How do I write busted unit tests for functions that return ok,err?

Question

I have a Lua function that returns false followed by error message and want to test its behavior using the busted testing framework. Right now I am doing it a bit like this:

function safe_divide(a, b)
    if b > 0 then -- buggy! should be b ~= 0 
        return a / b
    else
        return false, "division by zero"
    end
end

describe("safe_divide", function()
    it("can divide by positive numbers", function()
        local ok, err = safe_divide(10.0, 5.0)
        assert.truthy(ok)
        assert.are.same(2.0, ok)
    end)

    it("errors when dividing by zero", function()
        local ok, err = safe_divide(10.0, 0.0)
        assert.not_truthy(ok)
        assert.are.same("division by zero", err)
    end)

    it("can divide by negative numbers", function()
        local ok, err = safe_divide(-10.0, -5.0)
        assert.truthy(ok)
        assert.are.same(2.0, ok)
    end)
end)

There are two things that I don't like about my current approach:

• Each test is 3 lines instead of a single clean line
• When the 3rd test fails, busted just says that false is not a truthy value as expected, and never mentions the "divide by zero" error message.

Is there a way I can improve my test file to avoid these problems?

I think what I want to do is kind of similar to the has_error assertion in busted but that seems to be only for functions that actually raise exceptions, not for functions that return false followed by error message.

Answer 1

Busted makes use of luassert, that can be extended with your own assertions.

For instance, the following code uses a user-defined assertion answers that takes as first parameter a table of the expected result, and the effective function results as remaining parameters.

local assert = require "luassert"

local function safe_divide(a, b)
    if b > 0 then -- buggy! should be b ~= 0
        return a / b
    else
        return false, "division by zero"
    end
end

local function answers(state, arguments)
  local expected = arguments[1]
  assert(type(expected) == "table")
  for i = 2, #arguments do
      if arguments[i] ~= expected[i-1] then
          state.failure_message = "unexpected result " .. tostring (i-1) .. ": " .. tostring (arguments [i])
          return false
      end
  end
  return true
end
assert:register("assertion", "answers", answers)

describe("safe_divide", function()
    it("can divide by positive numbers", function()
        assert.answers({ 2.0 }, safe_divide(10.0, 5.0))
    end)

    it("errors when dividing by zero", function()
        assert.answers({ false, "division by zero" }, safe_divide(10.0, 0.0))
    end)

    it("can divide by negative numbers", function()
        assert.answers({ 2.0 }, safe_divide(-10.0, -5.0))
    end)
end)

This code lacks correct formatting for the assertion message. You can look at the luaassert documentation, or at the predefined assertions. It contains a say module for translations of messages.