Identifiers

@PL on Mar 19, 2021

Identifiers

We define Env to store the values of each variable. Env maps the name of the variable to the value of it.

typealias Env = [String : Value]

There are only one possible type of value at this point; an integer.

enum Value {
    case NumV(Int)
}

We define five arithmetic expressions.

indirect enum Expr {
    case Num(Int)
    case Add(Expr, Expr)
    case Sub(Expr, Expr)
    case Val(String, Expr, Expr)
    case Id(String)
}

We can implement the interpretation of each expressions like the following.

func interp(_ e: Expr, _ env: Env) -> Value
{
    switch e {
        case .Num(let n):
            return .NumV(n)
        case .Add(let l, let r):
            return NumVOperation(ADD, interp(l, env), interp(r, env))
        case .Sub(let l, let r):
            return NumVOperation(SUB, interp(l, env), interp(r, env))
        case .Val(let x, let i, let b):
            return interp(b, env + [x : interp(i, env)])
        case .Id(let x):
            return env[x]!
    }
}

All the materials are from CS320 class held on 2021 spring, KAIST.