[mercury-users] Re:[mercury-users] difference between is and =?

[Fergus Henderson]

On 27-Apr-2001, Terrence Brannon <princepawn at earthlink.net> wrote:
> in the last clause of this function, is is used instead of =. Why is
> this?  In fact, what is is?

It's for "hysterical raisins": originally this code was written so that it
would work with a Prolog compiler.  In Prolog, you need to use "is"
rather than "=" for arithmetic expression evaluation.
Originally the Mercury compiler was written in the intersection of
Prolog and Mercury, so that we could use a Prolog compiler to bootstrap it.
So the Mercury compiler allowed the use of "is" in Mercury,
treating it just as a synonym for "=".

Nowadays we don't use a Prolog compiler for bootstrapping.
But the Mercury compiler still allows this use of "is".

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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