Jump To …


The CoffeeScript language has a good deal of optional syntax, implicit syntax, and shorthand syntax. This 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, and generally clean things up.

The Rewriter class is used by the Lexer, directly against its internal array of tokens.

class exports.Rewriter

Helpful snippet for debugging: console.log (t[0] + '/' + t[1] for t in @tokens).join ' '

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) ->

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.

  scanTokens: (block) ->
    {tokens} = this
    i = 0
    i += block.call this, token, i, tokens while token = tokens[i]

  detectEnd: (i, condition, action) ->
    {tokens} = this
    levels = 0
    while token = tokens[i]
      return action.call this, token, i     if levels is 0 and condition.call this, token, i
      return action.call this, token, i - 1 if not token or levels < 0
      if token[0] in EXPRESSION_START
        levels += 1
      else if token[0] in EXPRESSION_END
        levels -= 1
      i += 1
    i - 1

Leading newlines would introduce an ambiguity in the grammar, so we dispatch them here.

  removeLeadingNewlines: ->
    break for [tag], i in @tokens when tag isnt 'TERMINATOR'
    @tokens.splice 0, i if i

Some blocks occur in the middle of expressions -- when we're expecting this, remove their trailing newlines.

  removeMidExpressionNewlines: ->
    @scanTokens (token, i, tokens) ->
      return 1 unless token[0] is 'TERMINATOR' and @tag(i + 1) in EXPRESSION_CLOSE
      tokens.splice i, 1

The lexer has tagged the opening parenthesis of a method call. Match it with its paired close. We have the mis-nested outdent case included here for calls that close on the same line, just before their outdent.

  closeOpenCalls: ->

    condition = (token, i) ->
      token[0] in [')', 'CALL_END'] or
      token[0] is 'OUTDENT' and @tag(i - 1) is ')'

    action = (token, i) ->
      @tokens[if token[0] is 'OUTDENT' then i - 1 else i][0] = 'CALL_END'

    @scanTokens (token, i) ->
      @detectEnd i + 1, condition, action if token[0] is 'CALL_START'

The lexer has tagged the opening parenthesis of an indexing operation call. Match it with its paired close.

  closeOpenIndexes: ->

    condition = (token, i) ->
      token[0] in [']', 'INDEX_END']

    action = (token, i) ->
      token[0] = 'INDEX_END'

    @scanTokens (token, i) ->
      @detectEnd i + 1, condition, action if token[0] is 'INDEX_START'

Object literals may be written with implicit braces, for simple cases. Insert the missing braces here, so that the parser doesn't have to.

  addImplicitBraces: ->

    stack       = []
    start       = null
    startsLine  = null
    sameLine    = yes
    startIndent = 0
    startIndex  = 0

    condition = (token, i) ->
      [one, two, three] = @tokens[i + 1 .. i + 3]
      return no if 'HERECOMMENT' is one?[0]
      [tag] = token
      sameLine = no if tag in LINEBREAKS
      return (
        (tag in ['TERMINATOR', 'OUTDENT'] or 
          (tag in IMPLICIT_END and sameLine and not (i - startIndex is 1))) and
        ((!startsLine and @tag(i - 1) isnt ',') or
          not (two?[0] is ':' or one?[0] is '@' and three?[0] is ':'))) or
        (tag is ',' and one and
          one[0] not in ['IDENTIFIER', 'NUMBER', 'STRING', '@', 'TERMINATOR', 'OUTDENT']

    action = (token, i) ->
      tok = @generate '}', '}', token[2]
      @tokens.splice i, 0, tok

    @scanTokens (token, i, tokens) ->
      if (tag = token[0]) in EXPRESSION_START
        stack.push [(if tag is 'INDENT' and @tag(i - 1) is '{' then '{' else tag), i]
        return 1
      if tag in EXPRESSION_END
        start = stack.pop()
        return 1
      return 1 unless tag is ':' and
        ((ago = @tag i - 2) is ':' or stack[stack.length - 1]?[0] isnt '{')
      sameLine = yes
      startIndex = i + 1
      stack.push ['{']
      idx =  if ago is '@' then i - 2 else i - 1
      idx -= 2 while @tag(idx - 2) is 'HERECOMMENT'
      prevTag = @tag(idx - 1)
      startsLine = not prevTag or (prevTag in LINEBREAKS)
      value = new String('{')
      value.generated = yes
      tok = @generate '{', value, token[2]
      tokens.splice idx, 0, tok
      @detectEnd i + 2, condition, action

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.

  addImplicitParentheses: ->

    noCall = seenSingle = seenControl = no

    condition = (token, i) ->
      [tag] = token
      return yes if not seenSingle and token.fromThen
      seenSingle  = yes if tag in ['IF', 'ELSE', 'CATCH', '->', '=>', 'CLASS']
      seenControl = yes if tag in ['IF', 'ELSE', 'SWITCH', 'TRY', '=']
      return yes if tag in ['.', '?.', '::'] and @tag(i - 1) is 'OUTDENT'
      not token.generated and @tag(i - 1) isnt ',' and (tag in IMPLICIT_END or
        (tag is 'INDENT' and not seenControl)) and
        (tag isnt 'INDENT' or
          (@tag(i - 2) not in ['CLASS', 'EXTENDS'] and @tag(i - 1) not in IMPLICIT_BLOCK and
          not ((post = @tokens[i + 1]) and post.generated and post[0] is '{')))

    action = (token, i) ->
      @tokens.splice i, 0, @generate 'CALL_END', ')', token[2]

    @scanTokens (token, i, tokens) ->
      tag     = token[0]
      noCall  = yes if tag in ['CLASS', 'IF', 'FOR', 'WHILE']
      [prev, current, next] = tokens[i - 1 .. i + 1]
      callObject  = not noCall and tag is 'INDENT' and
                    next and next.generated and next[0] is '{' and
                    prev and prev[0] in IMPLICIT_FUNC
      seenSingle  = no
      seenControl = no
      noCall      = no if tag in LINEBREAKS
      token.call  = yes if prev and not prev.spaced and tag is '?'
      return 1 if token.fromThen
      return 1 unless callObject or
        prev?.spaced and (prev.call or prev[0] in IMPLICIT_FUNC) and
        (tag in IMPLICIT_CALL or not (token.spaced or token.newLine) and tag in IMPLICIT_UNSPACED_CALL)
      tokens.splice i, 0, @generate 'CALL_START', '(', token[2]
      @detectEnd i + 1, condition, action
      prev[0] = 'FUNC_EXIST' if prev[0] is '?'

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.

  addImplicitIndentation: ->

    starter = indent = outdent = null

    condition = (token, i) ->
      token[1] isnt ';' and token[0] in SINGLE_CLOSERS and
      not (token[0] is 'ELSE' and starter not in ['IF', 'THEN'])

    action = (token, i) ->
      @tokens.splice (if @tag(i - 1) is ',' then i - 1 else i), 0, outdent

    @scanTokens (token, i, tokens) ->
      [tag] = token
      if tag is 'TERMINATOR' and @tag(i + 1) is 'THEN'
        tokens.splice i, 1
        return 0
      if tag is 'ELSE' and @tag(i - 1) isnt 'OUTDENT'
        tokens.splice i, 0, @indentation(token)...
        return 2
      if tag is 'CATCH' and @tag(i + 2) in ['OUTDENT', 'TERMINATOR', 'FINALLY']
        tokens.splice i + 2, 0, @indentation(token)...
        return 4
      if tag in SINGLE_LINERS and @tag(i + 1) isnt 'INDENT' and
         not (tag is 'ELSE' and @tag(i + 1) is 'IF')
        starter = tag
        [indent, outdent] = @indentation token, yes
        indent.fromThen   = true if starter is 'THEN'
        tokens.splice i + 1, 0, indent
        @detectEnd i + 2, condition, action
        tokens.splice i, 1 if tag is 'THEN'
        return 1
      return 1

Tag postfix conditionals as such, so that we can parse them with a different precedence.

  tagPostfixConditionals: ->

    original = null

    condition = (token, i) ->
      token[0] in ['TERMINATOR', 'INDENT']

    action = (token, i) ->
      if token[0] isnt 'INDENT' or (token.generated and not token.fromThen)
        original[0] = 'POST_' + original[0]

    @scanTokens (token, i) ->
      return 1 unless token[0] is 'IF'
      original = token
      @detectEnd i + 1, condition, action

Generate the indentation tokens, based on another token on the same line.

  indentation: (token, implicit = no) ->
    indent  = ['INDENT', 2, token[2]]
    outdent = ['OUTDENT', 2, token[2]]
    indent.generated = outdent.generated = yes if implicit
    [indent, outdent]

Create a generated token: one that exists due to a use of implicit syntax.

  generate: (tag, value, line) ->
    tok = [tag, value, line]
    tok.generated = yes

Look up a tag by token index.

  tag: (i) -> @tokens[i]?[0]


List of the token pairs that must be balanced.

  ['(', ')']
  ['[', ']']
  ['{', '}']

The inverse mappings of BALANCED_PAIRS we're trying to fix up, so we can look things up from either end.

exports.INVERSES = INVERSES = {}

The tokens that signal the start/end of a balanced pair.


for [left, rite] in BALANCED_PAIRS
  EXPRESSION_END  .push INVERSES[left] = rite

Tokens that indicate the close of a clause of an expression.


Tokens that, if followed by an IMPLICIT_CALL, indicate a function invocation.


If preceded by an IMPLICIT_FUNC, indicates a function invocation.

  '@', '->', '=>', '[', '(', '{', '--', '++'


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.


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']

Tokens that end a line.