50 lines
1.3 KiB
Ruby
50 lines
1.3 KiB
Ruby
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# Either positive or negative lookahead, doesn't consume its input.
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#
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# Example:
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#
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# str('foo').present? # matches when the input contains 'foo', but leaves it
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#
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class Parslet::Atoms::Lookahead < Parslet::Atoms::Base
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attr_reader :positive
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attr_reader :bound_parslet
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def initialize(bound_parslet, positive=true)
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super()
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# Model positive and negative lookahead by testing this flag.
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@positive = positive
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@bound_parslet = bound_parslet
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@error_msgs = {
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:positive => ["Input should start with ", bound_parslet],
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:negative => ["Input should not start with ", bound_parslet]
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}
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end
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def try(source, context, consume_all)
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pos = source.pos
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success, value = bound_parslet.apply(source, context, consume_all)
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if positive
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return succ(nil) if success
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return context.err_at(self, source, @error_msgs[:positive], pos)
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else
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return succ(nil) unless success
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return context.err_at(self, source, @error_msgs[:negative], pos)
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end
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# This is probably the only parslet that rewinds its input in #try.
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# Lookaheads NEVER consume their input, even on success, that's why.
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ensure
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source.pos = pos
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end
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precedence LOOKAHEAD
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def to_s_inner(prec)
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char = positive ? '&' : '!'
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"#{char}#{bound_parslet.to_s(prec)}"
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end
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end
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