[Haskell-cafe] Type checking of partial programs

[Henning Thielemann]

On Thu, 20 Mar 2008, Roberto Zunino wrote:

> ac wrote:
>> foo :: [Foo] -> <placeholder 1>
>> foo xs = map <placeholder 2> xs
>>
>> What are the possible type signatures for placeholder 1 and the possible
>> expressions for placeholder 2?
>
> A nice GHCi trick I learned from #haskell:
>
>> :t let foo xs = map ?placeholder2 xs in foo
>
>  forall a b. (?placeholder2::a -> b) => [a] -> [b]
>
> Also, the djinn tool might provide some actual expression for placeholder 2.

http://www.haskell.org/haskellwiki/Determining_the_type_of_an_expression