peg-1-parse-pt1.fs

let parseExpression (grammar:Map<string,Expression>) start (input:string) =
    let rec parse offset = function
    | Terminal x -> 
        let e = offset + x.Length - 1
        if e < input.Length && input.[offset..e] = x then (TerminalSymbol x, e + 1) else (Unmatched, offset)
    | NonTerminal x -> parse offset grammar.[x]
    | Epsilon -> if offset = input.Length then (EmptyMatch, offset) else (Unmatched, offset)