Special versus general

• Sometimes you want to pick out constructs:

  • simplify (IfThenElse b x y) =
  • if b == constant "True" then x ...
  • simplify (Apply f [x]) =
  • if f == function "id" then x ...

• Sometimes you want generic tree traversal:

  • freeVars (Node sym subtrees scope) =
  • if null subtrees then set [sym] else
  • union (map freeVars subtrees)

• The best approach is to arrange for both styles to be as convenient as possible