Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction

30 Mar 2012


      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