Modal

Modal is a matrioshka language based on pattern-matching to rewrite trees.

Modal programs are represented as a series of rules, formatted as tokens delimited by brackets and parentheses, applied to a given tree which gets continually modified until no rules match any given part of the tree.

To define a new rule, start a line with <>, followed by two tokens. The program evaluation starts at the first character of the string and walks through to the end trying to match a transformation rule from that location:

<> hello bye

.. hello world
00 bye world

A new rule definition syntax can also be created to suit a specific notation, for instance, here is a rule to define new rules with an infix syntax:

<> ((?x -> ?y)) (<> ?x ?y)
(a -> apple)
(b -> banana)
((apple banana) -> (fruit-salad))

.. a b
01 apple b
02 apple banana
03 fruit-salad