[Haskell-cafe] Why does Haskell not infer most general type?

[Thomas Schilling]

Yes, it has to do with mutually recursive bindings.  If you add a type
signature, you break the mutual recursion.  Mutually recursive
functions are type-checked together and then generalised.  Similarly,
polymorphic recursion cannot be inferred either, but is possible by
adding a type signature.

HTH

On 6 April 2010 20:56, Job Vranish <job.vranish at gmail.com> wrote:
>
> Is haskell supposed to always infer the most general type (barring
> extensions)?
>
> I found a simple case where this is not true:
>
> f _ = undefined
>   where
>     _ = y :: Int -> Int
>
> y x = undefined
>   where
>     _ = f x
>
> Haskell infers the types of 'y' and 'f' as:
> f :: Int -> a
> y :: Int -> Int
>
> This confused me at first, but after thinking about it a while it seemed to
> make sense. But then my friend John pointed out that you can add type sigs
> for 'f' and 'y':
> f :: a -> b
> y :: a -> b
> and have it still typecheck!
>
> This thoroughly confused me.
>
> Why does haskell not infer the most general type for these functions? Is it
> a limitation of the algorithm? a limitation of the recursive let binding?
>
> Any insight would be appreciated :)
>
> - Job
>
>
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