rewriter.coffee | |
|---|---|
| The CoffeeScript language has a decent amount of optional syntax, implicit syntax, and shorthand syntax. These things can greatly complicate a grammar and bloat the resulting parse table. Instead of making the parser handle it all, we take a series of passes over the token stream, using this Rewriter to convert shorthand into the unambiguous long form, add implicit indentation and parentheses, balance incorrect nestings, and generally clean things up. | |
| Set up exported variables for both Node.js and the browser. | this.exports: this unless process? |
| The Rewriter class is used by the Lexer, directly against its internal array of tokens. | exports.Rewriter: class Rewriter |
| Rewrite the token stream in multiple passes, one logical filter at a time. This could certainly be changed into a single pass through the stream, with a big ol' efficient switch, but it's much nicer to work with like this. The order of these passes matters -- indentation must be corrected before implicit parentheses can be wrapped around blocks of code. | rewrite: (tokens) ->
@tokens: tokens
@adjust_comments()
@remove_leading_newlines()
@remove_mid_expression_newlines()
@close_open_calls_and_indexes()
@add_implicit_indentation()
@add_implicit_parentheses()
@ensure_balance(BALANCED_PAIRS)
@rewrite_closing_parens()
@tokens |
| Rewrite the token stream, looking one token ahead and behind. Allow the return value of the block to tell us how many tokens to move forwards (or backwards) in the stream, to make sure we don't miss anything as tokens are inserted and removed, and the stream changes length under our feet. | scan_tokens: (block) ->
i: 0
while true
break unless @tokens[i]
move: block(@tokens[i - 1], @tokens[i], @tokens[i + 1], i)
i += move
true |
| Massage newlines and indentations so that comments don't have to be correctly indented, or appear on a line of their own. | adjust_comments: ->
@scan_tokens (prev, token, post, i) =>
return 1 unless token[0] is 'COMMENT'
after: @tokens[i + 2]
if after and after[0] is 'INDENT'
@tokens.splice(i + 2, 1)
@tokens.splice(i, 0, after)
return 1
else if prev and prev[0] isnt 'TERMINATOR' and prev[0] isnt 'INDENT' and prev[0] isnt 'OUTDENT'
@tokens.splice(i, 0, ['TERMINATOR', "\n", prev[2]])
return 2
else
return 1 |
| Leading newlines would introduce an ambiguity in the grammar, so we dispatch them here. | remove_leading_newlines: ->
@tokens.shift() while @tokens[0][0] is 'TERMINATOR' |
| Some blocks occur in the middle of expressions -- when we're expecting this, remove their trailing newlines. | remove_mid_expression_newlines: ->
@scan_tokens (prev, token, post, i) =>
return 1 unless post and include(EXPRESSION_CLOSE, post[0]) and token[0] is 'TERMINATOR'
@tokens.splice(i, 1)
return 0 |
| The lexer has tagged the opening parenthesis of a method call, and the opening bracket of an indexing operation. Match them with their paired close. | close_open_calls_and_indexes: ->
parens: [0]
brackets: [0]
@scan_tokens (prev, token, post, i) =>
switch token[0]
when 'CALL_START' then parens.push(0)
when 'INDEX_START' then brackets.push(0)
when '(' then parens[parens.length - 1] += 1
when '[' then brackets[brackets.length - 1] += 1
when ')'
if parens[parens.length - 1] is 0
parens.pop()
token[0]: 'CALL_END'
else
parens[parens.length - 1] -= 1
when ']'
if brackets[brackets.length - 1] == 0
brackets.pop()
token[0]: 'INDEX_END'
else
brackets[brackets.length - 1] -= 1
return 1 |
| Methods may be optionally called without parentheses, for simple cases. Insert the implicit parentheses here, so that the parser doesn't have to deal with them. | add_implicit_parentheses: ->
stack: [0]
@scan_tokens (prev, token, post, i) =>
tag: token[0]
stack.push(0) if tag is 'INDENT'
if tag is 'OUTDENT'
last: stack.pop()
stack[stack.length - 1] += last
if !post? or include IMPLICIT_END, tag
return 1 if tag is 'INDENT' and prev and include IMPLICIT_BLOCK, prev[0]
if stack[stack.length - 1] > 0 or tag is 'INDENT'
idx: if tag is 'OUTDENT' then i + 1 else i
stack_pointer: if tag is 'INDENT' then 2 else 1
for tmp in [0...stack[stack.length - stack_pointer]]
@tokens.splice(idx, 0, ['CALL_END', ')', token[2]])
size: stack[stack.length - stack_pointer] + 1
stack[stack.length - stack_pointer]: 0
return size
return 1 unless prev and include(IMPLICIT_FUNC, prev[0]) and include IMPLICIT_CALL, tag
@tokens.splice(i, 0, ['CALL_START', '(', token[2]])
stack[stack.length - 1] += 1
return 2 |
| Because our grammar is LALR(1), it can't handle some single-line expressions that lack ending delimiters. The Rewriter adds the implicit blocks, so it doesn't need to. ')' can close a single-line block, but we need to make sure it's balanced. | add_implicit_indentation: ->
@scan_tokens (prev, token, post, i) =>
return 1 unless include(SINGLE_LINERS, token[0]) and
post[0] isnt 'INDENT' and
not (token[0] is 'ELSE' and post[0] is 'IF')
starter: token[0]
@tokens.splice(i + 1, 0, ['INDENT', 2, token[2]])
idx: i + 1
parens: 0
while true
idx += 1
tok: @tokens[idx]
pre: @tokens[idx - 1]
if (not tok or
(include(SINGLE_CLOSERS, tok[0]) and tok[1] isnt ';') or
(tok[0] is ')' && parens is 0)) and
not (starter is 'ELSE' and tok[0] is 'ELSE')
insertion: if pre[0] is "," then idx - 1 else idx
@tokens.splice(insertion, 0, ['OUTDENT', 2, token[2]])
break
parens += 1 if tok[0] is '('
parens -= 1 if tok[0] is ')'
return 1 unless token[0] is 'THEN'
@tokens.splice(i, 1)
return 0 |
| Ensure that all listed pairs of tokens are correctly balanced throughout the course of the token stream. | ensure_balance: (pairs) ->
levels: {}
@scan_tokens (prev, token, post, i) =>
for pair in pairs
[open, close]: pair
levels[open] ||= 0
levels[open] += 1 if token[0] is open
levels[open] -= 1 if token[0] is close
throw new Error("too many ${token[1]} on line ${token[2] + 1}") if levels[open] < 0
return 1
unclosed: key for key, value of levels when value > 0
throw new Error("unclosed ${unclosed[0]}") if unclosed.length |
| We'd like to support syntax like this: In order to accomplish this, move outdents that follow closing parens inwards, safely. The steps to accomplish this are:
| rewrite_closing_parens: ->
stack: []
debt: {}
(debt[key]: 0) for key, val of INVERSES
@scan_tokens (prev, token, post, i) =>
tag: token[0]
inv: INVERSES[token[0]]
if include EXPRESSION_START, tag
stack.push token
return 1
else if include EXPRESSION_END, tag
if debt[inv] > 0
debt[inv] -= 1
@tokens.splice i, 1
return 0
else
match: stack.pop()
mtag: match[0]
return 1 if tag is INVERSES[mtag]
debt[mtag] += 1
val: if mtag is 'INDENT' then match[1] else INVERSES[mtag]
@tokens.splice i, 0, [INVERSES[mtag], val]
return 1
else
return 1 |
Constants | |
| List of the token pairs that must be balanced. | BALANCED_PAIRS: [['(', ')'], ['[', ']'], ['{', '}'], ['INDENT', 'OUTDENT'],
['PARAM_START', 'PARAM_END'], ['CALL_START', 'CALL_END'],
['INDEX_START', 'INDEX_END'], ['SOAKED_INDEX_START', 'SOAKED_INDEX_END']] |
| The inverse mappings of | INVERSES: {}
for pair in BALANCED_PAIRS
INVERSES[pair[0]]: pair[1]
INVERSES[pair[1]]: pair[0] |
| The tokens that signal the start of a balanced pair. | EXPRESSION_START: pair[0] for pair in BALANCED_PAIRS |
| The tokens that signal the end of a balanced pair. | EXPRESSION_END: pair[1] for pair in BALANCED_PAIRS |
| Tokens that indicate the close of a clause of an expression. | EXPRESSION_CLOSE: ['CATCH', 'WHEN', 'ELSE', 'FINALLY'].concat EXPRESSION_END |
| Tokens that, if followed by an | IMPLICIT_FUNC: ['IDENTIFIER', 'SUPER', ')', 'CALL_END', ']', 'INDEX_END'] |
| If preceded by an | IMPLICIT_CALL: ['IDENTIFIER', 'NUMBER', 'STRING', 'JS', 'REGEX', 'NEW', 'PARAM_START',
'TRY', 'DELETE', 'TYPEOF', 'SWITCH',
'TRUE', 'FALSE', 'YES', 'NO', 'ON', 'OFF', '!', '!!', 'NOT',
'@', '->', '=>', '[', '(', '{'] |
| Tokens indicating that the implicit call must enclose a block of expressions. | IMPLICIT_BLOCK: ['->', '=>', '{', '[', ','] |
| Tokens that always mark the end of an implicit call for single-liners. | IMPLICIT_END: ['IF', 'UNLESS', 'FOR', 'WHILE', 'TERMINATOR', 'INDENT', 'OUTDENT'] |
| Single-line flavors of block expressions that have unclosed endings. The grammar can't disambiguate them, so we insert the implicit indentation. | SINGLE_LINERS: ['ELSE', "->", "=>", 'TRY', 'FINALLY', 'THEN']
SINGLE_CLOSERS: ['TERMINATOR', 'CATCH', 'FINALLY', 'ELSE', 'OUTDENT', 'LEADING_WHEN'] |
Utility Functions | |
| Does a list include a value? | include: (list, value) ->
list.indexOf(value) >= 0
|