From: Gabriel Scherer <gabriel.scherer@gmail.com>
To: Philippe Wang <mail@philippewang.info>
Cc: Dan Bensen <danbensen@att.net>, caml-list@inria.fr
Subject: Re: [Caml-list] polymorphic parameters
Date: Thu, 6 Sep 2012 11:59:20 +0200 [thread overview]
Message-ID: <CAPFanBGFgQUfiG4kpvk27=qaFMGdn0hkLa0C90sULTk6j30AOg@mail.gmail.com> (raw)
In-Reply-To: <CAPFanBENEXKofW63nC6Amet6ZMeg=g4GaJ5Mzivcqy_E7WY0Sw@mail.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
>>
>> --
>> Caml-list mailing list. Subscription management and archives:
>> https://sympa-roc.inria.fr/wws/info/caml-list
>> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>> Bug reports: http://caml.inria.fr/bin/caml-bugs
>>
prev parent reply other threads:[~2012-09-06 10:00 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-06 9:20 Dan Bensen
2012-09-06 9:35 ` Philippe Wang
2012-09-06 9:49 ` David House
2012-09-06 13:19 ` Philippe Wang
2012-09-06 9:55 ` Gabriel Scherer
2012-09-06 9:59 ` Gabriel Scherer [this message]
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