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Re: Common Lisp solved this problem 20 years ago



Thomas Bushnell BSG wrote:
Per Bothner <per@xxxxxxxxxxx> writes:


Taylor Campbell wrote:

I'm a bit unclear on one part of your proposal: is the type
declaration syntax merely a suggestion to the compiler, or does it
actually affect the semantics of a program?

The latter, but perhaps not quite the way you're thinking.
(let ((V :: TYPE init)) ...)
has semantics like:
(let ((V (coerce-to-TYPE init))) ...)

I dislike this, because it's a hidden semantic in the type
declaration.

Huh?  What's hidden about it?  That's the *primary* semantics
of a non-hidden syntax.

Why not declare that (let ((V :: TYPE init)) ...) is an error if the
init doesn't return something of the proper type?

It does that, if the value isn't convertible.

But if you go along with the basic model that modular arithmetic is
selected based on the type of the operands, then we need a way
to convert a normal integer to a modular integer, and back again.
The coercion model provides that in a convenient way.
--
	--Per Bothner
per@xxxxxxxxxxx   http://per.bothner.com/