[Haskell-cafe] learning advanced haskell

Ivan Lazar Miljenovic ivan.miljenovic at gmail.com
Mon Jun 14 09:56:54 EDT 2010

Martijn van Steenbergen <martijn at van.steenbergen.nl> writes:
> 1) What's a type of this function? I say *a* type because there are
> multiple correct answers.
>> debugWith f = do
>>   putStrLn (f True)
>>   putStrLn (f 'c')
> Don't ask the compiler to infer the type for you; it won't be able
> to. One of the characteristics of RankNTypes is that the compiler
> needs you, the programmer, to supply the type.

I randomly guess something like "(forall a. a -> String) -> IO ()" (but
haven't done anything myself with existentials so I'm quite possibly

> 2) What would be a good argument to debugWith?

Does "show" suffice?

> 3) What's one reason the compiler can't infer the type for you?

foralls are tricksy little devils? :p

Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com

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