Re: [Caml-list] Problem with universal functions in a module

[Ben Millwood <bmillwood@janestreet.com>]

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Parametric polymorphic values in OCaml's type system can be thought of as
"choose an assignment of the variables in this type, and you can use this
value with that monomorphic type". But there's no monomorphic type that
lets 'helper' do the right thing, so you actually need something more
expressive than ordinary polymorphic types.

In Haskell there's higher-rank polymorphism, which in this case would look
something like 'helper :: (forall a. T a -> b) -> Helper b', so that you
could only pass sufficiently-polymorphic arguments to helper. However, as
Jeremy said, there's a lot of added complexity involved.

On 7 January 2015 at 17:26, Jeremy Yallop <yallop@gmail.com> wrote:

> On 7 January 2015 at 13:50, Goswin von Brederlow <goswin-v-b@web.de>
> wrote:
> > Why are function arguments less general than their original function?
>
> Polymorphic arguments complicate type checking and make type inference
> impossible, so OCaml doesn't allow them.
>
> > B) is there some way around this I'm not seeing?
>
> I don't think that there's any way of passing polymorphic arguments
> that works out simpler than using a record.
>
> Of course, in your simple example code it's possible to eliminate the
> GADT altogether, which would also eliminate the need for polymorphic
> arguments.  Perhaps GADTs play a more essential role in your real
> code, though.
>
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