git @ Cat's Eye Technologies Fountain / 06bc2be
Add some basic documentation on lookahead assertions. Chris Pressey 11 months ago
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0 Lookahead Assertions
1 ====================
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3 Fountain has (or will have) a general notion of lookahead assertions.
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5 A lookahead assertion is a constraint that says, "at this point, if
6 we tried to parse S (where S is some sub-grammar), we would succeed".
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8 There are a few things to note about them.
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10 A lookahead assertion does not consume any input.
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12 In the presence of lookahead assertions, an ordered choice operator
13 is not necessary, because ordered choice can be simulated. Suppose
14 ordered choice is denoted by `/`, and `?A` is our lookahead assertion
15 operator. Then
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17 A / B == ?B A | B
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19 Lookahead assertions can also serve the same function as conjunctive
20 grammars; we can assert that the tokens to come conform to one
21 syntactic structure, and actually parse a second syntactic structure,
22 meaning the parse will only succeed if the string conforms to both
23 structures -- the same as conjunction in conjuctive grammars.
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25 Lookahead assertions only make sense during parsing, so in the context
26 of Fountain, we need to figure out what to do with them during generation.
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28 We will disgregard them completely during generation. They do not
29 evaluate to either true or false; they are simply not evaluated at all.
30 They are removed from the constraint expressions that will be evaluated.