learning lpeg module to write a mini expression executor.

p2.lua

--
-- Created by IntelliJ IDEA.
-- User: andi
-- Date: 14-4-27
-- Time: 下午7:07
-- To change this template use File | Settings | File Templates.
--
local sqrt = require("math").sqrt
local re = require("re")

local function printIndent(v, n)
    n = n or 0
    print(string.rep("  ", n)..v);
end

local function p(t, indent)
    indent = indent or 0
    if indent > 5 then return end
    if not  t then
        printIndent('<nil>', indent)
        return
    end

    if type(t) == 'table' then
        printIndent("{", indent)
        for k, v in pairs(t) do
            if type(v) == 'table' then
                printIndent(k .. ":", indent + 1)
                p(v, indent + 1)
            else
                printIndent(k .. ":" .. v, indent + 1)
            end
        end
        printIndent("}", indent)
    else
        printIndent(t, indent)
    end

end

local grammer = [[
    stmt <- <expr> (%nl/!.)
    expr <- (s (<term>) {| (s (<termOp>) s (<term>) s)* |}) -> doTerm
    termOp <-  (('+' / '-')) -> Op
    term <- (s (<factor>) {| (s (<factorOp>) s (<factor>) s)* |}) -> doFactor
    factorOp <- (('*' / '/')) -> Op
    factor <- <num> / '(' <expr> ')' / <func> -> doNum
    func <- (<id> '(' <args> ')') -> doFunc
    id <- {[A-Za-z_][A-Za-z0-9_]*} -> doId
    args <- ((<expr>) {| (','  (<expr>))* |}) -> doArgs
    num <- {[0-9]+} -> tonumber
    s <- %s*
]]

local defs = {
    tonumber = tonumber,
    doTerm = function(left, rights)
        local ret = left

        if not rights then
            return ret
        end

        for i = 1, #rights, 2 do
            local operator = rights[i]
            local right = rights[i + 1]
            ret = operator == '+' and (ret + right) or (ret - right)
        end
        return ret
    end,

    Op = function(operator)
        return operator
    end,

    doFactor = function(left, rights)
        local ret = left

        if not rights then
            return ret
        end

        for i = 1, #rights, 2 do
            local operator = rights[i]
            local right = rights[i + 1]
            ret = operator == '*' and (ret * right) or (ret / right)
        end
        return ret
    end,

    doFunc = function(id, args)
        return id(args)
    end,

    doId = function(name)
        if name == 'sqrt' then
            return sqrt
        end
        error("invalid functiion name")
    end,

    doArgs = function(args)
        return args
    end,

    doNum = function(num)
        return num
    end,
}

local matcher = re.compile(grammer, defs)

p(matcher:match('1+2'))
p(matcher:match('1*3'))
p(matcher:match('1+1+2'))
p(matcher:match('1+4/1*4+2*2-sqrt(16)'))