Re: [Caml-list] polymorphic parameters

[Gabriel Scherer <gabriel.scherer@gmail.com>]

PS: the use of the wording "syntactic sugar" here is inappropriate:
"forall 'a ..." is not valid OCaml syntax. I meant that the formal
type this notation is referring to is quantified in this way. For
concrete syntax to denote polymorphism, ('a . foo) is used by
polymorphic field and methods or polymorphic recursion, and the blog
post David House linked discusses another syntax -- and there is yet
something different, a fusion of these two, for GADTs; unfortunately
this aspect of OCaml syntax is getting a bit crowded.

On Thu, Sep 6, 2012 at 11:55 AM, Gabriel Scherer
<gabriel.scherer@gmail.com> wrote:
> Philippe, the function parameter is not really polymorphic in your
> example: it's the whole function type that is.
> `f` is given the type (('a -> 'b) -> 'a -> 'b) by the OCaml
> type-checker, but this is syntactic sugar for (forall 'a 'b. ('a ->
> 'b) -> 'a -> 'b).
>
> Note how the universal quantifiers are at the beginning of the type,
> and not inside the function parameter.
> One could expect to be able to express the type forall 'a . (forall 'b
> . 'a -> 'b) -> 'a -> int for example.
>
> See the following FAQ item for a short answer to this question:
>   http://caml.inria.fr/resources/doc/faq/core.en.html#polymorphic-arguments
>
> On Thu, Sep 6, 2012 at 11:35 AM, Philippe Wang <mail@philippewang.info> wrote:
>> On Thu, Sep 6, 2012 at 11:20 AM, Dan Bensen <danbensen@att.net> wrote:
>>> How do you indicate that a function parameter is a polymorphic function?
>>
>> I hope these 2 lines will answer your question:
>>
>> let f g x = g x
>>
>> let f (g: 'a -> 'b) x = g x
>>
>> --
>> Philippe Wang
>>    mail@philippewang.info
>>
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