522 lines
17 KiB
JavaScript
522 lines
17 KiB
JavaScript
/**
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* Bootstrap Gibble Lisp ("Prebirth")
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*
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* Copyright (C) 2017 Mike Gerwitz
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*
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* This file is part of Gibble.
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*
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* Gibble is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as
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* published by the Free Software Foundation, either version 3 of the
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* License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*
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* THIS IS TEMPORARY CODE that will be REWRITTEN IN GIBBLE LISP ITSELF after
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* a very basic bootstrap is complete. It is retained as an important
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* artifact for those who wish to build Gibble from scratch without using
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* another version of Gibble itself. This is called "self-hosting".
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*
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* Rather than producing a sophisticated self-hosting language, this
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* language will be a terribly incomplete and inadequate version of what
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* will ultimately become a formidable and competent language.
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*
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* I refer to this entire complication process as "Prebirth".¹ The "Birth"
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* of Gibble is the act of reimplementing this Prebirth in a Prebirth
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* version of Gibble Lisp itself. It's the chicken-and-egg paradox, without
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* the paradox.²
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*
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* Gibble Lisp is _not_ the most primitive language that will be understood
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* by the system---it is too high-level. After Birth, the language can
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* devolve into something more powerful and workable.
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*
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* Some minor terminology:
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* - AST: Abstract Syntax Tree, a processed form of the CST.
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* - CST: Concrete Syntax Tree, a 1-1 conversion of source input to
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* tokens.
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* - token: an object produced by the lexer that represents a portion of
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* the input language
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* - lexer: sometimes called a ``tokenizer''---produces tokens by applying
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* the grammar to a string of input.
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* - grammar: a definition of the language (syntax).
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* - lexeme: the portion of the original source string associated with a
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* given token.
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* - LL(0): Left-to-right, Leftmost derivation, 0 tokens lookahead
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* - sexp: symbolic expression, (involving (lots (of (((parentheses))))))
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*
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* Excited? Great! My extemporaneous rambling is costing me more time than
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* I spent making this damn thing! (No, really, it is.)
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*/
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'use strict';
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/**
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* A very rudimentary (and extremely permissive) LL(0) Lisp parser
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*
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* This provides just enough to get by. It transforms lists into nested
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* arrays of tokens with some very basic error checking (e.g. for proper
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* nesting). This is not a general-purpose lisp parser.
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*/
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class Parser
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{
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/**
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* Produce an AST from the given string SRC of sexps
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*
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* This is essentially the CST with whitespace removed. It first
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* invokes the lexer to produce a token string from the input
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* sexps SRC. From this, it verifies only proper nesting (that SRC does
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* not close sexps too early and that EOF isn't reached before all sexps
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* are closed) and produces an AST that is an isomorphism of the
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* original sexps.
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*
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* @param {string} src input Lisp
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*
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* @throws {SyntaxError} on improper sexp nesting
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*
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* @return {Array} primitive abstract syntax tree of SRC
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*/
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parseLisp( src )
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{
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// token string from lexing
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const toks = this._lex( src );
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// perform a leftmost reduction on the token string
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const [ depth, ast ] = toks.reduce( ( result, token ) =>
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{
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const [ depth, xs, stack ] = result;
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const { type, pos } = token;
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// there are very few token types to deal with (again, this is
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// a very simple bootstrap lisp)
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switch ( type )
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{
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// ignore comments
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case 'comment':
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return result;
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// closing parenthesis (end of sexp)
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case 'close':
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if ( depth === 0 ) {
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this._error(
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src, pos, `Unexpected closing parenthesis`
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);
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}
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// the sexp is complete; add to the AST, reduce depth
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const top = stack.pop();
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top.push( xs );
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return [ ( depth - 1 ), top, stack ];
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// opening parenthesis (start of sexp)
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case 'open':
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stack.push( xs );
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return [ ( depth + 1 ), [], stack ];
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// symbol or primitive; just copy the token in place
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case 'string':
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case 'symbol':
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xs.push( token );
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return [ depth, xs, stack ];
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// should never happen unless there's a bug in the tokenizer
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// or we forget a token type above
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default:
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this._error( src, pos, `Unexpected token '${type}'` );
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}
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}, [ 0, [], [] ] );
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// if we terminate at a non-zero depth, that means there
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// are still open sexps
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if ( depth > 0 ) {
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throw SyntaxError(
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`Unexpected end of input at depth ${depth}`
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);
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}
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// the result is a set of tokens organized into ES arrays
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// isomorphic to the original sexp structure (the same structure)
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return ast;
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}
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/**
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* Throw a SyntaxError with a window of surrounding source code
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*
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* The "window" is simply ten characters to the left and right of the
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* first character of the source input SRC that resulted in the error.
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* It's a little more than useless.
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*
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* @param {string} src source code (sexps)
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* @param {number} pos position of error
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* @param {string} msg error message
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*
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* @throws {SyntaxError}}
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*
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* @return {undefined}
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*/
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_error( src, pos, msg )
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{
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const window = src.substr( pos - 10, pos + 10 )
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.replace( "\n", " " );
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throw new SyntaxError( `${msg}: '${window}'` );
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}
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/**
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* Convert source input into a string of tokens
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*
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* This is the lexer. Whitespace is ignored. The grammar consists of
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* simple s-expressions.
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*
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* This function is mutually recursive with `#_token'. It expects that
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* the source SRC will be left-truncated as input is
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* processed. POS exists for producing metadata for error
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* reporting---it has no impact on parsing.
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*
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* @param {string} src source code
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* @param {number} pos position (character offset) in source
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*
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* @return {Array} string of tokens
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*/
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_lex( src, pos = 0 )
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{
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// ignore whitespace, if any
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const ws = src.match( /^\s+/ ) || [ "" ];
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const trim = src.substr( ws[ 0 ].length );
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// adjust position to account for any removed whitespace
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pos += ws[ 0 ].length;
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// EOF and we're done
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if ( trim === '' ) {
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return [];
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}
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// comment until end of line
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if ( trim[ 0 ] === ';' ) {
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const eol = trim.match( /^(.*?)(\n|$)/ );
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return this._token( 'comment', eol[ 1 ], trim, pos );
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}
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// left and right parenthesis are handled in the same manner: they
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// produce distinct tokens with single-character lexemes
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if ( trim[ 0 ] === '(' ) {
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return this._token( 'open', '(', trim, pos );
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}
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if ( trim[ 0 ] === ')' ) {
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return this._token( 'close', ')', trim, pos );
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}
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// strings are delimited by opening and closing ASCII double quotes,
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// which can be escaped with a backslash
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if ( trim[ 0 ] === '"' ) {
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const str = trim.match( /^"(|.*?[^\\])"/ );
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if ( !str ) {
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this._error( src, pos, "missing closing string delimiter" );
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}
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// a string token consists of the entire string including quotes
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// as its lexeme, but its value will be the value of the string
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// without quotes due to the `str' match group (see `#_token')
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return this._token( 'string', str, trim, pos );
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}
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// anything else is considered a symbol up until whitespace or any
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// of the aforementioned delimiters
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const symbol = trim.match( /^[^\s()"]+/ );
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return this._token( 'symbol', symbol, trim, pos );
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}
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/**
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* Produce a token and recurse
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*
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* The token will be concatenated with the result of the mutually
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* recursive method `_lex'.
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*
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* For the record: I'm not fond of mutual recursion from a clarity
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* standpoint, but this is how the abstraction evolved to de-duplicate
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* code, and I don't much feel like refactoring it.
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*
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* @param {string} type token type
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* @param {string|Array} match lexeme match
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* @param {string} src source code string, left-truncated
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* @param {number} pos offset relative to original src
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*
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* @return {Array} string of tokens
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*/
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_token( type, match, src, pos )
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{
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const parts = ( Array.isArray( match ) )
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? match
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: [ match ];
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// the value is the first group of the match (indicating what we
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// are actually interested in), and the lexeme is the full match,
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// which might include, for example, string delimiters
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const [ lexeme, value ] = parts;
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const token = {
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type: type,
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lexeme: lexeme,
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value: value || lexeme,
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pos: pos
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};
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// continue producing tokens by recursing, left-truncating the
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// source string to discard what we have already processed
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return [ token ].concat(
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this._lex(
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src.substr( lexeme.length ),
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( pos + lexeme.length )
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)
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);
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}
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};
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/**
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* Dumb compiler to transform AST into ECMAScript
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*
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* This is a really dumb code generator: it takes the AST and essentially
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* transforms it 1:1 wherever possible into the target language.
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*
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* This is nothing like what we actually want the _ultimate_ compiler to do
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* after Birth, but it gets us to a point where we can self-host on a basic
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* Prebirth language and evolve from there.
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*
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* The code generation can be pretty much summed up by the last line of
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* `Compiler#_cdfn'.
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*/
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class Compiler
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{
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/**
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* Compile AST into ECMAScript
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*
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* Every function is mapped 1:1 to a function in ECMAScript. So, we
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* just map all root children (which are expected to be Scheme-style
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* shorthand function definitions) to functions.
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*
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* @param {Array} tree root containing top-level function definitions
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*/
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compile( tree )
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{
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// map every definition to a ES function definition and delimit them
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// (for readability) by two newlines
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return tree.map( this._cdfn.bind( this ) )
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.join( "\n\n" ) + "\n";
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}
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/**
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* Compile function definition into a ES function definition
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*
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* This will fail if the given token is not a `define'.
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*
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* @param {Object} t token
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*
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* @return {string} compiled function definition
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*/
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_cdfn( t )
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{
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// an application must be an s-expression
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if ( !Array.isArray( t ) ) {
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throw Error(
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`\`${name}' application expected, found symbol \`${t.value}'`
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);
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}
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// if it's not a definition, then it's a top-level application
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if ( t[ 0 ].value !== 'define' )
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{
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return this._sexpToEs( t ) + ';';
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}
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// e.g. (define (foo ...) body)
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const [ , [ { value: name }, ...params ], ...body ] = t;
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const id = this._idFromName( name, true );
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const paramjs = this._paramsToEs( params );
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const bodyjs = this._bodyToEs( body );
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// this is the final format---each function becomes its own function
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// definition in ES
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return `function ${id}(${paramjs})\n{\n${bodyjs}\n};`;
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}
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/**
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* Compile parameter list
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*
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* This simply takes the value of the symbol and outputs it (formatted),
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* delimited by commas.
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*
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* @param {Array} args token parameter list
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*
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* @return {string} compiled parameter list
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*/
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_paramsToEs( args )
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{
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return args.map( ({ value: name }) => this._idFromName( name ) )
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.join( ", " );
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}
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/**
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* Generate ECMAScript-friendly name from the given id
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*
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* @param {string} name source name
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* @param {boolean} global whether identifier should be globally unique
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*
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* @return {string} ES-friendly identifier
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*/
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_idFromName( name, global )
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{
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return ( global ? '$$' : '' )
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+ name.replace( /[^a-zA-Z0-9_]/g, '$' );
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}
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/**
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* Compile body s-expressions into ECMAScript
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*
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* This produces a 1:1 mapping of BODY s-expressions to ES statements,
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* recursively. The heavy lifting is done by `#_sexpToEs'.
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*
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* @param {Array} body s-expressions representing function body
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*
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* @return {string} compiled BODY
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*/
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_bodyToEs( body )
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{
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// the body must be an array of expressions (this should always be
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// the case unless we have a bug in the compiler)
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if ( !Array.isArray( body ) ) {
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throw Error( "body must be an Array" );
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}
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// process each s-expression in BODY
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const js = body.map( this._sexpToEs.bind( this ) );
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// the result (that is, an array of compiled s-expressions) is
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// joined semicolon-delimited, with a `return' statement preceding
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// the final expression
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return js.reduce( ( result, s, i ) =>
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{
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const ret = ( i === ( js.length - 1 ) ) ? "return " : "";
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return result + " " + ret + s + ";";
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}, "" );
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}
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/**
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* Convert s-expression or scalar into ECMAScript
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*
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* T may be either an array of tokens or a primitive token (e.g. string,
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* symbol). This method is applied recursively to T as needed if T is
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* an array.
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*
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* @param {Array|Object} t tokens representing s-expressions/scalars
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*
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* @return {string} compiled s-expression/scalar
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*/
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_sexpToEs( t )
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{
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// just output symbols as identifiers as-is for now
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if ( !Array.isArray( t ) ) {
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switch ( t.type )
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{
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// strings are output as-is (note that we don't escape
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// double quotes, because the method of escaping them is the
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// same in Scheme as it is in ECMAScript---a backslash)
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case 'string':
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return `"${t.value}"`;
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// symbols have the same concerns as function definitions: the
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// identifiers generated need to be ES-friendly
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case 'symbol':
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return this._idFromName( t.value );
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default:
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throw Error( `Cannot compile unknown token \`${t.type}'` );
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}
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}
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// simple function application (fn ...args)
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const [ { value: fn }, ...args ] = t;
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// convert all remaining symbols (after the symbol representing the
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// function application) into arguments by parsing their sexps or
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// scalar values
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const idfn = this._idFromName( fn, true );
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const argstr = args.map( arg => this._sexpToEs( arg ) ).join( ", " );
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// final function application
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return `${idfn}(${argstr})`;
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}
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}
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/*
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* Prebirth was originally intended to be run via the command line using
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* Node.js. But it doesn't have to be. If you want, feel free to run it in
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* your web browser; you'll just have to instantiate your own objects.
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*/
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( function ()
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{
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if ( typeof process === 'undefined' )
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{
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return;
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}
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const p = new Parser();
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const c = new Compiler();
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const fs = require( 'fs' );
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const src = fs.readFileSync( '/dev/stdin' ).toString();
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const tree = p.parseLisp( src );
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const lib = fs.readFileSync( __dirname + '/libprebirth.js' ).toString();
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// output libprebirth and compiled output, wrapped in a self-executing
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// function to limit scope
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process.stdout.write( "(function(){" );
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process.stdout.write( lib + '\n\n' );
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process.stdout.write( c.compile( tree ) );
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process.stdout.write( "})();\n" );
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} )();
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/*
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* Now that we have output, the next step is the hard part: rewriting this
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* file in Prebirth Lisp. As I mentioned, this process is called
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* "Birth". It's at this point that we have to decide on basic
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* abstractions---we are starting from scratch. The initial implementation
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* is therefore unlikely to be as concise and elegant as Prebirth
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* itself---it will be refactored.
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*
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* Here is an example Hello, World!:
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*
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* (define (hello x)
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* (js:console "Hello," x, "!"))
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*
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*
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* ¹ This term should invoke visuals of an abstract being entering existence
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* in some strange nonlinear-time² kind of way. If you thought of
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* something less pleasant, well, I'm sorry you went through that.
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*
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* ² Because we're dealing with nonlinear time!¹ This would be some bizarre
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* recursive footnote crap if it weren't for that.²
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*/
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