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

[Ting Lei]

Ketil, Thanks for the response. It seems that defining them as a pair only postphones the error.GHC will give an error when you extract the components of the pair, no matter whether you addthe "NoMonomorphismRestriction" flag or not. --{-# LANGUAGE NoMonomorphismRestriction #-}p :: (Show a, Ord b) => (a -> String, b -> b -> Bool)
p = (id . show, flip (<))f1 = fst p
f2 = snd p-----------------------Without NoMonomorphismRestriction, I got:
D:\work\test1.hs:6:10:
    Ambiguous type variable `a0' in the constraint:
      (Show a0) arising from a use of `p'
    Possible cause: the monomorphism restriction applied to the following:
      f1 :: a0 -> String (bound at D:\work\hsOcaml\test1.hs:6:1)
    Probable fix: give these definition(s) an explicit type signature
                  or use -XNoMonomorphismRestriction
    In the first argument of `fst', namely `p'
    In the expression: fst p
    In an equation for `f1': f1 = fst pD:\work\hsOcaml\test1.hs:6:10:
    Ambiguous type variable `b0' in the constraint:
      (Ord b0) arising from a use of `p'
    Probable fix: add a type signature that fixes these type variable(s)
...... Failed, modules loaded: none. ------------------With NoMonomorphismRestriction, I got: 
D:\work\test1.hs:6:10:
    Ambiguous type variable `b0' in the constraint:
      (Ord b0) arising from a use of `p'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `fst', namely `p'
    In the expression: fst p
    In an equation for `f1': f1 = fst pD:\work\test1.hs:7:10:
    Ambiguous type variable `a0' in the constraint:
      (Show a0) arising from a use of `p'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `snd', namely `p'
    In the expression: snd p
    In an equation for `f2': f2 = snd p
Failed, modules loaded: none.  Thanks,
 Ting > From: ketil at malde.org
> To: tinlyx at hotmail.com
> CC: haskell-cafe at haskell.org
> Subject: Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction
> Date: Thu, 29 Mar 2012 12:27:04 +0200
> 
> Ting Lei <tinlyx at hotmail.com> writes:
> 
> > (f1, f2) =
> >     let commond_definitions = undefined in
> >     let f1 = id.show 
> >         f2 x = (< x) 
> >     in
> >       (f1, f2)
> 
> I think the type signatures should be:
> 
>   f1 :: Show a => a -> String
> 
> and 
> 
>   f2 :: Ord b => b -> b -> Bool 
> 
> When I define these separately, this works:
> 
>   f1 :: Show a => a -> String
>   f1 = id . show
> 
>   f2 :: Ord b => b -> b -> Bool 
>   f2 = flip (<)
> 
> 
> But when I define them as a pair
> 
>   f1 :: Show a => a -> String
>   f2 :: Ord b => b -> b -> Bool 
>   (f1,f2) = (id . show, flip (<))
> 
> I get an error message:
> 
> Line 9: 1 error(s), 0 warning(s)
> 
> Couldn't match expected type `forall a. Show a => a -> String'
>             with actual type `a -> String'
> When checking that `f1'
>   has the specified type `forall a1. Show a1 => a1 -> String'
> 
> Defining the pair at once works:
> 
>   p :: (Show a, Ord b) => (a -> String, b -> b -> Bool)
>   p = (id . show, flip (<))
> 
> I guess that didn't help a lot, somebody with deeper GHC-fu than me will
> have to step in.
> 
> -k
> -- 
> If I haven't seen further, it is by standing in the footprints of giants
 		 	   		  
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