245 lines
12 KiB
Scheme
245 lines
12 KiB
Scheme
;;; Macro support for Rebirth Lisp
|
|
;;;
|
|
;;; Copyright (C) 2017, 2018 Mike Gerwitz
|
|
;;;
|
|
;;; This file is part of Ulambda Scheme.
|
|
;;;
|
|
;;; Ulambda Scheme is free software: you can redistribute it and/or modify
|
|
;;; it under the terms of the GNU Affero General Public License as
|
|
;;; published by the Free Software Foundation, either version 3 of the
|
|
;;; License, or (at your option) any later version.
|
|
;;;
|
|
;;; This program is distributed in the hope that it will be useful,
|
|
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
;;; GNU General Public License for more details.
|
|
;;;
|
|
;;; You should have received a copy of the GNU Affero General Public License
|
|
;;; along with this program. If not, see <http://www.gnu.org/licenses/>.
|
|
;;;
|
|
;;; THIS IS BOOTSTRAP CODE INTENDED FOR USE ONLY IN REBIRTH.
|
|
;;;
|
|
;;;
|
|
;;; === STEP 1 ===
|
|
;;;
|
|
;;; Did you read Step 0 first? If not, start there; see `rebirth.scm'.
|
|
;;;
|
|
;;; Without macro support, anything that involves producing code with
|
|
;;; variable structure at compile-time must be hard-coded in the
|
|
;;; compiler. Perhaps the greatest power in Lisp is the ability to extend
|
|
;;; the language through its own facilities---its ability to parse itself
|
|
;;; and treat itself as data.
|
|
;;;
|
|
;;; So we need to introduce macro support.
|
|
;;;
|
|
;;; This is not a trivial task: RⁿRS has a rich and powerful system that
|
|
;;; would be quite a bit of work upfront to implement. Instead, we're
|
|
;;; going to focus on traditional Lisp macros, which are conceptually
|
|
;;; rather simple---they produce a list that, when expanded, is treated as
|
|
;;; Lisp code as if the user had typed it herself.
|
|
;;;
|
|
;;; Macros hold the full power of Lisp---macro expansion _is_
|
|
;;; compilation. This means that we need to compile macro expansions as
|
|
;;; their own separate programs during the normal compilation process and
|
|
;;; splice in the result. But to execute the macro, we need to execute
|
|
;;; ECMAScript code that we just generated. In other words: the evil eval.
|
|
;;;
|
|
;;; ECMAScript has two ways of evaluating ES code contained in a string:
|
|
;;; through the `eval' function and by instantiating `Function' with a
|
|
;;; string argument representing the body of the function (or something
|
|
;;; that can be cast into a string). Good thing, otherwise we'd find
|
|
;;; ourselves painfully writing a Lisp interpreter in Rebirth Lisp.
|
|
;;;
|
|
;;; This implementation is very simple---there's very little code but a
|
|
;;; great deal of comments. They describe important caveats and hopefully
|
|
;;; enlighten the curious reader.
|
|
|
|
(cond-expand
|
|
(string->es
|
|
(define (cdfn-macro sexp)
|
|
(define (%make-macro-proc sexp)
|
|
;; The syntax for a macro definition is the same as a procedure
|
|
;; definition. In fact, that's exactly what we want, since a macro is
|
|
;; a procedure that, when applied, produces a list. But we want an
|
|
;; anonymous function, so override the id to the empty string.
|
|
(let* ((proc-es (cdfn-proc sexp "")))
|
|
;; Rather than outputting the generated ES function, we're going to
|
|
;; immediately evaluate it. This is a trivial task, but how we do
|
|
;; it is important: we need to maintain lexical scoping. This
|
|
;; means that we must use `eval'---`new Function' does not create a
|
|
;; closure.
|
|
;;
|
|
;; The only thing we need to do to ensure that eval returns a
|
|
;; function is to enclose the function definition in
|
|
;; parenthesis. This results in something along the lines of:
|
|
;; eval("(function(args){...})")
|
|
;;
|
|
;; If you're confused by the execution environment (compiler
|
|
;; runtime vs. compiler output), don't worry, you're not
|
|
;; alone. We're actually dealing with a number of things here:
|
|
;;
|
|
;; 1. Use `string->es' below to produce _compiler output_ for the
|
|
;; next version of a Rebirth Lisp compiler that will be
|
|
;; responsible for actually running the `eval'.
|
|
;; 2. That next version of the compiler will then compile
|
|
;; ECMAScript function definition from macro procedure source
|
|
;; using `cdfn-proc' as above.
|
|
;; 3. This will then be run by the compiler _at runtime_ by
|
|
;; running the `eval' statement below (which is part of the
|
|
;; program just as if it were Lisp).
|
|
;; 4. The result will be the procedure `proc-es' available to the
|
|
;; compiler at runtime rather than produced as compiler output.
|
|
;;
|
|
;; There's a lot of words here for so little code! We currently
|
|
;; lack the language features necessary to produce the types of
|
|
;; abstractions that would make this dissertation unnecessary.
|
|
(string->es "eval('(' + $$proc$_$es + ')')")))
|
|
|
|
;; We then store the macro by name in memory in `_env.macros'. When
|
|
;; invoked, it will apply the result of the above generated procedure
|
|
;; to `macro-compile-result' (defined below), which will produce the
|
|
;; ECMAScript code resulting from the macro application.
|
|
;;
|
|
;; There are consequences to this naive implementation. Rebirth is a
|
|
;; dumb transpiler that relies on features of ECMAScript to do its
|
|
;; job. In particular, we don't have any dependency graph or lexical
|
|
;; scoping or any of those necessary features---we let ECMAScript take
|
|
;; care of all of that. That means that we have no idea what is
|
|
;; defined or even what has been compiled; we just transpile and move
|
|
;; on blindly. Any errors resulting from undefined procedures, for
|
|
;; example, occur at runtime in the compiled output.
|
|
;;
|
|
;; These are features that will be implemented in Ulambda Scheme;
|
|
;; that's not something to distract ourselves with now.
|
|
;;
|
|
;; So there are some corollaries:
|
|
;;
|
|
;; 1. Macros must be defined _before_ they are called. Order
|
|
;; matters.
|
|
;; 2. Macros can only make use of what is defined in the compiler
|
|
;; runtime environment---if a procedure is defined, it won't be
|
|
;; available to macros until the next compilation pass. This is
|
|
;; because we have no dependency graph and cannot automatically
|
|
;; eval dependencies so that they are available in the execution
|
|
;; context.
|
|
;; - To work around that, procedures can be defined within the
|
|
;; macro body. Of course, then they're encapsulated within it,
|
|
;; which is not always desirable.
|
|
;;
|
|
;; While this implementation is crippled, it does still provide good
|
|
;; foundation with which we can move forward. Our use of recursive
|
|
;; Reⁿbirth passes and `cond-expand' makes this less of an issue as
|
|
;; well, since we're recursing anyway.
|
|
(let ((macro-proc (%make-macro-proc sexp))
|
|
(macro-id (token-value (caadr sexp)))) ; XXX
|
|
(string->es
|
|
"_env.macros[$$macro$_$id] = function(){
|
|
return $$macro$_$compile$_$result(
|
|
$$macro$_$proc.apply(this,arguments))};")
|
|
;; Because the macro procedure was evaluated at runtime, it would
|
|
;; never actually itself be output. This makes debugging difficult,
|
|
;; so we'll output it as a comment. This is admittedly a little bit
|
|
;; dangerous, as we're assuming that no block comments will ever
|
|
;; appear in `macro-proc'. But at the time of writing, this
|
|
;; assumption is perfectly valid.
|
|
(string-append "/*macro " macro-id ": " macro-proc "*/")))
|
|
|
|
|
|
;; Compile the S-expression resulting from the macro application into
|
|
;; ECMAScript.
|
|
;;
|
|
;; This simply converts the given S-expression SEXP into an AST and
|
|
;; compiles it using the same procedures that we've been using for all
|
|
;; other code. See below for details.
|
|
(define (macro-compile-result sexp)
|
|
(sexp->es (list->ast sexp)))
|
|
|
|
|
|
;; Produce a Rebirth List AST from an internal list form.
|
|
;;
|
|
;; Up until this point, the only way to represent Rebirth Lisp was using
|
|
;; a typical Lisp form. With macros, however, we have bypassed that
|
|
;; source form---we're working with our own internal representation of a
|
|
;; list.
|
|
;;
|
|
;; The structure of the AST is already done---it mirrors that of the list
|
|
;; itself. What we need to do is map over the list, recursively, and
|
|
;; convert each item into a token.
|
|
;;
|
|
;; Consider the tokens processed by `toks->ast': comments,
|
|
;; opening/closing delimiters, strings, and symbols. We don't need to
|
|
;; worry about comments since we aren't dealing with source code. We
|
|
;; also don't need to worry about opening/closing delimiters since we
|
|
;; already have our list. This leaves only two token types to worry
|
|
;; about: strings and symbols.
|
|
;;
|
|
;; And then there's the fascinating case of macro arguments. When a
|
|
;; macro or procedure application are encountered during compilation, the
|
|
;; arguments are represented as tokens (see `apply-proc-or-macro'). As
|
|
;; just mentioned, the end goal is to convert our list SEXP into tokens
|
|
;; for the AST. But the arguments are _already_ tokens, so they need no
|
|
;; additional processing---we just splice them in as-is! This trivial
|
|
;; operation yields the powerful Lisp macro ability we're looking for:
|
|
;; the ability to pass around chunks of the AST.
|
|
;;
|
|
;; Consequently, we have Rebirth-specific syntax to deal with when
|
|
;; processing the AST within macros. Up until this point, in place of
|
|
;; macros, we have used `fnmap', which operates on tokens. That is the
|
|
;; case here as well: if a macro wishes to assert on or manipulate any
|
|
;; syntax it is given, it must use the Rebirth token API that the rest of
|
|
;; the system uses. For example, say we have a macro `foo' that asserts
|
|
;; on its first argument as a string:
|
|
;;
|
|
;; (foo "moo") => "cow"
|
|
;; (foo "bar") => "baz"
|
|
;;
|
|
;; This will _not_ work:
|
|
;;
|
|
;; (define-macro (foo x)
|
|
;; (if (string=? x "moo") "cow" "baz"))
|
|
;;
|
|
;; The reason is that `x' is not a string---it is a `token?'. Instead,
|
|
;; we must do this:
|
|
;;
|
|
;; (define-macro (foo x)
|
|
;; (if (string=? (token-value x) "moo") "cow" "baz"))
|
|
;;
|
|
;; Of course, if you do not need to make that determination at
|
|
;; compile-time, you can defer it to runtime instead and use `string=?':
|
|
;;
|
|
;; (define-macro (foo x)
|
|
;; (quasiquote (if (string=? (unquote x) "moo") "cow" "baz")))
|
|
;;
|
|
;; Simple implementation, complex consequences. Scheme uses syntax
|
|
;; objects; we'll provide that abstraction over our implementation at
|
|
;; some point.
|
|
;;
|
|
;; Okay! That's trivial enough, isn't it?
|
|
(define (list->ast sexp)
|
|
;; Anything that is not a string is considered to be a symbol
|
|
;; token. But note that a symbol token does not necessarily mean an
|
|
;; ECMAScript Symbol object.
|
|
(define (%list-item item)
|
|
(case (es:typeof item)
|
|
(("string")
|
|
(list "string" item))
|
|
(("symbol")
|
|
(list "symbol" (string->es "Symbol.keyFor($$item)")))
|
|
(else
|
|
(list "symbol" (string->es "''+$$item")))))
|
|
|
|
;; Recursively create tokens for each item. Note that we will not have
|
|
;; any useful source code or source location information---just use the
|
|
;; empty string and 0 for them, respectively.
|
|
;;
|
|
;; The lexeme will simply be the item converted into a string, whatever
|
|
;; that happens to be.
|
|
(if (token? sexp)
|
|
sexp
|
|
(if (list? sexp)
|
|
(map list->ast sexp)
|
|
(let* ((item-parts (%list-item sexp))
|
|
(type (car item-parts))
|
|
(lexeme (cadr item-parts)))
|
|
(car (make-token type lexeme "" 0))))))))
|