Re: [Caml-list] Odd Type Checking Problem

[Alain Frisch <frisch@clipper.ens.fr>]

On Wed, 6 Feb 2002, Jonathan D Eddy wrote:

> (* type checks *)
> let mAny = fun succ0 input -> succ0 in
>     let ans0 = true in
>     let x = mAny (mAny ans0) in
>         x 1 2
>
> (* does not type check *)
> let mAny: 'a -> int -> 'a = fun succ0 input -> succ0 in
>     let ans0 = true in
>     let x = mAny (mAny ans0) in
>         x 1 2

I guess this is a problem of understanding type variable scoping rules.
The scope of the 'a variable above is all the phrase, including
the (mAny (mAny ans0)). So the type annotation makes mAny monomorphic,
but you want to use it with two different types.

It seems that explicitly introduced type variables are generalized only
at the (syntactic) level above their introduction; this together with
unclear scoping rules may be confusing ...


-- Alain

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