macro system

[alandipert]

TBD, see Notes and Questions

• Macros are globally-defined functions for which there is an entry in the list gherkin.core/*macros*. Macros are passed their arguments, unevaluated, and whatever they return is evaluated.

(def unless
  (fn (pred conseq alt)
    (list 'if pred alt conseq)))

(set! *macros* (cons 'user/unless *macros*))

(unless t 1 2) ;=> 2

• Support quasi-quoting via the reader macro, which expands to thesyntax-quote` special form.
• Elements within a syntax-quoted expression may be selectively unquoted via the ~ reader macro, which expands to the gherkin.core/unquote sentinel.
  • ~ is not valid outside of syntax-quote

(def unless
  (fn (pred conseq alt)
    `(if ~pred ~alt ~conseq)))

• Support splicing via ~@, the which expands to the gherkin.core/unquote-splicing sentinel.
  • ~@ is not valid outside of syntax-quote

(def unless
  (fn (pred & xs)
    `(if ~pred ~@(reverse xs))))

Notes

• syntax-quote doesn't need to resolve anything (yet)
• We can support symbol-macrolet if fns themselves may appear in the *macros* list
  • lexically bind the fn to a name
  • set! *macros* with the fn object
  • evaluate symbol-macrolet body
  • remove the fn object from the *macros* list
• We already have gensym

Questions

• property lists on symbols?
  • then we can take the traditional approach of '(macro . t) metadata
  • need property lists on conses to do symbol-macrolet?
    • then, macro meta goes directly on the function object, not its current name
      • as it could be tricky to manage property lists across lexical bindings