Meta-Clojure

Overview

Meta-Clojure provides staged compilation for Clojure. It includes a form of syntax quoting that is aware of both local environments and special-forms. Among other things, this makes many macros easier to write. Perhaps more importantly, it simplifies control over when code gets evaluated or compiled.

Usage

(require '[metaclj.core :refer [defmeta defbn syntax] :as meta])

Meta-Macros

The defmeta form is analogous to defmacro, but is expected to return Syntax objects (forms plus their environments) instead of plain forms.

(defmeta my-if [test then else]
  (syntax (if test then else)))

Note that you don't need to unquote any of the parameters to if, since the syntax form is aware of the meta-macro's environment.

Call-By-Name

Since it's common for macros to have a body that always templates code with a syntax-quoter, the convenience macro defbn provides a way to create "call-by-name" macros:

(defbn my-if [test then else]
  (if test then else))

Both versions of my-if have correct "lazy" behavior: they will only evaluate one arm of the conditional.

Staged Compilation

The meta/do macro will perform meta-quoting on zero or more forms, then evaluate each of them:

(meta/do 1 2 3)
;;=> 3

Combined with unquoting, this enables you to perform arbitrary computation at compile time:

(let [x 2 y 4]
  (meta/do ~(+ x y)))
;;=> 6

Unquoting is syntax aware and provides automatic splicing:

(let [args (syntax 2 4)]
  (meta/do ~(+ args)))
;;=> 6

You can use function expressions to defer computation. Note that the unquoted expression will still be evaluated at compile time:

(let [x 2 y 4]
  (meta/do (fn [] ~(+ x y))))
;=> #<Fn@32b2ad39 user/eval15784$fn__15788>

You can prove this to yourself by using meta/translate, which is a cousin of macroexpand-all:

(let [x 2 y 4]
  (meta/translate (fn [] ~(+ x y))))
=> ((fn* ([] 6)))

Note that the returned value is wrapped in a seq, since Meta-Clojure uniformly supports multiple expressions with implicit splicing:

(let [x (syntax 2 3)]
  (meta/translate 1 x 4))
;=> (1 2 3 4)

Status

• The comments at the bottom of core.clj and the code in core_test.clj form my testbed.
• Many known bugs and incomplete behavior.
• Some special forms not yet supported: case, deftype, and reify.
• No progress yet on Exotypes
• Use of clojure.core/eval is unavoidable at the top level, but it could be compiled away for more interior forms.
• Maybe someday I'll revive EClj and build its compiler on Meta-Clojure.

References

• Multi-stage Programming
• Terralang
• MetaOCaml
• EClj
• Exotypes

License

Copyright © 2016 Brandon Bloom

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.