Brackets.fs

let validate (str: string) =
    let rec loop (i: int) (acc: char list) =
        if i >= str.Length then
            acc.IsEmpty
        else 
            let validateChar ch = 
                not acc.IsEmpty && acc.Head = ch && loop (i + 1) acc.Tail
            match str.[i] with        
            | '(' | '[' | '{' -> loop (i + 1) (str.[i]::acc)
            | ')' -> validateChar '('
            | ']' -> validateChar '['
            | '}' -> validateChar '{'
            | _ -> loop (i + 1) acc
    loop 0 []

[<EntryPoint>]
let main argv =
    assert validate "(){()()}{}[]"
    assert (not <| validate "(]")
    assert (not <| validate ")")
    0