Re: [Haskell-cafe] Vanishing polymorphism

On Tue, 8 May 2007, David House wrote:

On 08/05/07, Matthew Sackman wrote:

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:t let f r s = (return negate) >>= (\(fn::forall n . (Num n) => n -> n) -> return (fn r, fn s)) in f

<interactive>:1:35: Couldn't match expected type `a -> a' against inferred type `forall n. (Num n) => n -> n' In the pattern: fn :: forall n. (Num n) => n -> n In a lambda abstraction: \ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s) In the second argument of `(>>=)', namely `(\ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s))'

I.e. why does the polymorphism get destroyed?

Here fn is bound by a lambda abstraction, and is therefore monomorphic. I can't find anything in the Report about that,

This won't be in the Haskell 98 report. I have to enable -fglasgow-exts in GHCi to get this even parsed. Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be