ulambda/build-aux/bootstrap/rebirth.scm

;;; Rebirth Lisp implemented in Birth Lisp (self-hosting)
;;;
;;;  Copyright (C) 2017 Mike Gerwitz
;;;
;;;  This file is part of Gibble.
;;;
;;;  Gibble 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 TEMPORARY CODE that will be REWRITTEN IN GIBBLE LISP ITSELF after
;;; a very basic bootstrap is complete.  It is retained as an important
;;; artifact for those who wish to build Gibble from scratch without using
;;; another version of Gibble itself.  This is called "self-hosting".
;;;
;;; This is the compiler for Rebirth Lisp---it builds off of Birth by
;;; first eliminating the need for libprebirth; this allows _all_
;;; development to happen in a Lisp dialect, which liberates the last
;;; remaining process that isn't technically self-hosted.  So, Rebirth
;;; completes the raw, self-hosting bootstrapping process.
;;;
;;; To continue with the creepy birthing puns, you can consider libprebirth
;;  to be the umbilical cord.  After Birth, it's still attached---here we
;;; cut it.
;;;
;;; Of course, bootstrapping can't end there: we need a fully functioning
;;; Scheme compiler.  Rebirth may as well be called Rerebirth, or
;;; Rererebirth, or Re*birth, or Reⁿbirth---it is a recursively self-hosting
;;; compiler.  It adds features to itself each time it compiles itself.
;;;
;;; Note that we're dealing with a small subset of Scheme here, so certain
;;; things might be done differently given a proper implementation.
;;;
;;; This is an exact copy of `birth.scm', modified to introduce additional
;;; features.  This is important, since Birth is a 1:1 translation of the
;;; Prebirth compiler and needs to stay that way.  This fork allows us to
;;; vary as much as we want from the initial implementation.  See the commit
;;; history for this file for more information as to how it evolved (the
;;; first commit is the direct copy before actual code changes).


;; The first step in the Rebirth process is to liberate ourselves from
;; libprebirth.

;; Here we define the libprebirth primitives.  When we first compile
;; Rebirth with Birth, `string->es' is not yet available, because it is
;; only implemented in Rebirth.  Further, Birth includes libprebirth in
;; its output, so we cannot blindly redefine the procedures without
;; producing an error.
;;
;; Once Rebirth is compiled with Birth, Rebirth can then compile
;; itself.  Since Rebirth _does_ implement `string->es', and further _does
;; not_ include libprebirth in its output, we can define the libprebirth
;; primitives ourselves in Rebirth Lisp.  Cut the cord.
;;
;; Some of these definitions aren't valid: variable arguments, for
;; example, aren't represented _at all_---the `define' form will be
;; properly implemented in the future to correct this.
(cond-expand
  (string->es
   (define #t (string->es "true"))
   (define #f (string->es "false"))

   ;; _truep is used only internally and is still defined as a JS function
   ;; for brevity
   (string->es "const _truep = x => x !== false")

   ;; intended for whether a procedure is defined, mostly
   (define (es:defined? x)
     (let ((id (tname->id x)))
       (string->es "eval('typeof ' + $$id) !== 'undefined'")))

   (define (es:typeof x)
     (string->es "typeof $$x"))

   (define (symbol=? x y)
     (and (string=? (es:typeof x) "symbol")
          (eq? x y)))

   (define (es:arg->arr args)
     (string->es "Array.prototype.slice.call($$args)"))
   (define (list)
     (es:arg->arr (string->es "arguments")))

   ;; warning: only compares two values
   (define (= x y)
     (string->es "+$$x === +$$y"))
   (define (> x y)
     (string->es "+$$y > +$$x"))
   (define (< x y)
     (string->es "+$$y < +$$x"))

   ;; warning: doesn't verify that it's a pair
   (define (length xs)
     (string->es "$$xs.length"))

   (define (es:array? xs)
     (string->es "Array.isArray($$xs)"))

   (define (es:-assert-list xs)
     (or (es:array? xs)
         (error "expecting list")))

   (define (es:-assert-pair xs)
     (es:-assert-list xs)
     (if (= 0 (length xs))
         (error "expecting pair")
         #t))

   ;; ignore obj for now
   (define (error msg obj)
     (string->es "throw Error($$msg)")
     #f)  ; prevent above from being in tail position and prefixing "return"

   ;; warning: these only operate on arrays
   (define (cons obj1 obj2)
     (es:-assert-list obj2)
     (string->es "[$$obj1].concat($$obj2)"))
   (define (car pair)
     (es:-assert-pair pair)
     (string->es "$$pair[0]"))
   (define (cdr pair)
     (es:-assert-pair pair)
     (string->es "$$pair.slice(1)"))

   (define (append)
     (fold (lambda (x xs)
             (es:-assert-list x)
             (string->es "$$xs.concat($$x)"))
           (list)
           (es:arg->arr (string->es "arguments"))))

   ;; warning: these two are wholly inadequate
   (define (list? xs)
     (string->es "Array.isArray($$xs)"))
   (define (pair? xs)
     (and (list? xs)
          (> 0 (length xs))))

   ;; R7RS string
   (define (substring s start end)
     (string->es "$$s.substring($$start, $$end)"))
   (define (string-length s)
     (string->es "$$s.length"))
   (define (string=? s1 s2)
     (string->es "typeof $$s1 === 'string' && $$s1 === $$s2"))
   (define (string-ref s i)
     (string->es "$$s[$$i] || $$error(`value out of range: ${$$i}`)"))
   (define (string-append)
     (let ((args (es:arg->arr (string->es "arguments"))))
       (string->es "$$args.join('')")))

   (define (eq? x y)
     (string->es "$$x === $$y"))

   ;; R7RS math
   (define (+)
     (let ((args (es:arg->arr (string->es "arguments"))))
       (fold (lambda (y x)
               (string->es "$$x + $$y"))
             0
             args)))
   (define (-)
     (let ((args (es:arg->arr (string->es "arguments"))))
       (fold (lambda (y x)
               (string->es "$$x - $$y"))
             (car args)
             (cdr args))))
   (define (zero? x)
     (eq? x 0))

   ;; SRFI-1
   ;; warning: fold here only supports one list
   (define (fold f init xs)
     (string->es "$$xs.reduce((prev, x) => $$f(x, prev), $$init)"))

   ;; warning: map here uses the length of the first list, not the shortest
   ;; (we implement this in ES for now so that we don't have to augment
   ;; Prebirth Lisp to support the "rest" procedure definition syntax)
   (define (map f)
     (string->es "__a = arguments")  ; because let introduces a function
     (let* ((args (es:arg->arr (string->es "__a")))
            (xs (cdr args)))
       (string->es
        "$$xs[0].map((_, i) => $$f.apply(null, $$xs.map(x => x[i])))")))

   (define (js:regexp s opts)
     (string->es "new RegExp($$s, $$opts)"))
   (define (js:match r s)
     (string->es "$$s.match($$r) || false"))
   (define (js:replace r repl s)
     (string->es "$$s.replace($$r, $$repl)"))

   (define *fsdata*
     (if (string->es "typeof __fsinit === 'undefined'")
         (string->es "{}")
         (string->es "__fsinit")))

   (define *fs*
     (if (string->es "typeof require === 'undefined'")
         (string->es
          "{
             readFileSync(path)
             {
               throw Error(`Cannot load ${path} (no fs module)`);
             },
           }")
         (string->es "require('fs')")))

   ;; so that we do not have to modify existing compiler output (which would
   ;; break the first round of compilation before these are defined)
   (string->es "const fsdata = $$$k$fsdata$k$")
   (string->es "const fs     = $$$k$fs$k$")

   (define (js:file->string path)
     (if (string->es "fsdata[$$path] === undefined")
         (string->es
          "fsdata[$$path] = fs.readFileSync($$path).toString()")
         (string->es "fsdata[$$path]")))))


;; 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
   ;; Stores macros for compiler runtime.
   (string->es "const _macros = {}")

   (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 `_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 Gibble Lisp; 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
        "_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))))))))


;; pair selection
(define (cadr xs)
  (car (cdr xs)))
(define (caadr xs)
  (car (car (cdr xs))))
(define (caddr xs)
  (car (cdr (cdr xs))))
(define (cadddr xs)
  (car (cdr (cdr (cdr xs)))))
(define (caddddr xs)
  (car (cdr (cdr (cdr (cdr xs))))))
(define (cddr xs)
  (cdr (cdr xs)))

(define (not x)
  (if x #f #t))

;; for convenience
(define (js:match-regexp re s)
  (js:match (js:regexp re) s))



;; 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 `js:while' and `js: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)
  (let ((toks (list)))
    (js:while #t                              ; browser stack workaround
     (let* ((ws     (or (js: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)
           (js: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 (js: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 (js: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 (js: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 Prebirth 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 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 `$'
;; in a catch-all and therefore could result in conflicts and cannot be
;; reliably distinguished from one-another.  Remember: this is temporary
;; code.
(define (tname->id name)
  (if (js:match (js:regexp "^\\d+$") name)
      name
      (string-append
       "$$" (js:replace (js:regexp "[^a-zA-Z0-9_]" "g")
                        (lambda (c)
                          (case c
                            (("-") "$_$")
                            (("?") "$7$")
                            (("@") "$a$")
                            (("!") "$b$")
                            ((">") "$g$")
                            (("#") "$h$")
                            (("*") "$k$")
                            (("<") "$l$")
                            (("&") "$n$")
                            (("%") "$o$")
                            (("+") "$p$")
                            (("=") "$q$")
                            (("^") "$v$")
                            (("/") "$w$")
                            (("$") "$$")
                            (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 simply takes the value of the symbol and outputs it (formatted),
;; delimited by commas.
(define (params->es params)
  (join ", " (map (lambda (t)
                    (tname->id (token-value t)))
                  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


;; 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)))


;; Compile procedure definition into an ES function definition
;;
;; This will fail if the given token is not a `define'.
(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)))))
         (params (params->es (cdr dfn)))
         (body   (body->es (cddr t) #t)))
    ;; this is the final format---each procedure becomes its own function
    ;; definition in ES
    (string-append
     "function " id "(" params ")\n{\n" body "\n}")))


;; 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 `_macros' is consulted.
;;
;; See `cdfn-macro' for more information.
(define (macro? name)
  (cond-expand
    (string->es
     (string->es "_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 "_macros[$$fn].apply(null,$$args)")
      ;; Procedures are produced as part of the compiler output.
      (let* ((idfn   (tname->id fn))
             (argstr (join ", " (map sexp->es args))))
        (string-append idfn "(" argstr ")"))))


;; Function/procedure aliases and special forms
;;
;; And here we have what is probably the most grotesque part of this file.
;;
;; This map allows for a steady transition---items can be removed as they
;; are written in Prebirth Lisp.  This should give us a sane (but still
;; simple) environment with which we can start to self-host.
;;
;; 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 ARS is the list of arguments.
;;
;; These are by no means meant to be solid implementations; notable
;; deficiencies are documented, but don't expect this to work properly in
;; every case.  They will be replaced with proper R7RS implementations in
;; the future (Rebirth).
(define (fnmap fn args t)
  (case fn
    (("js:console")
     (string-append "console.log(" (map sexp->es args) ")"))
    (("js:error")
     (string-append "console.error(" (map sexp->es args) ")"))

    ;; very primitive cond-expand
    (("cond-expand") (expand-cond-expand args))

    ;; output raw code into the compiled ECMAScript (what could go wrong?)
    (("string->es")
     (token-value (car args)))

    ;; yes, there are more important things to do until we get to the
    ;; point where it's worth implementing proper tail calls
    (("js:while")
     (let ((pred (car args))
           (body (cdr args)))
       (string-append
        "(function(__whilebrk){"
        "while (" (sexp->es pred) "){\n"
        (body->es body #f) " if (__whilebrk) break;\n"
        "}\n"
        "})(false)")))
    (("js:break") "__whilebrk=true")

    ;; note that the unquote forms are only valid within a quasiquote; see
    ;; that procedure for the handling of those forms
    (("quote")      (quote-sexp      (car args)))
    (("quasiquote") (quasiquote-sexp (car args)))

    (("define")       (cdfn t))
    (("define-macro") (cdfn-macro t))  ; not defined until string->es cond

    (("lambda")
     (let ((fnargs (car args))
           (body   (cdr args)))
       (string-append
        "function(" (join ", " (map sexp->es fnargs)) "){\n"
        (body->es body #t)
        "}")))

    ;; simple if statement with optional else, wrapped in a self-executing
    ;; function to simplify code generation (e.g. returning an if)
    (("if")
     (let ((pred (car args))
           (t    (cadr args))
           (f    (and (pair? (cddr args))
                      (caddr args))))
       (string-append
        "(function(){"
        "if (_truep(" (sexp->es pred) ")){return " (sexp->es t) ";}"
        (if (pair? f)
            (string-append "else{return " (sexp->es f) ";}")
            "")
        "})()")))

    ;; and short-circuits, so we need to implement it as a special form
    ;; rather than an alias
    (("and")
     (string-append
      "(function(__and){\n"
      (join "" (map (lambda (expr)
                      (string-append
                       "__and = " (sexp->es expr) "; "
                       "if (!_truep(__and)) return false;\n"))
                    args))
      "return __and;})()"))

    ;; or short-circuits, so we need to implement it as a special form
    ;; rather than an alias
    (("or")
     (string-append
      "(function(__or){\n"
      (join "" (map (lambda (expr)
                      (string-append
                       "__or = " (sexp->es expr) "; "
                       "if (_truep(__or)) return __or;\n"))
                    args))
      "return false;})()"))

    ;; (let ((binding val) ...) ...body), compiled as a self-executing
    ;; function which allows us to easily represent the return value of
    ;; the entire expression while maintaining local scope
    (("let*")
     (let ((bindings (car args))
           (body     (cdr args)))
       (string-append
        "(function(){\n"
        (join "" (map (lambda (binding)
                        (let ((var  (car binding))
                              (init (cadr binding)))
                          (string-append "    let " (sexp->es var)
                                         " = " (sexp->es init) ";\n")))
                      bindings))
        (body->es body #t) "\n"
        "    })()")))

    ;; similar to the above, but variables cannot reference one-another
    (("let")
     (let* ((bindings (car args))
            (body     (cdr args))
            (fparams  (join ", " (map sexp->es
                                      (map car bindings))))
            (fargs    (join ", " (map sexp->es
                                      (map cadr bindings)))))
       (string-append "(function(" fparams "){\n"
                      (body->es body #t) "\n"
                      "})(" fargs ")")))

    ;; and here I thought Prebirth Lisp would be simple...but having
    ;; `case' support really keeps things much more tidy, so here we are
    ;; (note that it doesn't support the arrow form, nor does it support
    ;; expressions as data)
    (("case")
     (let ((key     (car args))
           (clauses (cdr args)))
       (string-append
        "(function(){const _key=" (sexp->es key) ";\n"
        "switch (_key){\n"
        (join ""
              (map (lambda (data exprs)
                     (string-append
                      (if (and (token? data)
                                (string=? "else" (token-lexeme data)))
                          "default:\n"
                          (join ""
                                (map (lambda (datum)
                                       (string-append
                                        "case " (sexp->es datum) ":\n"))
                                     data)))
                      (body->es exprs #t) "\n"))
                   (map car clauses)
                   (map cdr clauses)))
        "}})()")))

    (("set!")
     (let ((varid (car args))
           (val   (cadr args)))
       (string-append (sexp->es varid) " = " (sexp->es val))))

    ;; procedure or macro
    (else (apply-proc-or-macro fn args))))


;; 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") (tname->id (token-value t)))

        (else (error
               (string-append
                "cannot compile unknown token `" (token-type t) "'"))))

      ;; otherwise, process the expression
      (fnmap (token-value (car t))
             (cdr t)
             t)))


;; Compile Rebirth Lisp AST into ECMAScript.
;;
;; The AST can be generated with `parse-lisp'.
(define (rebirth->ecmascript ast)
  ;; compiled output, wrapped in a self-executing function to limit scope
  ;; (note that we no longer depend on libprebirth)
  (string-append "(function(){"
                 (join "\n\n" (map sexp->es ast))
                 "})();"))


;; at this point, this program can parse itself and output a CST (sans
;; whitespace)
(js:console (rebirth->ecmascript
             (parse-lisp
              (js:file->string "/dev/stdin"))))