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Add prebirth.js 

This is hopefully the beginning of a good thing that I'll actually
finish.  I began planning this project formally just before the beginning of
Aug 2017.

* build-aux/bootstrap/prebirth.js: New file.
master
Mike Gerwitz 2017-08-21 02:20:03 -04:00
parent ecd8b6d9e7
commit 7998296a20
Signed by: mikegerwitz
GPG Key ID: 8C917B7F5DC51BA2
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/**
* Bootstrap Gibble Lisp ("Prebirth")
*
* 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".
*
* Rather than producing a sophisticated self-hosting language, this
* language will be a terribly incomplete and inadequate version of what
* will ultimately become a formidable and competent language.
*
* I refer to this entire complication process as "Prebirth".¹ The "Birth"
* of Gibble is the act of reimplementing this Prebirth in a Prebirth
* version of Gibble Lisp itself. It's the chicken-and-egg paradox, without
* the paradox.²
*
* Gibble Lisp is _not_ the most primitive language that will be understood
* by the system---it is too high-level. After Birth, the language can
* devolve into something more powerful and workable.
*
* Some minor terminology:
* - AST: Abstract Syntax Tree, a processed form of the CST.
* - CST: Concrete Syntax Tree, a 1-1 conversion of source input to
* tokens.
* - token: an object produced by the lexer that represents a portion of
* the input language
* - lexer: sometimes called a ``tokenizer''---produces tokens by applying
* the grammar to a string of input.
* - grammar: a definition of the language (syntax).
* - lexeme: the portion of the original source string associated with a
* given token.
* - LL(0): Left-to-right, Leftmost derivation, 0 tokens lookahead
* - sexp: symbolic expression, (involving (lots (of (((parentheses))))))
*
* Excited? Great! My extemporaneous rambling is costing me more time than
* I spent making this damn thing! (No, really, it is.)
*/
'use strict';
/**
* A very rudimentary (and extremely permissive) LL(0) Lisp parser
*
* This provides just enough to get by. It transforms lists into nested
* arrays of tokens with some very basic error checking (e.g. for proper
* nesting). This is not a general-purpose lisp parser.
*/
class Parser
{
/**
* 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.
*
* @param {string} src input Lisp
*
* @throws {SyntaxError} on improper sexp nesting
*
* @return {Array} primitive abstract syntax tree of SRC
*/
parseLisp( src )
{
// token string from lexing
const toks = this._lex( src );
// perform a leftmost reduction on the token string
const [ depth, ast ] = toks.reduce( ( result, token ) =>
{
const [ depth, xs, stack ] = result;
const { type, pos } = token;
// there are very few token types to deal with (again, this is
// a very simple bootstrap lisp)
switch ( type )
{
// closing parenthesis (end of sexp)
case 'close':
if ( depth === 0 ) {
this._error(
src, pos, `Unexpected closing parenthesis`
);
}
// the sexp is complete; add to the AST, reduce depth
const top = stack.pop();
top.push( xs );
return [ ( depth - 1 ), top, stack ];
// opening parenthesis (start of sexp)
case 'open':
stack.push( xs );
return [ ( depth + 1 ), [], stack ];
// symbol or primitive; just copy the token in place
case 'string':
case 'symbol':
xs.push( token );
return [ depth, xs, stack ];
// should never happen unless there's a bug in the tokenizer
// or we forget a token type above
default:
this._error( src, pos, `Unexpected token '${type}'` );
}
}, [ 0, [], [] ] );
// if we terminate at a non-zero depth, that means there
// are still open sexps
if ( depth > 0 ) {
throw SyntaxError(
`Unexpected end of input at depth ${depth}`
);
}
// the result is a set of tokens organized into ES arrays
// isomorphic to the original sexp structure (the same structure)
return ast;
}
/**
* Throw a SyntaxError with a window of surrounding source code
*
* The "window" is simply ten characters to the left and right of the
* first character of the source input SRC that resulted in the error.
* It's a little more than useless.
*
* @param {string} src source code (sexps)
* @param {number} pos position of error
* @param {string} msg error message
*
* @throws {SyntaxError}}
*
* @return {undefined}
*/
_error( src, pos, msg )
{
const window = src.substr( pos - 10, pos + 10 )
.replace( "\n", " " );
throw new SyntaxError( `${msg}: '${window}'` );
}
/**
* Convert source input into a string of tokens
*
* This is the lexer. Whitespace is ignored. The grammar consists of
* simple s-expressions.
*
* This function is mutually recursive with `#_token'. It expects that
* 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.
*
* @param {string} src source code
* @param {number} pos position (character offset) in source
*
* @return {Array} string of tokens
*/
_lex( src, pos = 0 )
{
// ignore whitespace, if any
const ws = src.match( /^\s+/ ) || [ "" ];
const trim = src.substr( ws[ 0 ].length );
// adjust position to account for any removed whitespace
pos += ws[ 0 ].length;
// EOF and we're done
if ( trim === '' ) {
return [];
}
// left and right parenthesis are handled in the same manner: they
// produce distinct tokens with single-character lexemes
if ( trim[ 0 ] === '(' ) {
return this._token( 'open', '(', trim, pos );
}
if ( trim[ 0 ] === ')' ) {
return this._token( 'close', ')', trim, pos );
}
// strings are delimited by opening and closing ASCII double quotes,
// which can be escaped with a backslash
if ( trim[ 0 ] === '"' ) {
const str = trim.match( /^"(|.*?[^\\])"/ );
if ( !str ) {
this._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')
return this._token( 'string', str, trim, pos );
}
// anything else is considered a symbol up until whitespace or any
// of the aforementioned delimiters
const symbol = trim.match( /^[^\s()"]+/ );
return this._token( 'symbol', symbol, trim, pos );
}
/**
* Produce a token and recurse
*
* The token will be concatenated with the result of the mutually
* recursive method `_lex'.
*
* For the record: I'm not fond of mutual recursion from a clarity
* standpoint, but this is how the abstraction evolved to de-duplicate
* code, and I don't much feel like refactoring it.
*
* @param {string} type token type
* @param {string|Array} match lexeme match
* @param {string} src source code string, left-truncated
* @param {number} pos offset relative to original src
*
* @return {Array} string of tokens
*/
_token( type, match, src, pos )
{
const parts = ( Array.isArray( match ) )
? match
: [ match ];
// 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
const [ lexeme, value ] = parts;
const token = {
type: type,
lexeme: lexeme,
value: value || lexeme,
pos: pos
};
// continue producing tokens by recursing, left-truncating the
// source string to discard what we have already processed
return [ token ].concat(
this._lex(
src.substr( lexeme.length ),
( pos + lexeme.length )
)
);
}
};
/**
* Dumb compiler to transform AST into ECMAScript
*
* This is a really dumb code generator: it takes the AST and essentially
* transforms it 1:1 wherever possible into the target language.
*
* This is nothing like what we actually want the _ultimate_ compiler to do
* after Birth, but it gets us to a point where we can self-host on a basic
* Prebirth language and evolve from there.
*
* The code generation can be pretty much summed up by the last line of
* `Compiler#_cdfn'.
*/
class Compiler
{
/**
* Compile AST into ECMAScript
*
* Every block is mapped 1:1 to a function in ECMAScript. So, we just
* map all root children (which are expected to be block definitions) to
* functions.
*
* @param {Array} tree root of tree containing top-level block definitions
*/
compile( tree )
{
// map every definition to a ES function definition and delimit them
// (for readability) by two newlines
return tree.map( this._cdfn.bind( this ) )
.join( "\n\n" ) + "\n";
}
/**
* Compile block definition into a ES function definition
*
* This will fail if the given token is not a `define-block'.
*
* @param {Object} t token
*
* @return {string} compiled block definition
*/
_cdfn( t )
{
this.assertApply( t, 'define-block' );
// e.g. (define-block <foo> ((input ...)) body)
const [ , { value: name }, desc, ...body ] = t;
const id = this._idFromName( name );
const bodyjs = this._bodyToEs( body );
// this is the final format---each block becomes its own function
// definition
return `function ${id}()\n{\n${bodyjs}\n};`;
}
/**
* Generate ECMAScript-friendly name from the given id
*
* @param {string} name source name
*
* @return {string} ES-friendly identifier
*/
_idFromName( name )
{
return name.replace( /[^a-zA-Z0-9_]/g, '$' );
}
/**
* Compile body s-expressions into ECMAScript
*
* This produces a 1:1 mapping of BODY s-expressions to ES statements,
* recursively. The heavy lifting is done by `#_sexpToEs'.
*
* @param {Array} body s-expressions representing block body
*
* @return {string} compiled BODY
*/
_bodyToEs( body )
{
// the body must be an array of expressions (this should always be
// the case unless we have a bug in the compiler)
if ( !Array.isArray( body ) ) {
throw Error( "body must be an Array" );
}
// process each s-expression in BODY
const js = body.map( this._sexpToEs.bind( this ) );
// the result (that is, an array of compiled s-expressions) is
// joined semicolon-delimited, with a `return' statement preceding
// the final expression
return js.reduce( ( result, s, i ) =>
{
const ret = ( i === ( js.length - 1 ) ) ? "return " : "";
return result + " " + ret + s + ";";
}, "" );
}
/**
* Convert s-expression or scalar into ECMAScript
*
* T may be either an array of tokens or a primitive token (e.g. string,
* symbol). This method is applied recursively to T as needed if T is
* an array.
*
* @param {Array|Object} t tokens representing s-expressions/scalars
*
* @return {string} compiled s-expression/scalar
*/
_sexpToEs( t )
{
// just output symbols as identifiers as-is for now
if ( !Array.isArray( t ) ) {
switch ( t.type )
{
// strings are output as-is (note that we don't escape
// double quotes, because the method of escaping them is the
// same in Scheme as it is in ECMAScript---a backslash)
case 'string':
return `"${t.value}"`;
// symbols have the same concerns as block definitions: the
// identifiers generated need to be ES-friendly
case 'symbol':
return this._idFromName( t.value );
default:
throw Error( "Cannot compile unknown token `${t.type}'" );
}
}
// only support block form for now, and assume that `fn' is a
// string value (in the future, this doesn't have to be the
// case---fn should be able to be an arbitrary sexp)
const [ { value: fn }, argmap ] = t;
if ( !this._isBlockForm( t ) ) {
throw Error( `\`${fn}' application is not in block form`)
}
// convert all remaining symbols (after the symbol representing the
// function application) into arguments by parsing their sexps or
// scalar values; we're not going to worry about mapping them for
// now; they will be compiled in the order in which they appear
const idfn = this._idFromName( fn );
const args = argmap.map( ([ , v ]) => this._sexpToEs( v ) );
const argstr = args.join( ", " );
// make the dangerous assumption that arguments are ordered
// for now
return `${idfn}(${argstr})`;
}
/**
* Determine whether T represents a block form
*
* Block form is an application of a block, which has a certain
* syntax. Specifically: `(<block> ((key value) ...))'.
*
* @param {*} t hopefully a token list
*
* @return {boolean} whether T represents a block form
*/
_isBlockForm( t )
{
// the first symbol is the function name, second is an sexp
// containing each of the key/value argument mappings
const [ fn, argmap ] = t;
// enforce block id convention (at least for now)
const isblockid = /^<[^>]+>$/.test( fn.value );
return (
Array.isArray( t )
&& isblockid
&& Array.isArray( argmap )
);
}
/**
* Determine whether T is an application of a symbol NAME, or error
*
* @param {*} t hopefully a token or token list
* @param {string} name block name to assert against
*/
assertApply( t, name )
{
// an application must be an s-expression
if ( !Array.isArray( t ) ) {
throw Error(
`\`${name}' application expected, found symbol \`${t.value}'`
);
}
// if there's a match, we can stop here
if ( t[ 0 ].value === name ) {
return;
}
// otherwise, provide an informative error of what we found and what
// we should have found
throw Error(
`\`${name}' expected, found \`${t[ 0 ].value}'`
);
}
}
/*
* Prebirth was originally intended to be run via the command line using
* Node.js. But it doesn't have to be. If you want, feel free to run it in
* your web browser; you'll just have to instantiate your own objects.
*/
( function ()
{
if ( typeof process === 'undefined' )
{
return;
}
const p = new Parser();
const c = new Compiler();
const src = require( 'fs' ).readFileSync( '/dev/stdin' ).toString();
const tree = p.parseLisp( src );
process.stdout.write( c.compile( tree ) );
} )();
/*
* Now that we have output, the next step is the hard part: rewriting this
* file in Prebirth Lisp. As I mentioned, this process is called
* "Birth". It's at this point that we have to decide on basic
* abstractions---we are starting from scratch. The initial implementation
* is therefore unlikely to be as concise and elegant as Prebirth
* itself---it will be refactored.
*
* Here is an example Hello, World!:
*
* (define-block <hello-world>
* ()
* (<js:console> ((message "Hello, world!"))))
*
*
* ¹ This term should invoke visuals of an abstract being entering existence
* in some strange nonlinear-time² kind of way. If you thought of
* something less pleasant, well, I'm sorry you went through that.
*
* ² Because we're dealing with nonlinear time!¹ This would be some bizarre
* recursive footnote crap if it weren't for that.²
*/