Balanced brackets: Difference between revisions

The simplest solution exploits the idea of stack-based automaton, which could be implemented by a fold.

<lang haskell>

isMatching :: String -> Bool

isMatching = null . foldl aut []

where

aut ('[':s) ']' = s

-- aut ('{':s) '}' = s -- automaton could be extended

aut s x = x:s

</lang>

This generates an infinite stream of correct balanced brackets expressions:

<lang haskell>

brackets = filter isMatching

$ [1.. ] >>= (`replicateM` "[]{}")

</lang>

In case the index of unmatched opening bracket is need to be found, following solution is suitable.