[Haskell-cafe] Inferred type less polymorphic than expected

[Joel Reymont]

This gets me pretty close. I now have to figure out how to return a  
PU Props instead of a PU [Prop] which is what it's returning. Still,  
it's a huge step forward.

props :: Props -> PU Props
props m = props' $ sort $ M.toList m
     where props' [] = lift []
           props' ((_, (_, x)):xs) =
               wrap (\(a, b) -> a : b,
                     \(a : b) -> (a, b))
                        (pair (prop x) (props' xs))

./Script/Prop.hs:82:10:
     Couldn't match `Props' against `[Prop]'
       Expected type: PU Props
       Inferred type: PU [Prop]
     In the expression: props' $ (sort $ (Data.Map.toList m))
     In the definition of `props':
         props m = props' $ (sort $ (Data.Map.toList m))
                 where
                     props' [] = lift []
                     props' ((_, (_, x)) : xs)
                              = wrap (\ (a, b) -> a : b, \ (a : b) ->  
(a, b)) (pair (prop x) (props' xs))

Big thanks to Andrew and Malcolm on #haskell for pointing me in the  
right direction!

	Thanks, Joel

On Nov 8, 2005, at 5:23 PM, Andrew Pimlott wrote:

> You might have better luck getting if you send a complete example,  
> since
> it's hard for me to figure out exactly what this code is supposed  
> to do.
> However, I'll take a quick stab.
>
> On Tue, Nov 08, 2005 at 04:46:56PM +0000, Joel Reymont wrote:
>> props :: Props -> PU Props
>> props m = props' $ sort $ M.toList m
>>     where props' [] = lift []
>>           props' ((_, (Attr _ _ pp := _)):xs) =
>>               wrap (\(a, b) -> a : b,
>>                     \(a : b) -> (a, b))
>>                        (pair pp (props' xs))
>>
>>
>> ./Script/Prop.hs:80:10:
>>     Inferred type is less polymorphic than expected
>>       Quantified type variable `a' is mentioned in the environment:
>>         props' :: [(a1, Prop)] -> PU [a] (bound at ./Script/Prop.hs:
>> 79:10)
>
> Recall that pp is a (PU a) for some a.  However, the inferred type for
> props' is fully polymorphic in a.  It might help first of all to write
> down the type you expect for props', and then add type annotations to
> different pieces until you find the one that's going awry.
>
> Andrew

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