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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.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Multiple
Architecture	Multiple