rewriter.coffee

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rewriter.coffee
¶

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.

Create a generated token: one that exists due to a use of implicit syntax.
```
generate = (tag, value) ->
    tok = [tag, value]
    tok.generated = yes
    tok
```
¶

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) ->
    @removeLeadingNewlines()
    @removeMidExpressionNewlines()
    @closeOpenCalls()
    @closeOpenIndexes()
    @addImplicitIndentation()
    @tagPostfixConditionals()
    @addImplicitBracesAndParens()
    @addLocationDataToGeneratedTokens()
    @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]
    true

  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
      0

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'
      1

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'
      1

Match tags in token stream starting at i with pattern, skipping HERECOMMENTs Pattern may consist of strings (equality), an array of strings (one of) or null (wildcard)

  matchTags: (i, pattern...) ->
    fuzz = 0
    for j in [0 ... pattern.length]
      fuzz += 2 while @tag(i + j + fuzz) is 'HERECOMMENT'
      continue if not pattern[j]?
      pattern[j] = [pattern[j]] if typeof pattern[j] is 'string'
      return no if @tag(i + j + fuzz) not in pattern[j]
    yes

yes iff standing in front of something looking like @: or :, skipping over 'HERECOMMENT's

  looksObjectish: (j) ->
    @matchTags(j, '@', null, ':') or @matchTags(j, null, ':')

yes iff current line of tokens contain an element of tags on same expression level. Stop searching at LINEBREAKS or explicit start of containing balanced expression.

  findTagsBackwards: (i, tags) ->
    backStack = []
    while i >= 0 and (backStack.length or
          @tag(i) not in tags and
          (@tag(i) not in EXPRESSION_START or @tokens[i].generated) and
          @tag(i) not in LINEBREAKS)
      backStack.push @tag(i) if @tag(i) in EXPRESSION_END
      backStack.pop() if @tag(i) in EXPRESSION_START and backStack.length
      i -= 1
    @tag(i) in tags

¶

Look for signs of implicit calls and objects in the token stream and add them.
```
  addImplicitBracesAndParens: ->
```

Track current balancing depth (both implicit and explicit) on stack.

    stack = []

    @scanTokens (token, i, tokens) ->
      [tag]     = token
      [prevTag] = if i > 0 then tokens[i - 1] else []
      [nextTag] = if i < tokens.length - 1 then tokens[i + 1] else []
      stackTop  = -> stack[stack.length - 1]
      startIdx  = i

¶

Helper function, used for keeping track of the number of tokens consumed and spliced, when returning for getting a new token.
```
      forward   = (n) -> i - startIdx + n
```

Helper functions

      inImplicit        = -> stackTop()?[2]?.ours
      inImplicitCall    = -> inImplicit() and stackTop()?[0] is '('
      inImplicitObject  = -> inImplicit() and stackTop()?[0] is '{'

Unclosed control statement inside implicit parens (like class declaration or if-conditionals)

      inImplicitControl = -> inImplicit and stackTop()?[0] is 'CONTROL'

      startImplicitCall = (j) ->
        idx = j ? i
        stack.push ['(', idx, ours: yes]
        tokens.splice idx, 0, generate 'CALL_START', '('
        i += 1 if not j?

      endImplicitCall = ->
        stack.pop()
        tokens.splice i, 0, generate 'CALL_END', ')'
        i += 1

      startImplicitObject = (j, startsLine = yes) ->
        idx = j ? i
        stack.push ['{', idx, sameLine: yes, startsLine: startsLine, ours: yes]
        tokens.splice idx, 0, generate '{', generate(new String('{'))
        i += 1 if not j?

      endImplicitObject = (j) ->
        j = j ? i
        stack.pop()
        tokens.splice j, 0, generate '}', '}'
        i += 1

Don't end an implicit call on next indent if any of these are in an argument

      if inImplicitCall() and tag in ['IF', 'TRY', 'FINALLY', 'CATCH',
        'CLASS', 'SWITCH']
        stack.push ['CONTROL', i, ours: true]
        return forward(1)

      if tag is 'INDENT' and inImplicit()

An INDENT closes an implicit call unless

We have seen a CONTROL argument on the line.
The last token before the indent is part of the list below

        if prevTag not in ['=>', '->', '[', '(', ',', '{', 'TRY', 'ELSE', '=']
          endImplicitCall() while inImplicitCall()
        stack.pop() if inImplicitControl()
        stack.push [tag, i]
        return forward(1)

Straightforward start of explicit expression

      if tag in EXPRESSION_START
        stack.push [tag, i]
        return forward(1)

Close all implicit expressions inside of explicitly closed expressions.

      if tag in EXPRESSION_END
        while inImplicit()
          if inImplicitCall()
            endImplicitCall()
          else if inImplicitObject()
            endImplicitObject()
          else
            stack.pop()
        stack.pop()

Recognize standard implicit calls like f a, f() b, f? c, h[0] d etc.

      if (tag in IMPLICIT_FUNC and token.spaced and not token.stringEnd or
          tag is '?' and i > 0 and not tokens[i - 1].spaced) and
         (nextTag in IMPLICIT_CALL or
          nextTag in IMPLICIT_UNSPACED_CALL and
          not tokens[i + 1]?.spaced and not tokens[i + 1]?.newLine)
        tag = token[0] = 'FUNC_EXIST' if tag is '?'
        startImplicitCall i + 1
        ret = forward(2)
        return ret

Implicit call taking an implicit indented object as first argument.

f
  a: b
  c: d

and

f
  1
  a: b
  b: c

Don't accept implicit calls of this type, when on the same line as the control strucutures below as that may misinterpret constructs like:

if f
   a: 1

if f(a: 1)

which is probably always unintended. Furthermore don't allow this in literal arrays, as that creates grammatical ambiguities.

      if tag in IMPLICIT_FUNC and @matchTags(i + 1, 'INDENT', null, ':') and
         not @findTagsBackwards(i, ['CLASS', 'EXTENDS', 'IF', 'CATCH',
          'SWITCH', 'LEADING_WHEN', 'FOR', 'WHILE', 'UNTIL'])
        startImplicitCall i + 1
        stack.push ['INDENT', i + 2]
        return forward(3)

¶

Implicit objects start here
```
      if tag is ':'
```

Go back to the (implicit) start of the object

        if @tag(i - 2) is '@' then s = i - 2 else s = i - 1
        s -= 2 while @tag(s - 2) is 'HERECOMMENT'

        startsLine = s is 0 or @tag(s - 1) in LINEBREAKS or tokens[s - 1].newLine

Are we just continuing an already declared object?

        if stackTop()
          [stackTag, stackIdx] = stackTop()
          if (stackTag is '{' or stackTag is 'INDENT' and @tag(stackIdx - 1) is '{') and
             (startsLine or @tag(s - 1) is ',' or @tag(s - 1) is '{')
            return forward(1)

        startImplicitObject(s, !!startsLine)
        return forward(2)

End implicit calls when chaining method calls like e.g.:

f ->
  a
.g b, ->
  c
.h a

      if prevTag is 'OUTDENT' and inImplicitCall() and tag in ['.', '?.', '::', '?::']
        endImplicitCall()
        return forward(1)

      stackTop()[2].sameLine = no if inImplicitObject() and tag in LINEBREAKS

      if tag in IMPLICIT_END
        while inImplicit()
          [stackTag, stackIdx, {sameLine, startsLine}] = stackTop()

Close implicit calls when reached end of argument list

          if inImplicitCall() and prevTag isnt ','
            endImplicitCall()

Close implicit objects such as: return a: 1, b: 2 unless true

          else if inImplicitObject() and sameLine and not startsLine
            endImplicitObject()

Close implicit objects when at end of line, line didn't end with a comma and the implicit object didn't start the line or the next line doesn't look like the continuation of an object.

          else if inImplicitObject() and tag is 'TERMINATOR' and prevTag isnt ',' and
                  not (startsLine and @looksObjectish(i + 1))
            endImplicitObject()
          else
            break

Close implicit object if comma is the last character and what comes after doesn't look like it belongs. This is used for trailing commas and calls, like:

x =
    a: b,
    c: d,
e = 2

and

f a, b: c, d: e, f, g: h: i, j

      if tag is ',' and not @looksObjectish(i + 1) and inImplicitObject() and
         (nextTag isnt 'TERMINATOR' or not @looksObjectish(i + 2))

¶

When nextTag is OUTDENT the comma is insignificant and should just be ignored so embed it in the implicit object.

When it isn't the comma go on to play a role in a call or array further up the stack, so give it a chance.
```
        offset = if nextTag is 'OUTDENT' then 1 else 0
        while inImplicitObject()
          endImplicitObject i + offset
      return forward(1)
```

Add location data to all tokens generated by the rewriter.

  addLocationDataToGeneratedTokens: ->
    @scanTokens (token, i, tokens) ->
      return 1 if     token[2]
      return 1 unless token.generated or token.explicit
      if token[0] is '{' and nextLocation=tokens[i + 1]?[2]
          {first_line: line, first_column: column} = nextLocation
      else if prevLocation = tokens[i - 1]?[2]
          {last_line: line, last_column: column} = prevLocation
      else
          line = column = 0
      token[2] =
        first_line:   line
        first_column: column
        last_line:    line
        last_column:  column
      return 1

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 isnt 'THEN') and
      not (token[0] in ['CATCH', 'FINALLY'] and starter in ['->', '=>'])

    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()...
        return 2
      if tag is 'CATCH'
        for j in [1..2] when @tag(i + j) in ['OUTDENT', 'TERMINATOR', 'FINALLY']
          tokens.splice i + j, 0, @indentation()...
          return 2 + j
      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 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) ->
      [tag] = token
      [prevTag] = @tokens[i - 1]
      tag is 'TERMINATOR' or (tag is 'INDENT' and prevTag not in SINGLE_LINERS)

    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
      return 1

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

  indentation: (implicit = no) ->
    indent  = ['INDENT', 2]
    outdent = ['OUTDENT', 2]
    indent.generated = outdent.generated = yes if implicit
    indent.explicit = outdent.explicit = yes if not implicit
    [indent, outdent]

  generate: generate

¶

Look up a tag by token index.
```
  tag: (i) -> @tokens[i]?[0]
```
¶

Constants

List of the token pairs that must be balanced.

BALANCED_PAIRS = [
  ['(', ')']
  ['[', ']']
  ['{', '}']
  ['INDENT', 'OUTDENT'],
  ['CALL_START', 'CALL_END']
  ['PARAM_START', 'PARAM_END']
  ['INDEX_START', 'INDEX_END']
]

¶

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.

EXPRESSION_START = []
EXPRESSION_END   = []

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

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_CALL, indicate a function invocation.

IMPLICIT_FUNC    = ['IDENTIFIER', 'SUPER', ')', 'CALL_END', ']', 'INDEX_END', '@', 'THIS']

If preceded by an IMPLICIT_FUNC, indicates a function invocation.

IMPLICIT_CALL    = [
  'IDENTIFIER', 'NUMBER', 'STRING', 'JS', 'REGEX', 'NEW', 'PARAM_START', 'CLASS'
  'IF', 'TRY', 'SWITCH', 'THIS', 'BOOL', 'NULL', 'UNDEFINED', 'UNARY', 'SUPER'
  'THROW', '@', '->', '=>', '[', '(', '{', '--', '++'
]

IMPLICIT_UNSPACED_CALL = ['+', '-']

Tokens that always mark the end of an implicit call for single-liners.

IMPLICIT_END     = ['POST_IF', 'FOR', 'WHILE', 'UNTIL', 'WHEN', 'BY',
  'LOOP', 'TERMINATOR']

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']

Tokens that end a line.

LINEBREAKS       = ['TERMINATOR', 'INDENT', 'OUTDENT']

iced additions

IMPLICIT_FUNC.push 'DEFER'
IMPLICIT_CALL.push 'DEFER'
IMPLICIT_END.push 'AWAIT'

rewriter.coffee

Constants