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

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