Question

I must come up with a function that evaluates an abstract syntax tree and returns the result of its evaluation.

evaluate will have type Exp -> int option

evaluate (Prod(Num 5, Diff(Num 6, Num 1)));;
val it : int option = Some 25

@John Palmer
How do i go about doing this? Can I use pattern matching? if so how would I do it in order to identify the operation that needs to be done.

Here is what i have come up with? I still dont understand the code per say.

let rec evaluate = function
 | Num n -> Some n
 | Neg e -> match evaluate e with
 |Sum (a,b) -> evaluate(a) + evaluate(b)
 |Diff(a,b) -> evaluate(a) - evaluate(b)
 | Prod (a,b) -> evaluate(a) * evaluate (b) 
 |Quot (a,b) -> evaluate(a) / evaluate(b) 

Answer 1

Here are some hints to get started

let rec evaluate AST =
   match AST with
   |Prod(a,b) -> evaluate(a) * evaluate(b)
   |Num(a) -> a
   ....

EDIT

So I assume your data type looks like

type AST =
|Num of int
|Neg of AST
|Prod of AST * AST
|Diff of AST * AST
|Quot of AST * AST
|Sum of AST * AST

so you set up your AST (presumably you are parsing this from somewhere – which is a whole other question) with something like

let ast = Prod(Num 5, Diff(Num 6, Num 1))

then you can define evaluate as

let rec evaluate arg =
    match arg with
    |Num n -> n
    |Neg tree -> - (evaluate tree)
    |Sum (a,b) -> (evaluate a) + (evaluate b)
    |Prod(a,b) -> (evaluate a) * (evaluate b)
    |Quot(a,b) -> (evaluate a) * (evaluate b)
    |Diff(a,b) -> (evaluate a) - (evaluate b)

then call it and print the result with

printfn "%i" (evaluate ast)

Note that evaluate has to have type AST -> int as otherwise you will need to handle option cases – for example what is 2 * None or 2 + None

I am not sure what you mean by

how should i go about calling the recursive method on smaller inputs.

but hopefully this explains everything