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[Caml-list] OCaml not automatically specialising a polymorphic function
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Date: -- (:)
From: Xavier Leroy <xavier.leroy@i...>
Subject: Re: [Caml-list] OCaml not automatically specialising a polymorphic function
> - max3.ml ------------------------------------------------------------
> let max a b =
>   if a > b then a else b
> in
> 
> print_int (max 2 3);;
> ----------------------------------------------------------------------
> 
> Looking at the assembler, OCaml doesn't work out that "max" has type
> max : int -> int -> int

Yes, because it has type 'a -> 'a -> 'a according to the ML typing rules...
But if you help OCaml with a type constraint, you'll get the more
efficient code that you expect:

  let max (a:int) (b:int) = ...

> , and so it generates very inefficient
> code. This is a bit surprising because (I think) max can't be called
> from outside the module, and the one place where it is called
> specifies the type.
> So I'm guessing here that OCaml doesn't really optimize across
> functions?

You're correct that OCaml doesn't do type specialization for
user-defined functions (only for some predefined operations like ">"). 
It does perform some inter-function optimizations such as function
inlining and known call optimizations.

- Xavier Leroy

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