Lua: how to verify that a table contains a specific function

Question

I'm developing a module that returns a table full of functions based on the arguments that are passed in. Specifically, the module returns a set of data transformation rules (functions) that need to be applied to a data set depending on which customer is sending it.

I decided to decouple my rule library (biz logic) from the code that decides which of the rules should be applied (config logic).

Here's the unit test I'm writing to verify that the ruleBuilder is adding the correct rule (function) based on one of my scenarios:

ruleBuilder = require("ruleBuilder")
ruleLibrary = require("ruleLibrary")
local rules = ruleBuilder.assembleRules("Customer1231")

assert(rules[1] == ruleLibrary.missingSSNRule)

Is this the correct way to do that verification? Will this work even if the ruleLibrary.missingSSNRule function has references to several other functions via a closure or parameter?

This is not a well-specified question. It's not clear what `assembleRules` is supposed to be doing, or how `someOtherCoolModule.coolFunction` might be placed within `rules`. What exactly are you attempting to "verify"? What is *supposed* to happen? — Nicol Bolas, Sep 19 '12 at 23:56
Question has been edited for clarity. Would you mind reversing your vote to close the question? — Mitch A, Sep 20 '12 at 02:30

score 2 · Accepted Answer · answered Sep 20 '12 at 00:08

To verify that a table contains a particular function you may use the fact that keys in Lua tables can be anything (including functions). In your assembleRules code you can write something like this:

function assembleRules(...)
  ...
  return {
    [someOtherCoolModule.coolFunction] = someOtherCoolModule.coolFunction,
    [yetAnotherModule.anotherFunction] = yetAnotherModule.anotherFunction,
  }
end

Then later you can simply check if the key exists:

local rules = ruleBuilder.assembleRules("somedata")
assert(rules[someOtherCoolModule.coolFunction])

score 1 · Answer 2 · answered Sep 20 '12 at 00:02

On the assumption that the return value of ruleBuilder.assembleRules is supposed to somehow know to put someOtherCoolModule.coolFunction in the 0-th index (note: Lua uses 1-based indices. Don't use 0 as an index) of its return value, then yes.

Will this work even if someOtherCoolModule.coolFunction is a closure?

All functions in Lua are closures. However, I'm going to assume that you mean that ruleBuilder.assembleRules is going to take someOtherCoolModule.coolFunction and build a new function around it.

A function is equal to itself. But it is only equal to itself. Just like two tables are only equal if they are the same table object, two functions are only equal if they are the same function. Functions are not equal to a different instantiation of the same function, nor is it equal to any other function. Here are examples of this.

I'm trying to write unit tests using the approach you recommended in this question: http://stackoverflow.com/questions/12344690/lua-can-i-use-require-as-a-form-of-dependency-injection I want to assert that a given function returns a table that contains the correct set of functions based on the arguments it was passed. If multiple instances of a given function can't be compared for equality, I'm at a loss on how to write this test — Mitch A, Sep 21 '12 at 01:36

Lua: how to verify that a table contains a specific function

2 Answers2