Goparsec: Maybe combinator

Parser combinator is a technique to build top down parsing logic in any language, for more detail on that refer to Parser Combinator wiki page, rest of this article describe a use case with goparsec tool written in golang.

Following code uses two types of combinator logic, And and Maybe:

• And combinator forces that all three tokens should match.
• Maybe combinator gives an exception on the second token, which can be optional.

Prior to this commit, there was an issue with goparsec tool, which failed to parse the second i/p text text: "hello" false:

text: "hello" 10 false
parsed 3 items
index:0 string
index:1 parsec.Terminal
index:2 parsec.Terminal
......
text: "hello" false

With a fix to goparsec tool, our parsec works fine with the second i/p text.

text: "hello" 10 false
parsed 3 items
index:0 string
index:1 parsec.Terminal
index:2 parsec.Terminal
......
text: "hello" false
parsed 3 items
index:0 string
index:1 parsec.MaybeNone
index:2 parsec.Terminal

Note that the second item, index:1, is parsed as parsec.MaybeNone, which indicates that the second token was not present in the input text. Whereas the first, index:0 and the third, index:2 nodes are parsed in same way as that of the first i/p.