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From: Gabriel Scherer <gabriel.scherer@gmail.com>
Date: Fri, 18 May 2012 18:00:13 +0200
Message-ID: <CAPFanBFm-0aSWtVS8BX2AuFjEFuxEweWQVp9bp7f0pXTE+_P-A@mail.gmail.com>
To: David.Teller@ens-lyon.org
Cc: Yitzhak Mandelbaum <yitzhakm@cs.princeton.edu>, OCaml mailing list <caml-list@inria.fr>
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Subject: Re: [Caml-list] optional functions in modules

You can regain that polymorphism at a moderate efficiency cost by eta-expan=
ding:

   let mmap2 f =3D match mmap with | None -> failwith "Nope" | Some x -> x =
f;;
   val mmap2 : ('a -> 'b) -> 'a list -> 'b list =3D <fun>

On Fri, May 18, 2012 at 5:24 PM, David Rajchenbach-Teller
<David.Teller@ens-lyon.org> wrote:
> On 5/17/12 7:43 PM, Yitzhak Mandelbaum wrote:
>> Thanks to everyone who responded. =A0FWIW, I found the argument in favor=
 of options most compelling as a general approach.
>>
>> Yitzhak
>>
>
> Note that there is a drawback to that approach: loss of polymorphism.
>
> # let mmap =3D Some List.map;;
> val mmap : (('a -> 'b) -> 'a list -> 'b list) option =3D Some <fun>
>
> # let mmap2 =3D match mmap with | None -> failwith "Nope" | Some x -> x;;
> val mmap2 : ('_a -> '_b) -> '_a list -> '_b list =3D <fun>
>
> I suspect that first-class modules can help here, but I have not really
> had time to toy with them yet.
>
> Cheers,
> =A0David
>