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RE: Undefined evaluation order
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Date: -- (:)
From: Jacques Garrigue <garrigue@k...>
Subject: RE: Undefined evaluation order: define it for constructors ?
Interesting that so many people would favour left-to-right order.

First a tiny addition: there seems to be a belief that evaluation in
ocaml is always right-to-left. This is true for ocaml2, but not always
for ocaml3, since (even in classic mode) optional arguments can be
passed in a different order. In such cases the compiler is free to
choose any evaluation order, in practice this is right-to-left, but
according to the type.

# let f ?(x=0) ?(y=0) () = x + y;;
val f : ?x:int -> ?y:int -> unit -> int = <fun>
# f ~y:(print_int 2; 2) ~x:(print_int 3; 3) ();;
23- : int = 5

As Xavier wrote, this would be pretty easy to automatically convert
the program as to provide strict left-to-right evaluation (according
to the text), but this is another example of potential tiny
innefficiency.
Moreover, strict left-to-right order also means that the function must
be evaluated first. For instance
    (f (f1 a1)) (f2 a2)
would lead to (flattening all computations in sub-expressions)
    let x1 = f1 a1 in let f0 = f x1 in let x2 = f2 a2 in f0 x2
rather than the expected
    let x1 = f1 a1 in let x2 = f2 a2 in f x1 x2
In bytecode the second form is more efficient, since it often avoids the
creation of a closure f0 (the binary application is detected at
runtime).


Another remark is that all the problems I see are due to
unspecified evaluation order in value construction, not in function
application. That is, tuples (Greg's example), records (Brian),
and lists (Pierre, old List.map), where the syntax really suggests
some sequencing.

On the other hand, function application is more symmetrical, and one
is less tempted by making assumptions about it.
The only caml program I've seen depending on evaluation order in
function application was by a friend of mine in Caml V3, about 10
years ago, and this was because he didn't know about ";" as a
sequencer :-)

Value construction and function application are distinguished
syntactically in Caml, so they might be considered independently.
I have a feeling that left-to-right order in value construction would
not be that expensive, even in the bytecode interpreter. How is it?

Regards,

Jacques
---------------------------------------------------------------------------
Jacques Garrigue      Kyoto University     garrigue at kurims.kyoto-u.ac.jp
		<A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>