Class: Parslet::Atoms::Lookahead

Inherits:
Base
  • Object
show all
Defined in:
lib/parslet/atoms/lookahead.rb,
lib/parslet/atoms/visitor.rb

Overview

Either positive or negative lookahead, doesn’t consume its input.

Example: 

str('foo').present? # matches when the input contains 'foo', but leaves it

Constant Summary

Constants included from Precedence

Precedence::ALTERNATE, Precedence::BASE, Precedence::LOOKAHEAD, Precedence::OUTER, Precedence::REPETITION, Precedence::SEQUENCE

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods inherited from Base

#apply, #inspect, #parse, #parse_with_debug, precedence, #setup_and_apply, #to_s

Methods included from CanFlatten

#flatten, #flatten_repetition, #flatten_sequence, #foldl, #merge_fold, #warn_about_duplicate_keys

Methods included from DSL

#>>, #absent?, #as, #maybe, #present?, #repeat, #|

Constructor Details

#initialize(bound_parslet, positive = true) ⇒ Lookahead

Returns a new instance of Lookahead.



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# File 'lib/parslet/atoms/lookahead.rb', line 11

def initialize(bound_parslet, positive=true)
  super()
  
  # Model positive and negative lookahead by testing this flag.
  @positive = positive
  @bound_parslet = bound_parslet
  
  @error_msgs = {
    :positive => ["Input should start with ", bound_parslet], 
    :negative => ["Input should not start with ", bound_parslet]
  }
end

Instance Attribute Details

#bound_parslet ⇒ Object (readonly)

Returns the value of attribute bound_parslet.



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# File 'lib/parslet/atoms/lookahead.rb', line 9

def bound_parslet
  @bound_parslet
end

#positive ⇒ Object (readonly)

Returns the value of attribute positive.



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# File 'lib/parslet/atoms/lookahead.rb', line 8

def positive
  @positive
end

Instance Method Details

#accept(visitor) ⇒ Object

Call back visitors #visit_lookahead method. See parslet/export for an example.



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# File 'lib/parslet/atoms/visitor.rb', line 69

def accept(visitor)
  visitor.visit_lookahead(positive, bound_parslet)
end

#to_s_inner(prec) ⇒ Object 



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# File 'lib/parslet/atoms/lookahead.rb', line 44

def to_s_inner(prec)
  char = positive ? '&' : '!'
  
  "#{char}#{bound_parslet.to_s(prec)}"
end

#try(source, context) ⇒ Object 



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# File 'lib/parslet/atoms/lookahead.rb', line 24

def try(source, context)
  pos = source.pos

  success, value = bound_parslet.apply(source, context)
  
  if positive
    return succ(nil) if success
    return context.err_at(self, source, @error_msgs[:positive], pos)
  else
    return succ(nil) unless success
    return context.err_at(self, source, @error_msgs[:negative], pos)
  end
  
# This is probably the only parslet that rewinds its input in #try.
# Lookaheads NEVER consume their input, even on success, that's why. 
ensure 
  source.pos = pos
end