optP -- making a parser optional

Synopsis

• 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

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