Re: (declare (pure ...))

Peter Bex <[email protected]> Sat, 10 Feb 2024 13:28:26 +0100

Newsgroups	gmane.lisp.scheme.chicken
Message-ID	<Zcdr6nIwP3aZl1kF@doggett>

On Sat, Feb 10, 2024 at 11:12:04AM +0000, siiky via wrote:
> Hi Al,
> 
> > On a practical level, I would be sad if vector-ref, for example, was
> > "impure", and thus compiling a vector-ref invalidated all previously-
> > checked globals for the current scope. Likewise, I would prefer to
> > declare a procedure using vector-ref as pure, to let csc know that it
> > does not modify globals (or the filesystem etc).
> 
> The function vector-ref doesn't stop being pure (i.e. referentially
> transparent) depending how you use it. The function is always referentially
> transparent -- the expression (vector-ref some-global-vec) isn't. Quoting
> from "Functional Programming in C++" by Ivan Čukić:

CHICKEN's type system differentiates between "pure" and "clean".  A "pure"
function has no side effects and will always return the same value given
the same input (excluding the environment).

A "clean" function does not modify any state, but may return different
values at different times depending on the value.

That's why vector-ref is marked as "clean" and not "pure" in types.db.
A procedure like "not" and all type predicates are marked as "pure".

Hope this clears things up a bit!

Cheers,
Peter

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