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optP -- making a parser optional

Usage:

optP p
Inputs:
- p, a parser
Outputs:
- a parser, a parser that accepts tokens accepted by p and returns the value returned by p, or accepts the empty sequence of tokens and returns the symbol nil. It prefers to do the former.

Description

After the first token, the parser is no slower than p would have been.

i1 : (optP constParser "abc" : charAnalyzer) "abc"

o1 = abc

i2 : (optP constParser "abc" : charAnalyzer) ""

o2 = nil

o2 : Symbol

See also

constParser -- produce a parser that accepts a fixed string, one character at a time
charAnalyzer -- a lexical analyzer that provides characters from a string one at a time
Parser : Analyzer -- combine a parser with a lexical analyzer to make a complete system

For the programmer

The object optP is a function closure.

The source of this document is in Parsing.m2:502:0.