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,

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.