[GHC] #1090: Bad monomorphism-restriction-related type error message

[GHC]

#1090: Bad monomorphism-restriction-related type error message
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    Reporter:  kirsten                  |       Owner:          
        Type:  bug                      |      Status:  new     
    Priority:  lowest                   |   Milestone:          
   Component:  Compiler (Type checker)  |     Version:  6.6     
    Severity:  trivial                  |    Keywords:          
  Difficulty:  Unknown                  |    Testcase:          
Architecture:  Multiple                 |          Os:  Multiple
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Consider the following program:
 {{{
 module Small2 where

 printFormatted :: IO ()
 printFormatted =
   let myPrint :: (a -> String) -> a -> IO ()
       myPrint f s = putStr (f s)
     --  myShow :: Show a => a -> IO ()
       myShow = myPrint show in
             myShow 1 >> myShow "foo"
 }}}
 If I compile this I get:
 {{{
 > ghc --make -no-recomp -c small2.hs

 [1 of 1] Compiling Small2           ( small2.hs, small2.o )

 small2.hs:9:19:
     No instance for (Num [Char])
       arising from the literal `1' at small2.hs:9:19
     Possible fix: add an instance declaration for (Num [Char])
     In the first argument of `myShow', namely `1'
     In the first argument of `(>>)', namely `myShow 1'
     In the expression:
         let
           myPrint :: (a -> String) -> a -> IO ()
           myPrint f s = putStr (f s)
           myShow = myPrint show
         in (myShow 1) >> (myShow "foo")
 }}}
 The fix, of course, is to either add -fno-monomorphism-restriction or to
 uncomment the type signature for {{{myShow}}}, but it took me a while to
 figure that out. It would be much better if the error message would
 suggest adding a type signature for {{{myShow}}}. As it is, the suggestion
 to add an instance for {{{Num [Char]}}} is none too helpful.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1090>
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