gustavoaca1997 / LuaExpressionGramar

A parser-interpreter for a simple arithmetic expression grammar made with LPegLabel (Lua)

Arithmetic expression parser and interpreter

This is an excercise of building a parser and an interpreter in Lua using LPegLabel, hence taking profit from error labels.

Grammar

Program -> (Cmd | Exp)*
Cmd      -> var '=' Exp
Exp       -> Exp '+' Term | Exp '-' Term | Term
Term     -> Term '*' Factor | Term '/' Factor | Factor
Factor   -> num | var | '('Exp')'

Usage

First import the higher order function genParser with

genParser = dofile "exp_grammar.lua"

genParser receives as parameter a function that "writes to an output" when an expression is evaluated as a statement. The print function is used by default.

For example:

parser = genParser()
parser("3")             --> would print `3`

Now look at this other example:

output = {}
parser = genParser(function (val) 
        table.insert(output, val) 
    end)
parser("3 4")
print(output[2])        --> would print `4`

Another example with default output function:

input = [[
            a = 1/2
            a * 4
        ]]
parser(input)           --> would print `2`

You can read more examples (including some syntatic wrong ones) on test.lua file.

Comments about the implementation

• Currently, error recovery is not implemented.
• Parser evaluates the expressions on the fly. This implies some output results before a syntax error.