Toy calculator in Lua, version 2

hoc2.lua

setmetatable(_ENV, { __index=lpeg })

VARS = {}

function eval(...)
    local args = {...}
    local accum = args[1]
    for i = 2, #args, 2 do
        local operator = args[i]
        local num2 = args[i+1]

        if operator == '+' then
            accum = accum + num2
        elseif operator == '-' then
            accum = accum - num2
        elseif operator == '*' then
            accum = accum * num2
        elseif operator == '/' then
            accum = accum / num2
        end
    end
    return accum
end

function assign(ref, value)
    ref.scope[ref.index] = value
    return value
end

function lookup(ref)
    return ref.scope[ref.index]
end

function makeref(...)
    local indices = {...}
    local tbl = VARS

    for i = 1, #indices-1 do
        tbl = tbl[indices[i]]
    end

    return {scope=tbl, index=indices[#indices]}
end

spc = S(" \t\n")^0

digit = R('09')
number = C( (P("-") + digit) *
                digit^0 *
                ( P('.') * digit^0 )^-1 ) / tonumber * spc

lparen = "(" * spc
rparen = ")" * spc

lbrack = "[" * spc
rbrack = "]" * spc
comma = "," * spc

expr_op = C( S('+-') ) * spc
term_op = C( S('*/') ) * spc
letter = R('AZ','az')
name = C( letter * (digit+letter+"_")^0 ) * spc

stmt = spc * P{
    "STMT";
    STMT =
        V("REF") * "=" * spc * V("VAL") / assign +
        V("EXPR"),
    EXPR = V("TERM") * ( expr_op * V("TERM") )^0 / eval,
    TERM = V("FACT") * ( term_op * V("FACT") )^0 / eval,
    REF = name * (lbrack * V("EXPR") * rbrack)^0 / makeref,
    FACT =
        number / eval +
        lparen * V("EXPR") * rparen / eval +
        V("REF") / lookup,
    ARRAY = lbrack * Ct( V("VAL_LIST")^-1 ) * rbrack,
    VAL_LIST = V("VAL") * (comma * V("VAL"))^0,
    VAL = V("EXPR") + V("ARRAY")
}

function test(expr)
    assert(expr:match(" 1 + 2 ") == 3)
    assert(expr:match("1+2+3+4+5") == 15)
    assert(expr:match("2*3*4 + 5*6*7") == 234)
    assert(expr:match(" 1 * 2 + 3") == 5)
    assert(expr:match("( 2 +2) *6") == 24)
    stmt:match("a=3"); assert(VARS.a == 3)
    assert(stmt:match("a") == 3)
    assert(stmt:match("a * 5") == 15); VARS.a=nil
    stmt:match("a = [ 4, 5, 6 ]");
    assert(VARS.a[1] == 4)
    assert(VARS.a[2] == 5)
    assert(VARS.a[3] == 6)
    VARS.a=nil
    stmt:match("b = [ ]");
    assert(VARS.b[1] == nil)
    VARS.b=nil
    stmt:match("c = [[1,2], [3,4]]")
    assert(VARS.c[1][1] == 1)
    assert(VARS.c[1][2] == 2)
    assert(VARS.c[2][1] == 3)
    assert(VARS.c[2][2] == 4)
    assert(stmt:match("c[4/2][1]") == 3)
    stmt:match("c[3] = 5")
    assert(VARS.c[3] == 5)
    VARS.c=nil
end

function repl(file)
    file = file or io.input()
    parser = stmt
    for line in file:lines() do
        print(parser:match(line))
    end
end

Yeah, exactly, what eric-2 said. :)