[Haskell-cafe] Why does Haskell not infer most general type?
Ross Paterson
ross at soi.city.ac.uk
Tue Apr 6 17:12:05 EDT 2010
On Tue, Apr 06, 2010 at 03:56:32PM -0400, Job Vranish wrote:
> f _ = undefined
> where
> _ = y :: Int -> Int
>
> y x = undefined
> where
> _ = f x
Because f and y are mutually recursive, their types are inferred together,
so y gets the type Int -> Int (as given), which forces f :: Int -> a.
If you add the type signature f :: a -> b, you break the cycle: that
type is used in inferring the type of y (namely a -> b), which is then
used in checking the typeof f. Ditto if you add y :: a -> b instead.
(This is not Haskell 98, but the implementations have done this for
years, and it will be in Haskell 2010.)
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