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Re: generic * and 0

From: Kevin Ryde
Subject: Re: generic * and 0
Date: Tue, 05 Dec 2006 11:00:28 +1100
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux)

"Mikael Djurfeldt" <address@hidden> writes:
> Good idea.

I made the change.

> Because of paragraph 6.2.2, a program cannot expect to get
> the result 0.0, and it seems like a strength of the implementation to
> provide the additional piece of information that the result is indeed
> *exactly* 0.

If you wanted an argument the other way, returning a flonum zero could
give negative zero for say "(* 0 -1.0)".  But I'd favour exactness,
since the result is certainly exactly zero.

> (Not entirely sure that the common zero is a good idea, but I tend
> to think so.)

I suppose it's a question of whether "*" should do that, or leave it
up to the application.

The only case I can think of where a common zero may not be good is
with matrices, where "(* 0 matrix) => matrix" could preserve the
dimensions of the input matrix in the output matrix.  Those dimensions
could be used later "(* matrix matrix)", to signal an error if the
dimensions were incompatible.

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