Support a custom reducer when using infix_expression

[chrismwendt]

This adds support for a custom reducer when using infix_expression. This allows you to change the way the tree is generated:

p = infix_expression(str('a'), [str('-'), 1, :right]) { |l,o,r| {:and=>[l,r]} }
p.parse("a-a-a")
# => {:and=>["a", {:and=>["a", "a"]}]}
# instead of {:l=>"a", :o=>"-", :r=>{:l=>"a", :o=>"-", :r=>"a"}}

This is more convenient that using Transforms because the reducer here is bundled with the parser and will follow the parser wherever the parser is used (more composable). Also, the reducer will only be applied to exactly the nodes of the tree where an infix expression occurs (not the whole tree).

[kschiess]

Good idea. Can you add an example of usage in the comments? This is otherwise ready for merge.

[chrismwendt]

@kschiess Thanks, and yep, I added an example.

[kschiess]

Thanks for this!