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

[Heinrich Apfelmus]

Ketil Malde wrote:
> 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.

The problem is that f1 and f2 are polymorphic functions. To put 
polymorphic functions in a pair, you need *impredicative polymorphism*.


Best regards,
Heinrich Apfelmus

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