let rec helper (n: (string * int) list) (text : string) =

    match n with

    | (x,y):: tail when x.[0] = text.[0] -> y

    | (x,y):: tail -> helper tail text

    | _-> 0

​

let eval vars expr =

    let i=0

    let stack = [3;4]

    let rec innerEval vars (stack: 'a list) (expr: string) = 

        printfn "%A" (String.length expr)

        if i = String.length expr then

            printfn "done"

        else

            let i=i+1

            let tempstr = "" + string expr.[i]

            //printfn "%A" tempstr

            //printfn "%A" (helper vars tempstr)

            match tempstr with

            | "@" -> printfn "@" 

            | "#" -> printfn "#"

            | "+" -> printfn "+"

            | "-" -> printfn "-"

            //| "a" | "b" | "c"| -> printfn "yey"

            | x -> printfn "op" tempstr

    innerEval vars stack expr

​

printfn "%A" (eval [("a",5);("b",2);("c",9)] "ab+")

//should return 7 (push 5, push 2, add)

​

printfn "%A" (eval [("a",5);("b",2);("c",9)] "cab+-")

//should return 2 (push 9, push 5, push 2, add, subtract)

​

//printfn "%A" (eval [("a",5);("b",2);("c",9)] "ab+cb-*")

//should return 49 (push 5, push 2, add, push 9, push2, subtract, multiply)

​

//printfn "%A" (eval [("a",5);("b",2);("c",9)] "ab+cb-* @d bd+")

//should return 51 (push 5, push 2, add, push 9, push2, subtract, multiply, bind 49 to d, push 2, push 49, add)

​

//printfn "%A" (eval [("a",5);("b",2);("c",9)] "ab-ab*ab+--")

//should return 0 (push 5, push 2, subtract [5-2->3], push 5, push 2, multiply [5*2->10], push 5, push 2, add [5+2->7], subtract [10-7->3], subtract [3-3->0])

​

//printfn "%A" (eval [("a",5);("b",2);("c",9)] "bbb @q bqbq*****")

//should return 64 (six 2's on the stack before the multiplies)

​

//printfn "%A" (eval [("a",5);("b",2);("c",9)] "#a #c ca- @b bc-")

//should return -5 (print a, print b, push 9, push 5, subtract, bind 4 to b, push 4, push 9, subtract)

​