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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, origin) ->
        tok = [tag, value]
        tok.generated = yes
        tok.origin = origin if origin
        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()
          @closeOpenCalls()
          @closeOpenIndexes()
          @normalizeLines()
          @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

- ¶

  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 ‘HERECOMMENT’s. `pattern` may consist of strings (equality), an array of strings (one of) or null (wildcard). Returns the index of the match or -1 if no match.

        indexOfTag: (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 -1 if @tag(i + j + fuzz) not in pattern[j]
          i + j + fuzz - 1

- ¶

  Returns `yes` if standing in front of something looking like `@<x>:`, `<x>:` or `<EXPRESSION_START><x>...<EXPRESSION_END>:`, skipping over ‘HERECOMMENT’s.

        looksObjectish: (j) ->
          return yes if @indexOfTag(j, '@', null, ':') > -1 or @indexOfTag(j, null, ':') > -1
          index = @indexOfTag(j, EXPRESSION_START)
          if index > -1
            end = null
            @detectEnd index + 1, ((token) -> token[0] in EXPRESSION_END), ((token, i) -> end = i)
            return yes if @tag(end + 1) is ':'
          no

- ¶

  Returns `yes` if 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 = []
          start = null

          @scanTokens (token, i, tokens) ->
            [tag]     = token
            [prevTag] = prevToken = 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', ')', ['', 'end of input', token[2]]
              i += 1

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

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

  1. We have seen a `CONTROL` argument on the line.
  2. 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()
              start = 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 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
              return forward(2)

- ¶

  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

  as

      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
               @indexOfTag(i + 1, 'INDENT') > -1 and @looksObjectish(i + 2) 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

              s = switch
                when @tag(i - 1) in EXPRESSION_END then start[1]
                when @tag(i - 2) is '@' then i - 2
                else i - 1
              s -= 2 while @tag(s - 2) is 'HERECOMMENT'

- ¶

  Mark if the value is a for loop

              @insideForDeclaration = nextTag is 'FOR'

              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

  and also

      f a
      .g b
      .h a

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

            newLine = prevTag is 'OUTDENT' or prevToken.newLine
            if tag in IMPLICIT_END or tag in CALL_CLOSERS and newLine
              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 not @insideForDeclaration and sameLine and
                        tag isnt 'TERMINATOR' and prevTag isnt ':'
                  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))
                  return forward 1 if nextTag is 'HERECOMMENT'
                  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
               not @insideForDeclaration 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. To keep the grammar clean and tidy, trailing newlines within expressions are removed and the indentation tokens of empty blocks are added.

        normalizeLines: ->
          starter = indent = outdent = null

          condition = (token, i) ->
            token[1] isnt ';' and token[0] in SINGLE_CLOSERS and
            not (token[0] is 'TERMINATOR' and @tag(i + 1) in EXPRESSION_CLOSE) and
            not (token[0] is 'ELSE' and starter isnt 'THEN') and
            not (token[0] in ['CATCH', 'FINALLY'] and starter in ['->', '=>']) or
            token[0] in CALL_CLOSERS and @tokens[i - 1].newLine

          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'
              if @tag(i + 1) is 'ELSE' and @tag(i - 1) isnt 'OUTDENT'
                tokens.splice i, 1, @indentation()...
                return 1
              if @tag(i + 1) in EXPRESSION_CLOSE
                tokens.splice i, 1
                return 0
            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 tokens[i]
              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: (origin) ->
          indent  = ['INDENT', 2]
          outdent = ['OUTDENT', 2]
          if origin
            indent.generated = outdent.generated = yes
            indent.origin = outdent.origin = origin
          else
            indent.explicit = outdent.explicit = yes
          [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']
        ['STRING_START', 'STRING_END']
        ['REGEX_START', 'REGEX_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', 'THEN', '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', 'STRING_START', 'JS', 'REGEX', 'REGEX_START'
        'NEW', 'PARAM_START', 'CLASS', 'IF', 'TRY', 'SWITCH', 'THIS', 'BOOL', 'NULL'
        'UNDEFINED', 'UNARY', 'YIELD', 'UNARY_MATH', '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']

- ¶

  Tokens that close open calls when they follow a newline.

      CALL_CLOSERS     = ['.', '?.', '::', '?::']