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Date: Thu, 12 Aug 1999 14:05:35 +0200
From: Damien Doligez <Damien.Doligez@inria.fr>
Message-Id: <199908121205.OAA22637@tobago.inria.fr>
To: caml-list@inria.fr, skaller@maxtal.com.au
Subject: Re:  Constructor/reference bug?
Sender: weis

>From: John Skaller <skaller@maxtal.com.au>

>let g x = 
>let lineno = ref (1,"") in
>let rec f x' = match x' with
>| NEWLINE p :: t -> lineno := p; f t
>| COLON :: t     -> CTRL !lineno :: f t
>| h :: t -> h :: f t
>| [] -> []
>in f x;;
>
>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:
>
>| COLON :: t -> let x = !lineno in CTRL x :: f t
>
>the code works as expected -- the CTRL refers to the
>value of lineno at the time the last NEWLINE was processed.
>
>What's happening? Are coproduct constructors lazy?


You have to be careful because f is a function with side effects.

In the line

    | COLON :: t     -> CTRL !lineno :: f t

the subexpression "!lineno" is not guaranteed to be evaluated before
"f t".  The current implementation happens to evaluate "f t" first.
Using "let x = !lineno in ..." is the right way to write your
function.

-- Damien