Alaric Snell-Pym scripsit:

> And the rule that made the macros second-class was that no macro values
> could remain in the input to the compiler once macro expansion was
> complete. In other words, all expressions that computed macros
> (generally in order to "apply" them, as they're not much use otherwise)
> had to be computable at compile time.
>
> So you could write "((if x a b) y z)" to choose which of two macros, a
> or b, to use; and as long as x could be reduced to a literal at compile
> time, that was JUST FINE. But if it couldn't, then the output of the
> expander would still include a and b (no longer just the symbols, of
> course, but the propagated actual macro values), and it'd be rejected by
> the compiler.

Very nice!

> This was all good fun, and easy enough to do in a pure functional
> dialect of Scheme (mutation really bungs up the kind of aggressive
> constant propagation and inlining I was doing), but would be hard to
> scale to a Scheme with mutation!

Well, you could limit mutation to run time only.  See Conall
Elliot's wonderful post "The C language is purely functional" at
<http://conal.net/blog/posts/the-c-language-is-purely-functional>.

--
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