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PEGParser

PEGParser is a PEG Parser for Julia with Packrat capabilties. PEGParser was inspired by pyparsing, parsimonious, boost::spirit, as well as several others.

Defining a grammar

To define a grammar you can write:

@grammar <name> begin
  rule1 = ...
  rule2 = ...
  ...
end

Allowed rules

The following rules can be used:

  • Terminals: Strings and characters
  • Or: a | b | c
  • And: a + b + c
  • Grouping: (a + b) | (c + d)
  • Optional: ?(a + b)
  • One or more: +((a + b) | (c + d))
  • Zero or more: *((a + b) | (c + d))
  • Regular expressions: r"[a-zA-Z]+"
  • Lists: list(a+b, c)

TODO

Multiple: (a+b)^(3, 5)

Example 1

Suppose you want a parser that takes input and converts [text] into <b>text<>. You can write the following grammar:

@grammar markup begin
  # this is the standard start rule
  start = bold_text

  # compose a sequence
  bold_text = bold_open + text + bold_code

  # use a regular expression to define the text
  text = r"[a-zA-z]"

  bold_open = '['
  bold_close = ']'
end

The first step in using the grammar is to create an AST from a given input:

(ast, pos, error) = parse(markup, "[test]")

The variable ast contains the AST which can be transformed to the desired result. To do so, first a mapping of the node names to transform has to established:

tohtml(node, cvalues, ::MatchRule{:bold_open}) = "<b>"
tohtml(node, cvalues, ::MatchRule{:bold_close}) = "</b>"
tohtml(node, cvalues, ::MatchRule{:text}) = node.value
tohtml(node, cvalues, ::MatchRule{:bold_text}) = join(cvalues)

And finally:

result = transform(tohtml, ast)
println(result) # "<b>test</b>"

Example 2

Transforms can also be used to calculate a value from the tree. Consider the standard calculator app:

@grammar calc begin
  start = expr
  number = r"([0-9]+)"
  expr = (term + op1 + expr) | term
  term = (factor + op2 + term) | factor
  factor = number | pfactor
  pfactor = lparen + expr + rparen
  op1 = '+' | '-'
  op2 = '*' | '/'
  lparen = "("
  rparen = ")"
end

And to use the grammar:

(node, pos, error) = parse(grammar, "5*(42+3+6+10+2)")

# A ::MatchRule{:default} can be specified and will be used for anything that isn't
# explicitely defined and is not on the ignore list
evaluate(node, cvalues, ::MatchRule{:number}) = float(node.value)
evaluate(node, cvalues, ::MatchRule{:expr}) = 
  length(children) == 1 ? children : eval(Expr(:call, cvalues[2], cvalues[1], cvalues[3]))
evaluate(node, cvalues, ::MatchRule{:factor}) = cvalues
evaluate(node, cvalues, ::MatchRule{:pfactor}) = cvalue 
evaluate(node, cvalues, ::MatchRule{:term}) = 
  length(children) == 1 ? children : eval(Expr(:call, cvalues[2], cvalues[1], cvalues[3]))
evaluate(node, cvalues, ::MatchRule{:op1}) = symbol(node.value)
evaluate(node, cvalues, ::MatchRule{:op2}) = symbol(node.value)

# Note: the ignore list -- these will produce no output when encountered.
result = transform(math, node, ignore=[:lparen, :rparen])

println(result) # 315.0

Caveats

This is still very much a work in progress and doesn't yet have as much test coverage as I would like.

The error handling still needs a lot of work. Currently only a single error will be emitted, but the hope is to allow multiple errors to be returned.