Lisp Clone

lisp_clone.js

const funs = {
  sqrt: (val) => Math.sqrt(val),
  sum: (a,b)=> a + b,
  mul: (a,b) => a * b
}

class Exp {
  constructor(fnName, params){
    this.fnName = fnName
    this.params = params
  }
  eval(){
    // evaluate function parameters (call-by-val)
    const paramVals = this.params.map(p=> typeof p === 'object' ? p.eval() : p)
    return funs[this.fnName](...paramVals)   
  }
}

function run(str){
  const tokens = lex(str)
  const rootExp = ast(tokens)
  console.log(rootExp)
  console.log(rootExp.eval())
}

function lex(str){
  const norm = str.replace(/\(/g, ' ')
                  .replace(/\)/g, ' ')
  return norm.split(' ').filter(f=> f.length>0)
}

function ast(tokens){
  let paramStack = []
  
  tokens.reverse().forEach(t => {    
    if(funs[t]){
      const exp = new Exp(t, [...paramStack.reverse()])
      paramStack = [exp]
    }
    else{
      const val = parseFloat(t) || t
      paramStack.push(val)
    }
  })
  
  return paramStack[0]
}

run(`sqrt(sum(2 mul(3 4)))`) // returns 3.7416573867739413