How can we implement an eval function case for the if-then-else expression in SML?

Question

How can we implement an eval function case for the if-then-else expression in SML? This subject in SML is challenging and new to me.

datatype exp = ...
  | Equal of exp * exp      
  | If of exp * exp * exp;

datatype value = BVal of bool | ...

fun print (Equal(x,y)) = "(" ^print(x)^ " = " ^print(y)^ ")"
  | print (If(x,y,z)) = "if " ^print(x)^ "
                         then " ^print(y)^ " else " ^print(z)
  | ...

fun eval (Equal(e1, e2)) ctx =
           let
                val (CVal x) = eval e1 ctx
                val (CVal y) = eval e2 ctx 
           in
                BVal (round x = round y)
           end
  | ...

Answer 1

How can we implement an eval function case for the if-then-else expression?

1. Make sure CVal is defined. It looks like it should be a part of datatype value, but it could also be defined in an area of the code that you have omitted. Please provide a minimal, reproducible example when you post on StackOverflow, since too much guessing can lead to nonsensical answers.
2. Extend the eval function with pattern matching to handle the If constructor in a similar way as it already supports the Equal constructor.

Here is something that might get you started:

datatype value = BVal of bool | CVal of int (* replacing "..." *)

fun eval (Equal (e1, e2)) ctx =
    let
      val (CVal x) = eval e1 ctx
      val (CVal y) = eval e2 ctx 
    in
      BVal (round x = round y)
    end

  | eval (If (cond, e1, e2)) =
    let
      val ... = eval cond ctx
    in
      ...
    end

Depending on how you perform the pattern matching, you will want to pick eval e1 ctx or eval e2 ctx to be the result. You may want to look into the case-of construct here.