Re: Better Syntax?

[Miroslav Janíček <mira.janicek@...>]

> On 4 Oct 2016, at 17:44, Sean Conner <sean@conman.org> wrote:
> 
> It was thus said that the Great Miroslav Janíček once stated:
>> 
>>> On 4 Oct 2016, at 0:04, Sean Conner <sean@conman.org> wrote:
>>> 
>>> It was thus said that the Great Mike Jones once stated:
>>>> Hi,
>>>> 
>>>> I am working on a project where I have basically an event style
>>>> framework, and I would like some way to "chain together" functions. 
>>> 
>>> I dug through my archives and I found the following bit of code (cleaned
>>> up---I was starting to learn Lua):
>>> 
>>> 	function fappend(f1,f2)
>>> 	  return function(...)
>>> 	    return f2(f1(...))
>>> 	  end
>>> 	end
>> 
>> It might be a good idea to handle cases in which f1 or f2 are undefined:
> 
>  I disagree [1].  It makes *no* sense to call the function with any of the
> parameters nil (it comes across as pointless) and at worse, it could hide
> bugs in the code elsewhere (why is f1 nil?  How could that happen?). [2]

You’re right in that for a general function composition function, this wouldn’t be correct (nil is not the identity function).

However, in this use-case, it might be practical. Suppose we have two modules, module1 and module2.

In module1:

    local function foo(...)
      -- etc
    end

    dosomething = fappend(dosomething, foo)

In module2:

    local function bar(...)
      -- etc
    end

    dosomething = fappend(dosomething, bar)

Calling dosomething() with both modules loaded would call both foo and bar (in the order in which they were loaded); if we only load one of them, things will still work.

I think something like this is what the OP wanted — but it’s quite likely that I misunderstood the problem!

>>      function fappend(f1, f2)
>>        if f1 ~= nil and f2 ~= nil then return function(...) return f2(f1(...)) end
>>        elseif f1 ~= nil then return f1
>>        else return f2
>>        end
>>      end
>> 
>> (And maybe also calling the function “compose” would be more indicative of its intent.)
> 
>  That I have no problem with.

So in the end, the nil-checking version should not be called “compose”, just the original, proper, composition function (the one you posted). Which is why I kept the name “fappend” in the snippets above.

  M.