply primitive to function not supported VPrim x % VPrim y = VLit (x `apply` y) And an example ValueType: data Prim = PInt Int | PSucc instance ValueType Prim where apply PSucc (PInt z) = PInt $! z+1 apply _ _ = error Type error This approach has been very nice and modular for experimenting with dynamically typed interpreters. You could support application of literals to functions with some more support from the type class, but it wasn't worth it to me (and would limit the interpretation strategies that I would be able to use). The decision about what suite of primitives to include and how they combine with each other is pushed out to the user, and the interpreter just focuses on the important things: functions. You could even write a little primitive combinator library (perhaps made more composable by switching to dictionary passing for ValueType instead of typeclass), so that users can easily specify any suite of primitives. Anyway, those were just some thoughts for you. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Removing polymorphism from type classes (viz. Functor)
George Pollard schrieb: Ok, so I have a small idea I'm trying to work on; call it a Prelude-rewrite if you want. For this I want to be able to have the hierarchy Functor → Applicative → Monad. For Functor, I would like to be able to implement it for a wider variety of types, as there are types which have aren't polymorphic which would also benefit from having an instance. My running example for this set of types is ByteString; the module contains the method: map ∷ (Word8 → Word8) → ByteString → ByteString However, we cannot use this for Functor because ByteString isn't polymorphic. To get around this, I devised the following: Introduce a type family which represents ‘points’ inside the type: type family Point f ∷ ★ For ByteString we have: type instance Point ByteString = Word8 For a polymorphic example (lists) we have: type instance Point [a] = a I had the same in mind for Data.Set with Ord constraint for elements, StorableVector with Storable constraint for the elements, and Control.Monad.Excepetion.Asynchronous monad with Monoid constraint for the monadic result. I tried to come up with a class hierarchy: http://code.haskell.org/~thielema/category-constrained/src/Control/Constrained/ but I encountered the same problem with the Applicative class. Different from what I tried in Applicative.hs I think that the most flexible approach is to convert the ByteString (or Data.Set or StorableVector) to an interim data structure first where you do, say 'liftA3' aka 'zipWith3', then convert back to the real data structure, here ByteString. The interim data structure can be stream-fusion:Data.Stream, i.e. not a real data structure but an algorithm to read from the ByteString. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Questions about haskell CPP macros
Malcolm Wallace wrote:
{-# LANGUAGE CPP #-}
main = putStrLn (__FILE__ ++ : ++ show __LINE__)
This outputs:
test.hs:2
if I had a module Foo.Bar.Car.MyModule, I would like to be able to
output something like this on error:
Foo.Bar.Car.MyModule:2
It works for me. If you place that text in Try/Me.hs and call
ghc -E Try/Me.hs
you get
Try/Me.hs:2
If you just want to turn slashes into dots, and remove the suffix, that
is a simple exercise in Haskell itself
main = putStrLn (mangle __FILE__)
where mangle ('/':cs) = '.': mangle cs
mangle .
Careful, '/' might be '\\' on another OS, the file might end with .hsc
instead of .hs, the line numbers might not fit in the .hsc case...
--
Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.
- Dieter Nuhr
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[Haskell-cafe] Example for formatted show in HStringTemplate
Hello, is there a working example of how to use the format clause with HStringTemplate, e.g. for Data.Time.Day? I think, if there is a parameter $day$, a reasonable template might contain e.g.: $day;format=%d.%b.%Y$ But I only get toModifiedJulianDay: [54960] as the result which corresponds to the unformatted show. Regards, Torsten ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Removing polymorphism from type classes (viz. Functor)
It does seem that having quantified contexts would make this *much* easier... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to Read this?
Hi,
I have a data structure, which shows like this: AttrBgColor {bgColor
= Color 0 0 0}
And the following is my Read code. But it failed parsing
31 instance Read Attribute where
32 readsPrec _ str = [ (mkAttr attr_ color, rest) | (attr_, rest1) - lex str
33 , (color, rest)
- if (isPrefixOf {bgColor = rest1)
34
|| (isPrefixOf {fgColor = rest1)
35
then case reads $ drop 12 rest1 of
36
[(color_, rest_)] - [(color_, rest_)]
37
_ - [(Color 0 0 0, rest1)]
38
else [(Color 0 0 0, rest1)] ]
39 where mkAttr AttrFgColor color = AttrFgColor color
40 mkAttr AttrBgColor color = AttrBgColor color
41 mkAttr AttrInverse _ = AttrInverse
42 mkAttr AttrWeak _ = AttrWeak
43 mkAttr AttrUnderline _ = AttrUnderline
--
竹密岂妨流水过
山高哪阻野云飞
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[Haskell-cafe] Re: Are GADTs what I need?
Ryan Ingram wrote: data Type a where TInt :: Type Int TBool :: Type Bool TChar :: Type Char TList :: Type a - Type [a] TFun :: Type a - Type b - Type (a - b) Type here is what I call a simple type witness. Simple type witnesses are useful because they can be compared by value, and if they have the same value, then they have the same type. So you can write this: data EqualType a b where MkEqualType :: EqualType t t matchWitness :: Type a - Type b - Maybe (EqualType a b) matchWitness TInt TInt = Just MkEqualType matchWitness TBool TBool = Just MkEqualType matchWitness TChar TChar = Just MkEqualType matchWitness (TList w1) (TList w2) = do MkEqualType - matchWitness w1 w2 return MkEqualType matchWitness (TFun wa1 wb1) (TFun wa2 wb2) = do MkEqualType - matchWitness wa1 wa2 MkEqualType - matchWitness wb1 wb2 return MkEqualType matchWitness _ _ = Nothing Now whenever you match some value with MkEqualType, the compiler will infer the identity of the two types. See my witness package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/witness -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
