[Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
This is a question about the use of type classes in Haskell.
I get an error (below) when trying to compile the code (below and at
https://github.com/chrisdew/haskell-sandbox/blob/master/not_working_but_clean.hs
).
As someone just learning Haskell, I have tried following GHC's advice,
but I think the cause is different.
I believe the problem is that either of the types 'IO String' or plain
'String' could be the type of 'lhello - lbracket', but GHC doesn't
know which.
The problem is that it doesn't matter, either type would work fine.
I have posted a working version of the code at
https://github.com/chrisdew/haskell-sandbox/blob/master/working_but_ugly.hs
. This replaces one of the - operators with a new (non type class)
operator '-' which forces 'lhello - lbracket' to be of type 'IO
String'.
* Is my analysis correct? Or is there something else going on here?
* Is there any way of informing GHC what the type of 'lhello -
lbracket' doen't matter and that is should just chose either of the
two possibilities. Or perhaps theres a LANGUAGE option which will let
me specify that 'lastest declared matching instance of the class wins'
if anything is undecided.
Thanks,
Chris.
Error:
chris@chris-linux-desktop:~/nonworkspace/haskell-sandbox$ ghc
not_working_but_clean.hs
not_working_but_clean.hs:40:16:
No instance for (Stream (IO String) (IO String) (IO String) d)
arising from a use of `-' at not_working_but_clean.hs:40:16-34
Possible fix:
add an instance declaration for
(Stream (IO String) (IO String) (IO String) d)
In the first argument of `(-)', namely `lhello - lbracket'
In the second argument of `($)', namely
`lhello - lbracket - putStrLn'
In a stmt of a 'do' expression:
forkIO $ lhello - lbracket - putStrLn
not_working_but_clean.hs:40:16:
No instance for (Stream d String (IO ()) (IO ()))
arising from a use of `-' at not_working_but_clean.hs:40:16-47
Possible fix:
add an instance declaration for (Stream d String (IO ()) (IO ()))
In the second argument of `($)', namely
`lhello - lbracket - putStrLn'
In a stmt of a 'do' expression:
forkIO $ lhello - lbracket - putStrLn
In the expression:
do { forkIO $ (bracket $ hello) - putStrLn;
forkIO $ lhello - lbracket - putStrLn;
forkIO $ bracket hello - putStrLn;
forkIO $ lbracket lhello - putStrLn;
}
not_working_but_clean.hs:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances,
TypeSynonymInstances, OverlappingInstances #-}
{-# OPTIONS_GHC #-}
module Main (
main
)
where
import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar,
takeMVar, ThreadId, threadDelay)
import Control.Monad (forever, liftM)
class Stream a b c d where
(-) :: a - (b - c) - d
instance Stream (IO d) d (IO c) (IO c) where
f - g = f = g
instance Stream d d (IO c) (IO c) where
f - g = g f
instance Stream d d c c where
x - y = y $ x
-- This simply wraps a string in brackets.
bracket :: String - String
bracket x = ( ++ x ++ )
lbracket :: IO String - IO String
lbracket x = liftM bracket x
hello :: String
hello = Hello World!
lhello :: IO String
lhello = do return hello
main :: IO ()
main = do
forkIO $ (bracket $ hello) - putStrLn
forkIO $ lhello - lbracket - putStrLn
forkIO $ bracket hello - putStrLn
forkIO $ lbracket lhello - putStrLn
threadDelay 1000 -- Sleep for at least 10 seconds before exiting.
working_but_ugly.hs:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances,
TypeSynonymInstances, OverlappingInstances #-}
{-# OPTIONS_GHC #-}
module Main (
main
)
where
import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar,
takeMVar, ThreadId, threadDelay)
import Control.Monad (forever, liftM)
class Stream a b c d where
(-) :: a - (b - c) - d
instance Stream (IO d) d (IO c) (IO c) where
f - g = f = g
instance Stream d d (IO c) (IO c) where
f - g = g f
instance Stream d d c c where
x - y = y $ x
x - y = y $ x
-- This simply wraps a string in brackets.
bracket :: String - String
bracket x = ( ++ x ++ )
lbracket :: IO String - IO String
lbracket x = liftM bracket x
hello :: String
hello = Hello World!
lhello :: IO String
lhello = do return hello
main :: IO ()
main = do
forkIO $ (bracket $ hello) - putStrLn
forkIO $ lhello - lbracket - putStrLn
forkIO $ bracket hello - putStrLn
forkIO $ lbracket lhello - putStrLn
threadDelay 1000 -- Sleep for at least 10 seconds before exiting.
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Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
On 14/04/11 13:00, Chris Dew wrote: class Stream a b c d where (-) :: a - (b - c) - d instance Stream (IO d) d (IO c) (IO c) where f - g = f= g instance Stream d d (IO c) (IO c) where f - g = g f instance Stream d d c c where x - y = y $ x I notice that in all your instances, the last two types are the same. So do you need the final type parameter? Could you not make it: class Stream a b c where (-) :: a - (b - c) - c I quickly tried this, and it fixes the errors you were getting. If that doesn't hold for all instances you have in mind, then you may want to use functional dependencies or type families to specify a relationship between the types. Thanks, Neil. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
Hi Chris What does the Stream class *do* though? class Stream a b c d where (-) :: a - (b - c) - d Even with Neil's change its still quite unusual: class Stream a b c where (-) :: a - (b - c) - c In the first formulation there is an input of type a, a function (b - c) and a result of a completely different type d. In Neil's class the function relates to the type of the answer but not to the input. The difficult type classes in Haskell - Applicative, Monad, and Arrows / Category - are related to some degree to fairly standard combinators on functions. But they generalize the combinators to operate on other types than the function type (-). As there isn't a relation between input and output, I don't quite see how the Stream type could start as a combinator. Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
@Neil Brown - That did it. It's not the ideal solution, as all - are 'coerced' into being 'IO x' (if the rightmost term is an 'IO x'. But it'll do for the time being. Many thanks, Chris. On 14 April 2011 13:50, Neil Brown nc...@kent.ac.uk wrote: On 14/04/11 13:00, Chris Dew wrote: class Stream a b c d where (-) :: a - (b - c) - d instance Stream (IO d) d (IO c) (IO c) where f - g = f= g instance Stream d d (IO c) (IO c) where f - g = g f instance Stream d d c c where x - y = y $ x I notice that in all your instances, the last two types are the same. So do you need the final type parameter? Could you not make it: class Stream a b c where (-) :: a - (b - c) - c I quickly tried this, and it fixes the errors you were getting. If that doesn't hold for all instances you have in mind, then you may want to use functional dependencies or type families to specify a relationship between the types. Thanks, Neil. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
@Stephen Tetley - The stream class exists simply to allow for the creation of a - operator which can be used to 'Stream' data through multiple pure and IO functions, on the way to some form of output. It's probably not a great idea, as there are more idiomatic solutions in Haskell - I'm sure someone will mention arrows. I want the result of (-) to be what the following function requires, either an 'a' or and 'IO a'. This is too unconstrained if the following function is flexible in it's input. (e.g. another application of (-)). Hence my original problem. a and b have are related, but not in a way I know how to express in Haskell. They are constrained to: a == b || IO a == b || a == IO b. c and d have a similar constraint. Could you suggest how these constraints could be expressed in the Haskell type system? Thanks, Chris. On 14 April 2011 14:28, Stephen Tetley stephen.tet...@gmail.com wrote: Hi Chris What does the Stream class *do* though? class Stream a b c d where (-) :: a - (b - c) - d Even with Neil's change its still quite unusual: class Stream a b c where (-) :: a - (b - c) - c In the first formulation there is an input of type a, a function (b - c) and a result of a completely different type d. In Neil's class the function relates to the type of the answer but not to the input. The difficult type classes in Haskell - Applicative, Monad, and Arrows / Category - are related to some degree to fairly standard combinators on functions. But they generalize the combinators to operate on other types than the function type (-). As there isn't a relation between input and output, I don't quite see how the Stream type could start as a combinator. Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
On 14 April 2011 20:35, Chris Dew cms...@gmail.com wrote: Could you suggest how these constraints could be expressed in the Haskell type system? Hi Chris I'm afriad I'd have to decline - generally in Haskell implicit lifters are problematic, so it isn't something I'd be looking to solve. There was a thread on Haskell Cafe about them last November called Making monadic code more concise, that you might find interesting - especially Oleg Kiselyov's comments: http://www.haskell.org/pipermail/haskell-cafe/2010-November/086445.html Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
Thanks, that link's very relevant to what I'm trying. For the time being I'll accept a partial solution where the last two types are now the same, and try to improve it when my knowledge of Haskell improves. I really want (hello - bracket) in (hello - bracket - putStrLn) to have a type of String. Using the partial solution which Neil Brown proposed, the code will work, but (hello - bracket) will have a type of IO String which *seems* like it will be less efficient. All the best, Chris. On 14 April 2011 21:22, Stephen Tetley stephen.tet...@gmail.com wrote: On 14 April 2011 20:35, Chris Dew cms...@gmail.com wrote: Could you suggest how these constraints could be expressed in the Haskell type system? Hi Chris I'm afriad I'd have to decline - generally in Haskell implicit lifters are problematic, so it isn't something I'd be looking to solve. There was a thread on Haskell Cafe about them last November called Making monadic code more concise, that you might find interesting - especially Oleg Kiselyov's comments: http://www.haskell.org/pipermail/haskell-cafe/2010-November/086445.html Best wishes Stephen ___ 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
