A simple parser for simple expression in math

Input:

1 + 2 * 3

Output: 7

Just a simple parser and caculator, it will parse the expression into trees and then calculate it.

Although this is just a toy for programming praticing. But this program is seriously usable.

Recursive decent parser written by hand, Combining the knowledge of precedence parsing.

At first, this program is designed to execute by iterating the abstract syntax tress(ASTs). And you can see the previous versions in the early commits. But at last I decide to use the virtual machine. The program iterator the AST to generate the liner VM codes, and VM execute the codes.

This vm is a stack machine. Use a virtual machine to calculate a math expression is overkill. But I just want to try.

Now you can assign value and expression to varible such as:

Example:

let a = 1 + 2 * 3
let b = 1 + a
a + b

Now you can add the parens to change the priority of the expression, such as

let a = 3
(a + 2) * 2  // output: 10

10 * (2 + 3) // output: 50