Interpreter pattern in Go using closures

I recently had to create some code using the interpreter pattern from GoF. However I didn't have my Design Pattern by the Gang of Four ready so I had to look it up at wikipedia. That got me looking at example implementations in C# and Java. Being all about Go for the time being it got me thinking what the same code would look like in Go using closures instead of classes. Just like I demonstrated with other patterns in previous posts.

The code below demonstrates the interpreter pattern by implementing an interpreter of Reverse Polish notation as described on the wiki page. The wiki example uses a class to implement each grammar rule. Here we use a closure. A higher order function will return a function which when run will evaluate the grammar in given context.

import "fmt"
import . "container/vector"
import . "strings"

type Expression func(variables map[string]int) int

func Number(number int) Expression {
  return func(variables map[string]int) int {
    return number
  }
}

func Plus(left, right Expression) Expression {
  return func(variables map[string]int) int {
    return left(variables) + right(variables)
  }
}

func Minus(left, right Expression) Expression {
  return func(variables map[string]int) int {
    return left(variables) - right(variables)
  }
}

func Variable(name string) Expression {
  return func(variables map[string]int) int {
    return variables[name]
  }
}

What is interesting to notice is that the Java version using classes to define grammars takes up 43 lines, while the Go version takes 25 lines. The C# version takes even more lines: 63. However that is in large part because the approach is a bit different. The Evaluator takes about the same number of lines.

func Evaluator(expression string) Expression {
  var expStack Vector
  for _, token := range Split(expression, " ", 0) {
    switch {
    case token == "+": 
      subExpression := Plus(expStack().(Expression), 
                            expStack().(Expression))
      expressionStack.Push(subExpression)
    case token == "-": 
      subExpression := Minus(expStack().(Expression), 
                             expStack().(Expression))
      expressionStack.Push(subExpression)
    default:
      expressionStack.Push(Variable(token))
    }    
  }
  syntaxTree := expStack().(Expression)
  
  return func(context map[string]int) int {
    return  syntaxTree(context)
  }
}

The test code saves some lines on having map as a builtin type.

func main() {
  expression := "w x z - +"
  sentence   := Evaluator(expression)
  variables  := map[string]int {"w" : 5, "x" : 10, "z" : 42}
  result     := sentence(variables)
  fmt.Println(result) 
}

One could change the code to have the + and - operator share some more code. But it wouldn't increase the code size as in the C# example.

func plus(a, b int)  int { return a + b }
func minus(a, b int) int{ return a - b }

func BinOp(left, right Expression, op func(a, b int) int) Expression {
  return func(variables map[string]int) int {
    return op(left(variables), right(variables))
  }
}