From: Jean-Christophe Filliatre <filliatr@lri.fr>
Date: Thu, 12 Aug 1999 13:22:59 +0200 (MEST)
To: John Skaller <skaller@maxtal.com.au>
Subject: Re: Constructor/reference bug?
In-Reply-To: <3.0.6.32.19990811180530.00985e40@mail.triode.net.au>
> | COLON :: t -> CTRL !lineno :: f t
>
> The code doesn't work as I expected: every
> CTRL value refers to the same lineno, the last one.
> If I change the COLON line to read:
That is only because the second argument is evaluated before the first
one in a cons (::). Indeed, you can try
======================================================================
# print_int 1 :: print_int 2 :: [];;
======================================================================
and you will get
======================================================================
21- : unit list = [(); ()]
======================================================================
It explains your problem. As you can read it in the ocaml reference
manual, the order of evaluation of tuples elements in the not
specified :
======================================================================
Products
The expression expr1 ,..., exprn evaluates to the n-tuple of the values of expressions expr1 to exprn. The
evaluation order for the subexpressions is not specified.
Variants
The expression ncconstr expr evaluates to the variant value whose constructor is ncconstr, and whose argument
is the value of expr.
======================================================================
so your second code with a "let in" should be used.
> | COLON :: t -> let x = !lineno in CTRL x :: f t
Best regards,
-- Jean-Christophe FILLIATRE mailto:Jean-Christophe.Filliatre@lri.fr http://www.lri.fr/~filliatr
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