f :: (Show a, Ord b) => (a -> String, b -> b -> Bool) f = let commond_definitions = undefined in let f1 = id.show f2 x = (< x) in (f1, f2)

From: Ting Lei To:  Cc: Date: Wed, 28 Mar 2012 23:42:30 -0700 Subject: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction Hi

I have met a piece of code that cannot be compiled whether I add or remove the NoMonomorphismRestriction flag (as of GHC 7.0.4, Haskell platform 2011.4.0.0). I have extracted a minimal example below:

{-# LANGUAGE NoMonomorphismRestriction #-} (f1, f2) =     let commond_definitions = undefined in     let f1 = id.show         f2 x = (< x)     in       (f1, f2)

I needed this format because there are many shared definitions in common_definitions for f1 and f2, and I want to keep them local.

If I compile them with NoMonomorphismRestriction, I get:

D:\work\test.hs:7:8:     Ambiguous type variable `a0' in the constraint:       (Show a0) arising from a use of `f1'     Possible cause: the monomorphism restriction applied to the following:       f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2)     Probable fix: give these definition(s) an explicit type signature     In the expression: f1     In the expression: (f1, f2)     In the expression:       let         f1 = id . show         f2 x = (< x)       in (f1, f2) D:\work\test.hs:7:12:     Ambiguous type variable `a1' in the constraint:       (Ord a1) arising from a use of `f2'     Possible cause: the monomorphism restriction applied to the following:       f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6)     Probable fix: give these definition(s) an explicit type signature     In the expression: f2     In the expression: (f1, f2)     In the expression:       let         f1 = id . show         f2 x = (< x)       in (f1, f2) Failed, modules loaded: none.

If I comment out -- {-# LANGUAGE NoMonomorphismRestriction #-} I get:

D:\work\hsOcaml\test.hs:4:17:     Ambiguous type variable `a0' in the constraint:       (Show a0) arising from a use of `show'     Possible cause: the monomorphism restriction applied to the following:       f1 :: a0 -> String (bound at D:\work\hsOcaml\test.hs:2:2)     Probable fix: give these definition(s) an explicit type signature                   or use -XNoMonomorphismRestriction     In the second argument of `(.)', namely `show'     In the expression: id . show     In an equation for `f1': f1 = id . show D:\work\hsOcaml\test.hs:7:12:     Ambiguous type variable `a1' in the constraint:       (Ord a1) arising from a use of `f2'     Possible cause: the monomorphism restriction applied to the following:       f2 :: a1 -> a1 -> Bool (bound at D:\work\hsOcaml\test.hs:2:6)     Probable fix: give these definition(s) an explicit type signature                   or use -XNoMonomorphismRestriction     In the expression: f2     In the expression: (f1, f2)     In the expression:       let         f1 = id . show         f2 x = (< x)       in (f1, f2) Failed, modules loaded: none.

Can anyone show me why this does not work and how to fix it (e.g. by adding type signature as the error message suggested)? I tried to add type signature by couldn't figure out the right way of doing it.

Thanks in advance!

Ting

Denis Moskvin