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A comment on Advanced Programming Languages (2009). Parent comment

Re:Advanced Programming Languages (2009) by thesz on Friday May 08, 2020 @09:15AM (#23114487)
Macros have local scope. They are evaluated to program text only once, probably, lazily.

Rewrite rules fire when compiler encounters a chance for them to fire, at any point of (optimizing) program transformation.

Due to this fact, rewrite rules are not local - compiler can transform the inner structure of a program changing the locality of program statements.

    bs = map g xs
    ... many other lines of program text not reassigning bs
    as = map f bs
still can be transformed into "as = map (f . g) xs" by rule "map f (map g xs) => map (f . g) xs" due to substitution.
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Re:Advanced Programming Languages (2009) by BerislavLopac on Friday May 08, 2020 @01:14PM (#23117069)
So they -- like macros -- rewrite the source code into something different before the compilation, but -- unlike macros -- only do that under certain circumstances. Would this be a sensible approximation? I'm just trying to understand what they're doing, not why or how. Thank you!
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