sexp-template/src/evaluator.js

const AST = require('./ast')
const Env = require('./env')

module.exports = class Evaluator {
  eval(tree, env) {
    let evaluatedTree = []

    tree.forEach(node => {
      evaluatedTree.push(this.evalNode(node, env))
    })

    return evaluatedTree
  }

  evalNode(node, env) {
    switch (node.constructor) {
      case AST.Boolean:
      case AST.Number:
      case AST.String:
        return node
      case AST.Identifier:
        return env.get(node.name)
      case AST.Application:
        switch (node.function.constructor) {
          case AST.Identifier:
            return this.evalNode(
              new AST.Application({
                function: env.get(node.function.name),
                args: node.args,
              }),
              env
            )
          case AST.Lambda:
            let innerEnv = new Env()

            node.function.parameters.forEach((param, index) => {
              innerEnv.set(param.name, node.args[index])
            })

            return this.evalNode(node.function.body, innerEnv)
          case Function:
            let args = node.args.map(arg => {
              return this.evalNode(arg, env)
            })
            args.unshift(this)
            return node.function.call(...args)
        }
    }

    return node
  }
}