Re: [Haskell-cafe] type class question
Ben schrieb: dear haskellers -- i'm trying this question again, in haskell-cafe. i got some responses in haskell-beginners but am looking for more guidance. also, i understand this functionality is encapsulated in the Workflow module in hackage, but i'd like to understand this myself. this email is an (il)literate haskell file. suppose i have class of computations a - State s b. for concreteness, let's say i'm writing a library of on-line statistical summary functions, like I used functions of such type to describe causal processes. In order make them an arrow, I had to hide the state s using existential quantification. http://code.haskell.org/synthesizer/core/src-4/Synthesizer/Causal/Process.hs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class question
Yes, the recursive calls of compCommand are supposed to be calls of compBlock. The intention of the program is a generic evaluator comp... of Sigma-terms in arbitrary Sigma-algebras. The signature Sigma is given by the first 4 types (and the corresponding functions in the class declaration), the terms are the objects of the types, and the algebras are the class instances. The problem with my implementation in terms of multiple-parameter classes seems to be - I conclude this from Ryan's comment - that the intended dependency among the parameters is not reflected here. But what are the alternatives? Roughly said, I need a construct that allows me gather several type variables such that an instance is always an instance of all of them. On Dec 3, 2007 7:43 AM, Peter Padawitz [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: What is wrong here? ghci tries (and fails) to deduce certain types for the comp functions that I did not expect. |type Block = [Command] data Command = Skip | Assign String IntE | Cond BoolE Block Block | Loop BoolE Block data IntE= IntE Int | Var String | Sub IntE IntE | Sum [IntE] | Prod [IntE] data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE class Java block command intE boolE where block_ :: [command] - block skip :: command assign :: String - intE - command cond :: boolE - block - block - command loop :: boolE - block - command intE_ :: Int - intE var :: String - intE sub :: intE - intE - intE sum_ :: [intE] - intE prod :: [intE] - intE boolE_ :: Bool - boolE greater :: intE - intE - boolE not_ :: boolE - boolE compBlock :: Block - block compBlock = block_ . map compCommand compCommand :: Command - command compCommand Skip = skip compCommand (Assign x e) = assign x (compIntE e) compCommand (Cond be c c') = cond (compBoolE be) (compCommand c) (compCommand c') compCommand (Loop be c)= loop (compBoolE be) (compCommand c)-} compIntE :: IntE - intE compIntE (IntE i) = intE_ i compIntE (Var x)= var x compIntE (Sub e e') = sub (compIntE e) (compIntE e') compIntE (Sum es) = sum_ (map compIntE es) compIntE (Prod es) = prod (map compIntE es) compBoolE :: BoolE - boolE compBoolE (BoolE b) = boolE_ b compBoolE (Greater e e') = greater (compIntE e) (compIntE e') compBoolE (Not be) = not_ (compBoolE be) | Well, first of all, the definition of compCommand should use calls to compBlock, not recursive calls to compCommand. But that's not the main source of your problems. What exactly are you trying to accomplish? And why do you need a type class? -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class question
Peter Padawitz wrote: Yes, the recursive calls of compCommand are supposed to be calls of compBlock. The intention of the program is a generic evaluator comp... of Sigma-terms in arbitrary Sigma-algebras. The signature Sigma is given by the first 4 types (and the corresponding functions in the class declaration), the terms are the objects of the types, and the algebras are the class instances. The problem with my implementation in terms of multiple-parameter classes seems to be - I conclude this from Ryan's comment - that the intended dependency among the parameters is not reflected here. But what are the alternatives? Roughly said, I need a construct that allows me gather several type variables such that an instance is always an instance of all of them. well, given class Java a b c d where if it is true that everything is determined by choice of any one of them, you could write class Java a b c d | a - b, b - c, c - d, d - a where... otherwise, well you can express whatever dependency network you want... Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class question
On Dec 3, 2007 7:43 AM, Peter Padawitz [EMAIL PROTECTED] wrote: What is wrong here? ghci tries (and fails) to deduce certain types for the comp functions that I did not expect. type Block = [Command] data Command = Skip | Assign String IntE | Cond BoolE Block Block | Loop BoolE Block data IntE= IntE Int | Var String | Sub IntE IntE | Sum [IntE] | Prod [IntE] data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE class Java block command intE boolE where block_ :: [command] - block skip :: command assign :: String - intE - command cond :: boolE - block - block - command loop :: boolE - block - command intE_ :: Int - intE var :: String - intE sub :: intE - intE - intE sum_ :: [intE] - intE prod :: [intE] - intE boolE_ :: Bool - boolE greater :: intE - intE - boolE not_ :: boolE - boolE compBlock :: Block - block compBlock = block_ . map compCommand compCommand :: Command - command compCommand Skip = skip compCommand (Assign x e) = assign x (compIntE e) compCommand (Cond be c c') = cond (compBoolE be) (compCommand c) (compCommand c') compCommand (Loop be c)= loop (compBoolE be) (compCommand c)-} compIntE :: IntE - intE compIntE (IntE i) = intE_ i compIntE (Var x)= var x compIntE (Sub e e') = sub (compIntE e) (compIntE e') compIntE (Sum es) = sum_ (map compIntE es) compIntE (Prod es) = prod (map compIntE es) compBoolE :: BoolE - boolE compBoolE (BoolE b) = boolE_ b compBoolE (Greater e e') = greater (compIntE e) (compIntE e') compBoolE (Not be) = not_ (compBoolE be) Well, first of all, the definition of compCommand should use calls to compBlock, not recursive calls to compCommand. But that's not the main source of your problems. What exactly are you trying to accomplish? And why do you need a type class? -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 7, 2007 4:34 PM, Nicholas Messenger [EMAIL PROTECTED] wrote: If you're willing to have an extra Typeable constraint, this does what you want: import Data.Typeable (Typeable, cast) import Data.Maybe (fromMaybe) toString :: (Show a, Typeable a) = a - String toString x = fromMaybe (show x) (cast x) Nice! Thank you for introducing me to the Typeable class (and by extension, Dynamic) -- I hadn't realized there was library support for dynamic types. G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
If you're willing to have an extra Typeable constraint, this does what you want:
import Data.Typeable (Typeable, cast)
import Data.Maybe (fromMaybe)
toString :: (Show a, Typeable a) = a - String
toString x = fromMaybe (show x) (cast x)
*Main toString blah
blah
*Main toString 1
1
*Main toString (Just 0.5)
Just 0.5
So Strings are just cast into the result. Non-strings become Nothing,
which fromMaybe turns into (show x).
--
Nicholas Messenger
[EMAIL PROTECTED]
On Nov 6, 2007 4:23 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 3:29 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] wrote:
Have you tried using -fglasgow-exts? That should enable all ghc
extensions.
If anyone's interested, I had best results when I added the flag
-fallow-incoherent-instances. Without it, I could not handle numbers
without declaring their types, e.g. 'toString (33 :: Int)' would work,
but 'toString 33' would lead to:
Ambiguous type variable `t' in the constraints:
`ToString t'
arising from use of `toString'
at /home/graham/tmp/ToString.hs:13:15-25
`Num t'
arising from the literal `33'
at /home/graham/tmp/ToString.hs:13:24-25
Probable fix: add a type signature that fixes these type variable(s)
Here's the code I ended up with.
{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-incoherent-instances -fallow-undecidable-instances #-}
module ToString (ToString(..)) where
class Show a = ToString a where toString :: a - String
instance ToString Stringwhere toString s = s
instance (Show a) = ToString a where toString s = show s
Thanks to all who responded; I learned a lot from this.
Graham
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RE: [Haskell-cafe] type/class question: toString
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Graham Fawcett
Is there a way to declare a 'toString' function, such that
toString x | x is a String = x
toString x | x's type is an instance of Show = show x
Perhaps, in the type system, there's a way to declare a ToString
class, and somehow inherit all instances of Show as ToString
instances?
I'm assuming you're not fond of the way the print function handles
Strings?
With GHC you can do this:
{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}
class Show a = MyShow a where show_ :: a - String
instance MyShow String where show_ s = s
instance (Show a) = MyShow a where show_ s = show s
Alistair
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Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 10:30 AM, Bayley, Alistair
[EMAIL PROTECTED] wrote:
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Graham Fawcett
Is there a way to declare a 'toString' function, such that
toString x | x is a String = x
toString x | x's type is an instance of Show = show x
I'm assuming you're not fond of the way the print function handles
Strings?
More a curiosity about the flexibility of the type system -- but yes,
I could see cases where such a thing could be useful.
With GHC you can do this:
{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}
class Show a = MyShow a where show_ :: a - String
instance MyShow String where show_ s = s
instance (Show a) = MyShow a where show_ s = show s
This doesn't want to compile for me:
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.6.1
$ ghc ToString.hs # your code, verbatim
ToString.hs:5:0:
Illegal instance declaration for `MyShow String'
(The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `MyShow String'
ToString.hs:6:0:
Illegal instance declaration for `MyShow a'
(The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `MyShow a'
I'll read up on those two GHC options, and try to figure it out myself
(but hints are welcome).
Thanks,
Graham
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Re: [Haskell-cafe] type/class question: toString
On 11/6/07, Graham Fawcett [EMAIL PROTECTED] wrote: ToString.hs:5:0: Illegal instance declaration for `MyShow String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyShow String' ToString.hs:6:0: Illegal instance declaration for `MyShow a' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `MyShow a' In ghc 6.8.1, the error messages are more helpful: foo.hs:5:0: Illegal instance declaration for `MyShow String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `MyShow String' foo.hs:6:0: Illegal instance declaration for `MyShow a' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `MyShow a' When I run with -XTypeSynonymInstances -XFlexibleInstances it works. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
I'm assuming you're not fond of the way the print function handles
Strings?
With GHC you can do this:
{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}
class Show a = MyShow a where show_ :: a - String
instance MyShow String where show_ s = s
instance (Show a) = MyShow a where show_ s = show s
I'm curious why this works. How does GHC know to pick the MyShow String
instance instead of the one coming from Show String?
I expect there's no way to do this without undecidable instances, is there?
I try to stay away from that flag nowadays, since I've seen some strange
unpredictable behavior from it in the past (the unpredictability of the
behavior may come from the fact that I don't know how the inference algorithm
works).
Luke
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Re: [Haskell-cafe] type/class question: toString
On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote: Hi folks, Is there a way to declare a 'toString' function, such that toString x | x is a String = x toString x | x's type is an instance of Show = show x Perhaps, in the type system, there's a way to declare a ToString class, and somehow inherit all instances of Show as ToString instances? I think the simpler solution (for your particular problem) is to tag strings that should be printed as-is: newtype Literal = Literal String instance Show Literal where show (Literal x) = x lit :: String - Literal lit = Literal I generally try to keep the law read . show == id Thus, for anything that needs to be printed in a nicer way I use something like this: class PPrint a where pretty :: a - Doc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
Hello, Have you tried using -fglasgow-exts? That should enable all ghc extensions. -Jeff [EMAIL PROTECTED] wrote on 11/06/2007 02:02:11 PM: On Nov 6, 2007 12:15 PM, David Benbennick [EMAIL PROTECTED] wrote: In ghc 6.8.1, the error messages are more helpful: foo.hs:5:0: Illegal instance declaration for `MyShow String' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `MyShow String' Thanks for the tip. I might give 6.8.1 a try; I still cannot get it to work in 6.6.1. The problem may exist between the keyboard and the chair. G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 12:03 PM, Thomas Schilling [EMAIL PROTECTED] wrote: On Tue, 2007-11-06 at 09:18 -0500, Graham Fawcett wrote: Hi folks, Is there a way to declare a 'toString' function, such that toString x | x is a String = x toString x | x's type is an instance of Show = show x I think the simpler solution (for your particular problem) is to tag strings that should be printed as-is: newtype Literal = Literal String instance Show Literal where show (Literal x) = x lit :: String - Literal lit = Literal I almost replied to ask, doesn't that solve a different problem? But I see that in practice, it leads to a similar result, and without type-system trickery. The type-system trickery is still devilishly interesting, though. ;-) Thanks, G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] wrote: Have you tried using -fglasgow-exts? That should enable all ghc extensions. Ah thanks, that does it. G ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type/class question: toString
On Nov 6, 2007 3:29 PM, Graham Fawcett [EMAIL PROTECTED] wrote:
On Nov 6, 2007 2:21 PM, Jeff Polakow [EMAIL PROTECTED] wrote:
Have you tried using -fglasgow-exts? That should enable all ghc
extensions.
If anyone's interested, I had best results when I added the flag
-fallow-incoherent-instances. Without it, I could not handle numbers
without declaring their types, e.g. 'toString (33 :: Int)' would work,
but 'toString 33' would lead to:
Ambiguous type variable `t' in the constraints:
`ToString t'
arising from use of `toString'
at /home/graham/tmp/ToString.hs:13:15-25
`Num t'
arising from the literal `33'
at /home/graham/tmp/ToString.hs:13:24-25
Probable fix: add a type signature that fixes these type variable(s)
Here's the code I ended up with.
{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-incoherent-instances -fallow-undecidable-instances #-}
module ToString (ToString(..)) where
class Show a = ToString a where toString :: a - String
instance ToString Stringwhere toString s = s
instance (Show a) = ToString a where toString s = show s
Thanks to all who responded; I learned a lot from this.
Graham
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Re: [Haskell-cafe] type class question
Tim, If I have a type class for conversion to a type X: class XType a where toX :: a - X [...] instance XType String where toX = ... results in: Illegal instance declaration for `XType String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `XType String' In addition to Derek's pointer, you could also consider extending the class definition: class XType a where toX :: a - X listToX :: [a] - X listToX = ... -- some default definition for listToX Of course, it depends on your type X whether a suitable default definition for listToX can be given. Assuming that it can, you can now, as before, have instance XType Int where toX = ... instance XType Double where toX = ... instance XType Tuple where toX = ... but also instance XType Char where toX c = ... -- your toX implementation for Char listToX s = ... -- your toX implementation for String This 'trick' is used in the standard libraries to accommodate a Show instance for String, for instance. Cheers, Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class question
On Tue, 22 May 2007, Tim Docker wrote: I think this must almost be a FAQ, or at least a PAQ (Previously AQ)... I think it too, thus I added your case to the Wiki: http://www.haskell.org/haskellwiki/List_instance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] type class question
Thanks for this - I only wonder if the page title List Instance would have suggested that this was a solution to me problem - I can't think of a better name however: Lists as type class instances perhaps? -Original Message- From: Henning Thielemann [mailto:[EMAIL PROTECTED] Sent: Tuesday, 22 May 2007 10:11 PM To: Tim Docker Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] type class question On Tue, 22 May 2007, Tim Docker wrote: I think this must almost be a FAQ, or at least a PAQ (Previously AQ)... I think it too, thus I added your case to the Wiki: http://www.haskell.org/haskellwiki/List_instance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class question
Tim Docker wrote: I think this must almost be a FAQ, or at least a PAQ (Previously AQ)... If I have a type class for conversion to a type X: class XType a where toX :: a - X I can define instances for instance XType Int where toX = ... instance XType Double where toX = ... instance XType Tuple where toX = ... but not for Strings, given that they are a synonym for [Char]. Hence: instance XType String where toX = ... results in: Illegal instance declaration for `XType String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `XType String' Is there some type class cleverness that can make this work in haskell 98? I can create a new wrapper type for strings: newtype StringWrap = StringWrap String and write an instance for that, but then I'll have to litter my code with calls to this constructor. I'm aware of the approach taken by class Show in the prelude, which adds a extra method to the class: class XType a where toX :: a - X listToX :: [a] - X but I believe this says that whenever we can convert a to an X we can also convert [a] to an X, whereas I only want [Char] to be acceptable. I believe there is a trick where essentially you end up with, instance IsChar a = XType [a] where ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] type class question
Derek Elkins wrote: I believe there is a trick where essentially you end up with, instance IsChar a = XType [a] where ... That is simple enough, and works fine. Thanks! Tim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
