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(int * int) <> int*int ?
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Date: 2006-02-23 (22:26)
From: Frédéric_Gava <gava@u...>
Subject: Re: [Caml-list] (int * int) <> int*int ?
>No.  There are cases where (int*int*int) is not the same as int*int*int.
>Specifically, int*(int*int*int) is different than int*int*int*int.

As a sub-part of a type, ok but I speak about general parentheses...

>You keep trying to assume that the parentheses are purely syntactic-
>they're not.

I have never say that...you can read that i have write int->int->int <>

>So, in terms of representation:
>type t = A of int * int
>is represented in memory as
>     tag * int * int
>type t' = B of (int * int)
>is represented in memory as
>     tag * (int * int)
>where in both cases tag is the tagging information added by the compiler.
>Note that in the first case it's a three word structure (two ints and a
>tag), while in the second case it's a two word structure (a tag and a
>reference to a tuple of two ints).

But where tag could represent information both int*int and (int*int) which
is the same things (modulo an indirection which could be deleted at
pattern-mathching). Morever, as Jon points of, variant types do not have
this curious things due to boxed values...Also when you have build a pair,
semantically you want to apply the constructor on this pair...because,
there no partial applications of the constructors : for a constructor A of
int*int you can not write (A 3) of type int->t (as in mini-ML or Haskell). I
understand (and agree) the performances reasons, but I thinks that could be
deleted since the only (that i see) case where the performances are
differents is in the case of pattern-matching...

>No.  Because in the case of B, I can snag the tuple as an independent
>peice of data.  For example, I can write:
>  let f = function B x -> fst x;;
>and it works.  On the other hand, if I try to write:
>  let g = function A x -> fst x;;
>this doesn't work- because the "tuple" in A doesn't really exist as an
>independent data structure (it's been unboxed).  Here, Ocaml is refusing
>to allow a pointer into the middle of a structure (the GC algorithm thanks
>you), and doesn't want to have to allocate a tuple to hold x (this
>seriously complicates pattern matching).

Peraps it complicates the pattern-matching but in the second case, x could
be quickly and automatically build from (A x) because tags of tuples and of
concrete types are close...and you avoid this curious construction  (many
years that I program in ocaml and curiously, never saw this things because
always build the different parameters of the concrete constructor in
different "variables" ;-)  so I think that it is not a problem that appear
many times in ocaml programs...some one always used the two differents
constructions ?)