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# Either positive or negative lookahead, doesn't consume its input.
#
# Example:
#
# str('foo').present? # matches when the input contains 'foo', but leaves it
#
class Parslet::Atoms::Lookahead < Parslet::Atoms::Base
attr_reader :positive
attr_reader :bound_parslet
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
def try(source, context, consume_all)
rewind_pos = source.bytepos
error_pos = source.pos
success, value = bound_parslet.apply(source, context, consume_all)
if positive
return succ(nil) if success
return context.err_at(self, source, @error_msgs[:positive], error_pos)
else
return succ(nil) unless success
return context.err_at(self, source, @error_msgs[:negative], error_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.bytepos = rewind_pos
end
precedence LOOKAHEAD
def to_s_inner(prec)
char = positive ? '&' : '!'
"#{char}#{bound_parslet.to_s(prec)}"
end
end
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