// matching (), [] and {} brackets.

open System;
let argv = Environment.GetCommandLineArgs();
let input = if argv.Length>2 then argv[2] else "";

let errpositions = Array.zeroCreate input.Length;
let mutable errors = 0;

let rec matchem stack positions index =
  if index >= input.Length then check_stack stack positions
  else
    match (stack,positions,input[index]) with
      | (s, p, '(') -> matchem ('('::s) (index::p) (index+1)
      | (s, p, '[') -> matchem ('['::s) (index::p) (index+1)
      | (s, p, '{') -> matchem ('{'::s) (index::p) (index+1)
      | ('('::s, i::p, ')') | ('['::s, i::p, ']') | ('{'::s, i::p, '}') ->
        matchem s p (index+1)
      | (a::s, i::p, (')' as c)) | (a::s, i::p, (']' as c)) | (a::s, i::p, ('}' as c)) ->
        errors <- errors + 1
        eprintfn "%d. mismatched %c and %c at indices %d and %d" errors a c i index
        errpositions[i] <- errors
        errpositions[index] <- errors
        matchem s p (index+1)
      | ([], [], (')' as c)) | ([],[],(']' as c)) | ([],[],('}' as c)) ->
        errors <- errors + 1        
        eprintfn "%d. unmatched %c at index %d" errors c index
        errpositions[index] <- errors
        matchem [] [] (index+1)
      | (s, p, x) -> matchem s p (index+1)

and

  check_stack stack positions =
    match (stack,positions) with
      | ([],p) ->
        if errors=0 then printfn "All brackets match"
        else printfn "There were %d errors" errors
      | ('(' as c::s, i::p) | ('[' as c::s, i::p) | ('{' as c::s, i::p) ->
        errors <- errors + 1
        eprintfn "%d. unmatched %c at index %d" errors c i
        errpositions[i] <- errors
        check_stack s p
      | (a::b, p) -> check_stack b p


matchem [] [] 0

if errors>0 then
  printfn "%s" input
  for i in 0..input.Length-1 do
    if errpositions[i]>0 && errpositions[i]<10 then printf "^" else printf " "
  printfn ""
  for i in 0..input.Length-1 do
    if errpositions[i]>0 && errpositions[i]<10 then printf "%d" errpositions[i]
    else printf " "
  printfn ""