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UNFINISHED: Years-long uncommitted changes These changes have been sitting here uncommitted for a almost a couple of years now. It's become clear that, while this project deeply interests me, I do not have time for it. Implementing a full-fledged Scheme is simply not something I have time to do. The original goal of this project is also something I'm still interested in, but will not have time to do any time soon. But it has been folded into broader, more grand ideas. I hope that this will, at the very least, be interested to someone else. It is terribly lacking in certain cases, e.g. it does not even implement pairs properly, because I never needed to. See also https://github.com/google/schism, which compiles to WASM. Happy hacking!
2020-02-22 23:59:21 -05:00
;;;; ECMAScript compiler 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/>.
;;;;
;;; Commentary:
;;; THIS IS BOOTSTRAP CODE INTENDED FOR USE ONLY IN REBIRTH.
;;;
;;;
;;; This defines the lexer, parser, and core of the compiler. Note that
;;; most of the actual compiling happens with the macros defined in
;;; `es.scm'.
;;;
;;; The core structure persists from Birth, along with much of the original
;;; code. Certain parts have been simplified or moved into other files,
;;; while other parts have been extended.
;;;
;;; Code:
;; Convert source input into a string of tokens.
;;
;; This is the lexer. Whitespace is ignored. The grammar consists of
;; simple s-expressions.
;;
;; Tokens are produced with `make-token'. The source SRC will be
;; left-truncated as input is processed. POS exists for producing metadata
;; for error reporting---it has no impact on parsing.
;;
;; This implementation was originally recursive and immutable, but the stack
;; was being exhausted, so it was refactored into an inferior
;; implementation. Note the use of `es:while' and `es:break'---these are
;; quick fixes to the problem of stack exhaustion in browsers (where we have
;; no control over the stack limit); proper tail call support will come
;; later when we have a decent architecture in place.
;;
;; The result is a list of tokens. See `token' for the format.
(define (lex src pos)
(define (es:match-regexp re s)
(es:match (es:regexp re) s))
(let ((toks (list)))
(es:while #t ; browser stack workaround
(let* ((ws (or (es:match-regexp "^\\s+"
src)
(list "")))
(ws-len (string-length (car ws)))
(trim (substring src ws-len)) ; ignore whitespace, if any
(newpos (+ pos ws-len))) ; adj pos to account for removed ws
(if (string=? "" trim)
(es:break) ; EOF and we're done
;; normally we'd use `string-ref' here, but then we'd have to
;; implement Scheme characters, so let's keep this simple and keep
;; with strings
(let* ((ch (substring trim 0 1))
(t (case ch
;; comments extend until the end of the line
((";") (let ((eol (es:match-regexp "^(.*?)(\\n|$)" trim)))
(make-token "comment" (cadr eol) trim newpos)))
;; left and right parenthesis are handled in the same
;; manner: they produce distinct tokens with
;; single-character lexemes
(("(") (make-token "open" ch trim newpos))
((")") (make-token "close" ch trim newpos))
;; strings are delimited by opening and closing ASCII
;; double quotes, which can be escaped with a
;; backslash
(("\"") (let ((str (es:match-regexp
"^\"(|(?:.|\\\n)*?[^\\\\])\""
trim)))
(or str (parse-error
src pos
"missing closing string delimiter"))
;; a string token consists of the entire
;; string including quotes as its lexeme,
;; but its value will be the value of the
;; string without quotes due to the `str'
;; match group (see `token')
(make-token "string" str trim newpos)))
(else
;; anything else is considered a symbol up until
;; whitespace or any of the aforementioned
;; delimiters
(let ((symbol (es:match-regexp "^[^\\s()\"]+"
trim)))
(make-token "symbol" symbol trim newpos))))))
;; yikes---see notes in docblock with regards to why
;; we're using mutators here
(set! toks (append toks (list (car t))))
(set! src (cadr t))
(set! pos (caddr t))))))
toks))
;; Throw an error with a window of surrounding source code.
;;
;; The "window" is simply ten characters to the left and right of the
;; first character of the source input SRC that resulted in the error.
;; It's a little more than useless.
(define (parse-error src pos msg)
(let ((window (substring src (- pos 10) (+ pos 10))))
(error (string-append msg " (pos " pos "): " window)
src)))
;; Produce a token, left-truncate src, and update pos.
;;
;; Unlike the JS Prebirth implementation which uses a key/value object,
;; we're just using a simple list.
;;
;; The expected arguments are: the token type TYPE, the match group or
;; string MATCH, left-truncated source code SRC, and the position POS
;; relative to the original source code.
(define (make-token type match src pos)
(let* ((parts (if (list? match) match (list match match)))
(lexeme (car parts))
;; the value is the first group of the match (indicating what we
;; are actually interested in), and the lexeme is the full match,
;; which might include, for example, string delimiters
(value (or (and (pair? (cdr parts))
(cadr parts))
lexeme))
(len (string-length lexeme)))
;; produce token and recurse on `lex', left-truncating the source
;; string to discard what we have already processed
(list (list (quote token) type lexeme value pos)
(substring src len)
(+ pos len))))
;; various accessor procedures for token lists (we're Birth Lisp here,
;; so no record support or anything fancy!)
(define (token? t) (and (pair? t) (symbol=? (quote token) (car t))))
(define (token-type t) (cadr t))
(define (token-lexeme t) (caddr t))
(define (token-value t) (cadddr t))
(define (token-pos t) (caddddr t))
;; Produce an AST from the given string SRC of sexps
;;
;; This is essentially the CST with whitespace removed. It first invokes
;; the lexer to produce a token string from the input sexps SRC. From this,
;; it verifies only proper nesting (that SRC does not close sexps too early
;; and that EOF isn't reached before all sexps are closed) and produces an
;; AST that is an isomorphism of the original sexps.
(define (parse-lisp src)
;; accessor methods to make you and me less confused
(define (ast-depth ast) (car ast))
(define (ast-tree ast) (cadr ast))
(define (ast-stack ast) (caddr ast))
;; perform a leftmost reduction on the token string
(define (toks->ast toks)
(fold
(lambda (token result)
(let ((depth (ast-depth result))
(xs (ast-tree result))
(stack (ast-stack result))
(type (token-type token))
(pos (token-pos token)))
;; there are very few token types to deal with (again, this is a
;; very simple bootstrap lisp)
(case type
;; ignore comments
(("comment") result)
;; when beginning a new expression, place the expression
;; currently being processed onto a stack, allocate a new list,
;; and we'll continue processing into that new list
(("open") (list (+ depth 1)
(list)
(cons xs stack)))
;; once we reach the end of the expression, pop the parent off of
;; the stack and append the new list to it
(("close") (if (zero? depth)
(parse-error src pos
"unexpected closing parenthesis")
(list (- depth 1)
(append (car stack) (list xs))
(cdr stack))))
;; strings and symbols (we cheat and just consider everything,
;; including numbers and such, to be symbols) are just copied
;; in place
(("string" "symbol") (list depth
(append xs (list token))
stack))
;; we should never encounter anything else unless there's a bug
;; in the tokenizer or we forget a token type above
(else (parse-error
src pos (string-append
"unexpected token `" type "'"))))))
(list 0 (list) (list)) ; initial 0 depth; empty tree; expr stack
toks))
;; lex the input SRC and pass it to `toks->ast' to generate the AST;
;; if the depth is non-zero after we're done, then we're unbalanced.
(let* ((toks (lex src 0))
(ast (toks->ast toks)))
(if (zero? (ast-depth ast))
(ast-tree ast)
;; if we terminate at a non-zero depth, that means there ar still
;; open sexps
(error (string-append
"unexpected end of input at depth "
(ast-depth ast))))))
;; Generate ECMAScript-friendly parameter name for the given token T.
;;
;; The generated name will not have any environment references and is
;; suitable only for the immediate scope.
(define (tparam->es t)
(tname->id (token-value t)))
;; Predicate determining whether NAME should be output verbatim as an
;; ECMAScript identifier.
;;
;; This only returns #t for numbers.
(define (tname-verbatim? name)
(es:match (es:regexp "^-?(\\d+(\\.\\d+)?|\\.\\d+)$") name))
;; Generate ECMAScript to reference the variable associated with the token T
;; in the current environment.
;;
;; The "current" environment is relative to whatever context into which the
;; caller places this generated code---that is, the environment is
;; resolved by the runtime environment.
;;
;; If macro support is not yet compiled in, then this returns the identifier
;; name _without_ the environment, just as Birth.
(define (env-ref t)
(let ((name (if (token? t)
(token-value t)
t)))
(if (tname-verbatim? name)
name
(cond-expand
(cdfn-macro
(string-append "_env." (tname->id name)))
(else
(tname->id name))))))
;; Generate ECMAScript-friendly name from the given id.
;;
;; A subset of special characters that are acceptable in Scheme are
;; converted in an identifiable manner; others are simply converted to `$'
;; in a catch-all and therefore could result in conflicts and cannot be
;; reliably distinguished from one-another. Remember: this is temporary
;; code.
(define (tname->id name)
(if (tname-verbatim? name)
name
(string-append
"$$" (es:replace (es:regexp "[^a-zA-Z0-9_]" "g")
(lambda (c)
(case c
(("-") "$_$")
(("?") "$7$")
(("@") "$a$")
(("!") "$b$")
((">") "$g$")
(("#") "$h$")
(("*") "$k$")
(("<") "$l$")
(("&") "$n$")
(("%") "$o$")
(("+") "$p$")
(("=") "$q$")
(("^") "$v$")
(("/") "$w$")
(("$") "$$")
(else "$")))
name))))
;; Join a list of strings XS on a delimiter DELIM
(define (join delim xs)
(if (pair? xs)
(fold (lambda (x str)
(string-append str delim x))
(car xs)
(cdr xs))
""))
;; Compile parameter list.
;;
;; This takes the value of the symbol and outputs it (formatted), delimited
;; by commas.
;;
;; Since we do not support actual pairs (yet), the "." syntax that normally
;; denotes the cdr is retained and presents itself here. The form "(arg1,
;; arg2 . rest)" creates a list `rest' containing all remaining arguments
;; after that point. Conveniently, ECMAScript Harmony supports this
;; natively with the "..." syntax.
(define (params->es params)
(define (%param-conv params)
(let* ((param (car params))
(name (token-value param))
(id (tname->id name))
(rest (cdr params)))
(if (string=? name ".")
(list (string-append
"..." (car (%param-conv rest))))
(if (pair? rest)
(cons id (%param-conv rest))
(list id)))))
(if (pair? params)
(join "," (%param-conv params))
""))
;; Compile body s-expressions into ECMAScript
;;
;; This produces a 1:1 mapping of body XS s-expressions to ES statements,
;; recursively. The heavy lifting is done by `sexp->es'.
(define (body->es xs ret)
;; recursively process body XS until we're out of pairs
(if (not (pair? xs))
""
(let* ((x (car xs))
(rest (cdr xs))
(more? (or (not ret) (pair? rest))))
;; the result is a semicolon-delimited string of statements, with
;; the final statement prefixed with `return' unless (not ret)
(string-append
" "
(if more? "" "return ") ; prefix with `return' if last body exp
(sexp->es x) ";" ; process current body expression
(if (pair? rest) "\n" "")
(body->es rest ret))))) ; recurse
;; Place parameters PARAMS into the current environment.
;;
;; This is ugly so that Rebirth can support multiple implementations at
;; once---those with environment support and those without.
(define (env-params params)
(join "\n"
(map (lambda (param)
(if (string=? (token-value param) ".")
"" ; next param is the cdr
(string-append (env-ref param) " = "
(tparam->es param) ";")))
params)))
;; Compile variable or procedure definition into ES
;;
;; This performs a crude check to determine whether a procedure definition
;; was supplied: if the cadr of the given token T is itself token, then it
;; is considered to be a variable.
(define (cdfn t)
(if (token? (cadr t))
(cdfn-var t) ;; (define foo ...)
(cdfn-proc t #f))) ;; (define (foo ...) ...)
;; Compile variable definition into ES
;;
;; This compiles the token T into a simple let-assignment.
(define (cdfn-var t)
(let* ((dfn (cadr t))
(id (tname->id (token-value dfn)))
(value (sexp->es (caddr t))))
(string-append "let " id "=" value ";_env." id " = " id)))
;; Compile procedure definition into an ES function definition
;;
;; This will fail if the given token is not a `define'.
;;
;; The output does something peculiar: it not only assigns to the active
;; scope, but also to the root of the environment, which has the effect of
;; making the procedure available to _everything_. The reason for this is a
;; kluge to make procedures available to macros during compilation without
;; having to wait for a rebirth repass. But this does have its issues and
;; it's important to understand that this is a temporary solution until
;; Ulambda has some level of static analysis.
;;
;; Care needs to be taken to make sure, regardless of scope, procedures of
;; the same name are not defined if used within macros, otherwise the latter
;; (again, regardless of scope) in the file will take precedence. This
;; behavior will not be observed by execution of compiled code, though,
;; because the scope will have the correct version. However, if a procedure
;; is _not_ in scope, then rather than being undefined, the one assigned to
;; root would be available.
;;
;; The generated ECMAScript is evaluated immediately to make it available to
;; macros during the compilation process.
(define (cdfn-proc t id-override)
;; e.g. (define (foo ...) body)
(let* ((dfn (cadr t))
(id (or id-override
(tname->id (token-value (car dfn)))))
(named? (not (string=? id "")))
(params (cdr dfn))
(fparams (params->es params))
(fenv (env-params params))
(body (body->es (cddr t) #t)))
;; this is the final format---each procedure becomes its own function
;; definition in ES
(let ((es (string-append
"function " id "(" fparams ")\n{"
"return (function(_env){\n" fenv "\n"
body
"\n})(Object.create(_env));}"
(if named?
(string-append ";_env." id " = " id
";_env.root." id " = " id ";")
""))))
;; Immediately evaluate to make available to macros during
;; compilation. See procedure notes above.
(cond-expand
(string->es
(if named? (string->es "eval($$es)"))))
es)))
;; Quote an expression
;;
;; If SEXP is a token, produce an ECMAScript Symbol. Otherwise,
;; recursively apply to each element in the list.
;;
;; TODO: This implementation isn't wholly correct---numbers, for example,
;; should not be converted to symbols, as they already are one.
(define (quote-sexp sexp)
(if (token? sexp)
(case (token-type sexp)
(("string") (sexp->es sexp))
(else
(string-append "Symbol.for('" (token-value sexp) "')")))
(string-append
"[" (join "," (map quote-sexp sexp)) "]")))
;; Quasiquote an expression
;;
;; A quasiquoted expression acts just like a quoted expression with one
;; notable exception---quoting can be escaped using special forms. For
;; example, each of these are equivalent:
;;
;; (quasiquote (a 1 2 (unquote (eq? 3 4))))
;; (list (quote a) 1 2 (eq? 3 4))
;; (quasiquote (a (unquote-splicing (list 1 2)) (unquote (eq? 3 4))))
;;
;; TODO/WARNING: Normally "(quasiquote a (unquote-splicing b))" would
;; produce "(a . b)" in a proper Lisp, but we do not yet support proper
;; pairs at the time that this procedure was written; all cdrs are assumed
;; to be lists. So do not do that---always splice lists.
(define (quasiquote-sexp sexp)
;; get type of token at car of pair, unless not a pair
(define (%sexp-maybe-type sexp)
(and (pair? sexp)
(token? (car sexp))
(token-value (car sexp))))
;; recursively process the sexp, handling various types of unquoting
(define (%quote-maybe sexp delim)
(if (pair? sexp)
(let* ((item (car sexp))
(rest (cdr sexp))
(type (%sexp-maybe-type item))
(add-delim (not (or (string=? type "unquote-splicing")
(string=? type "unquote@")))))
(string-append
(case type
;; escape quoting, nest within
(("unquote")
(string-append (if delim "," "")
(sexp->es (cadr item))))
;; escape quoting, splice list into parent expression
;; (lazy kluge warning), along with an alias for brevity
;; given that we lack the ",@" syntax right now
(("unquote-splicing" "unquote@")
(string-append
"]).concat(" (sexp->es (cadr item)) ").concat(["))
;; anything else, we're still quasiquoting recursively
(else (string-append (if delim "," "")
(quasiquote-sexp item))))
;; continue processing this list
(%quote-maybe rest add-delim)))
""))
;; tokens fall back to normal quoting
(if (token? sexp)
(quote-sexp sexp)
(string-append
"([" (%quote-maybe sexp #f) "])")))
;; Statically expand expressions based on implementation features
;;
;; Support for `cond-expand' allows Rebirth to introduce new features each
;; time that it is compiled. If matched, expressions will be evaluated as
;; if they were entered in place of the `cond-expand' itself; otherwise,
;; the entire `cond-expand' expression as a whole will be discarded.
;;
;; Birth will always discard `cond-expand' expressions unless they contain
;; an `else' clause, which permits us to compile on the first pass without
;; error.
(define (expand-cond-expand args)
(if (pair? args)
(let* ((clause (car args))
(feature (token-value (car clause)))
(body (cdr clause)))
;; now we get meta
(cond-expand
(string->es
(case feature
(("string->es" "else") (body->es body #f))
(else (if (es:defined? feature)
(body->es body #f)
(expand-cond-expand (cdr args))))))
;; if we're not yet compiled with Rebirth, then string->es will
;; not yet be available---but it _will_ be in Rebirth, so
;; compile cond-expand such that it marks it as supported
(else
(case feature
;; these two are always supported in Rebirth Lisp
(("string->es" "else") (body->es body #f))
;; keep recursing until we find something (this allows us to
;; short-circuit, most notably with "else")
(else
(expand-cond-expand (cdr args)))))))
""))
;; Determine whether the given name NAME represents a macro.
;;
;; If `string->es' is not yet supported, then this procedure always
;; yields `#f'. Otherwise, the compiler runtime `_env.macros' is consulted.
;;
;; See `cdfn-macro' for more information.
(define (macro? name)
(cond-expand
(string->es
(string->es "_env.macros[$$name] !== undefined"))
(else #f)))
;; Determine if FN is a procedure or macro and apply it accordingly with
;; arguments ARGS.
;;
;; These actions represent two separate environments: If a macro, then the
;; call needs to be executed immediately within the context of the compiler
;; runtime. Otherwise, procedure applications are simply compiled to be
;; produced with the rest of the compiler output and will be run at a later
;; time within the context of the compiled program.
(define (apply-proc-or-macro fn args)
(if (macro? fn)
(string->es "_env.macros[$$fn].apply(null,$$args)")
;; Procedures are produced as part of the compiler output.
(let ((argstr (join ", " (map sexp->es args))))
(string-append (env-ref fn) "(" argstr ")"))))
;; Primitive special forms.
;;
;; These are forms that cannot be re-written as macros because of
;; chicken-and-egg issues. Since the Rebirth compiler is temporary, we're
;; not going to worry about getting rid of the rest of these.
;;
;; String values are simple function aliases. Function values take over
;; the compilation of that function and allow for defining special forms
;; (in place of macro support). The first argument FN is the name of the
;; function/procedure/form, and ARGS is the list of arguments.
;;
;; For the forms previously defined here in Birth, see `fnmap-premacro' in
;; `es.scm'.
(define (fnmap fn args t)
(case fn
;; output raw code into the compiled ECMAScript (what could go wrong?)
(("string->es")
(token-value (car args)))
;; very primitive cond-expand
(("cond-expand") (expand-cond-expand args))
;; Note that the unquote forms are only valid within a quasiquote; see
;; that procedure for the handling of those forms. Since we do not
;; support the special prefix form, we also offer "`quote" as a
;; shorthand for quasiquote.
(("quote") (quote-sexp (car args)))
(("quasiquote" "`quote") (quasiquote-sexp (car args)))
(("define") (cdfn t))
(("define-macro") (cdfn-macro t)) ; not defined until string->es cond
;; Defining `include' is trivial right now since we're not doing any
;; sort of static analysis---we just need to start compilation of the
;; requested file and inline it right where we are. Note that this
;; doesn't enclose the file in `begin', so this isn't yet proper.
(("include") (rebirth->ecmascript
(parse-lisp
(es:file->string (token-value (car args))))))
;; If we have macro support (`cdfn-macro'), then assume that they exist
;; and try to use them; otherwise, continue to use built-in forms, which
;; have been moved into `fnmap-premacro').
(else
(cond-expand
(cdfn-macro (apply-proc-or-macro fn args))
(else (fnmap-premacro fn args t))))))
;; Convert s-expressions or scalar into ECMAScript
;;
;; T may be either an array of tokens or a primitive token (e.g. string,
;; symbol). This procedure is applied recursively to T as needed if T is
;; a list.
(define (sexp->es t)
(if (not (list? t))
(error "unexpected non-list for sexp->es token"))
(if (token? t)
(case (token-type t)
;; strings output as-is (note that we don't escape double quotes,
;; because the method of escaping them is the same in Scheme as it
;; is in ECMAScript---a backslash)
(("string") (string-append "\"" (token-value t) "\""))
;; symbols have the same concerns as procedure definitions: the
;; identifiers generated need to be ES-friendly
(("symbol") (env-ref t))
(else (error
(string-append
"cannot compile unknown token `" (token-type t) "'"))))
;; otherwise, process the expression
(fnmap (token-value (car t))
(cdr t)
t)))