[Haskell-cafe] Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?

[Chris Dew]

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 at 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 10000000 -- 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 10000000 -- Sleep for at least 10 seconds before exiting.