[Haskell-cafe] MPTCs and rigid variables

David House dmhouse at gmail.com
Tue Mar 6 09:34:32 EST 2007

On 06/03/07, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
> 4.  The "rigid type variable" thing isn't just an implementation question.  What *would* you like the error message to say when you write
>        f :: ([a], Int) -> Int
>        f (x:xs, y) = x+y
> Here we unify 'a' with Int, which is wrong.  What would a nicer error message say?

"Inferred type was monomorphic but a polymorphic type was given", or
something. Hugs says "Inferred type not as polymorphic as expected",
which is the right kind of area.

-David House, dmhouse at gmail.com

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