I just found this function in the project I’m working at:
-- Just returns the text unchanged.
-- Note: <text> may be nil, function must return nil in that case!
function Widget:wtr(text)
return text
end
Too sad, the coder does not work in the company anymore. Why would one make a function that does nothing, but returns the parameter it’s called with?
Is there any use to such a function, not specified to this example, but overall in any case?
Due to
function aFunction(parameter)
return parameter
end
Ends in
aFunction(parameter) == parameter
Why would I write something like
aFunction(parameter) == whatIWantToCheck
instead of
parameter == whatIWantToCheck
?
12
Your question is sort of like asking what’s the good of the number zero if whenever you add it to something you get the same value back. An identity function is like the zero for functions. Kind of useless by itself, but occasionally useful as part of an expression using higher-order functions, where you can take a function as a parameter or return it as a result. That’s why most functional programming languages have an id or identity in their standard library.
Put another way, it makes a handy default function. Just like you might want to set offset = 0 as a default integer, even though that makes an offset do nothing, it might come in handy to be able to set filterFunction = identity as a default function, even though that makes the filter do nothing.
5
Sure. Imagine an external API that allows you to retrieve things from somewhere that you absolutely need, but you have to specify filter criteria and output formatters for every query, even if you want all the results and want to get them exactly as stored.
Then you’d have to invent a trivial “accept everything” and a trivial “return unchanged” function to get at the data. wtr is precisely such a trivial pseudo-formatter. If you use that API a lot, it can be very useful to have this pseudo-helper function lying around rather than have to create an anonymous function whenever you query something. (You might consider calling it identity rather than wtr so that it’s immediately clear it does nothing.)
7
Isn’t that just a polymorphysm?
From my experience with Qt tr() and wtr() methods are supposed to be used (there, in Qt) in localization of Widget‘s text.
So, by returning unchanged text this method just says: Widget does no localization (translation of the text) by default. Override this function to enable your own logic.
1
A very common case for this is ‘later added functionality’. So the programmer thought that there could be some change to that function later. Since you access it as a function and not as the parameter itself, you can easily globally change this. Lets say you have an index with:
function getIndex(index)
return index
end
and since your index starts at 0 this is all fine. You later change a component somewhere else, this component needs you to translate your index to start at 1. You would have to add hundreds of index+1 everywhere in your code. But with a method like this, simply say:
function getIndex(index)
return index+1
end
and you are fine. Functions like this can quickly lead to over-engineering but at some points they make a lot of sense!
1
Stub functions like this are common in type-inheritance situations. The base class might do nothing useful, but derived classes might do various things with the inputs. Declaring the stub function in the base class allows all derived classes to have a function of that name, and individual derived classes to override it with something more elaborate and useful.
One additional scenario is similar to the use in python and C# of properties, but in languages that don’t have the feature.
In general, you can declare something as just a member of the class; let’s say, ‘name’ as a string. This means that you access it like this:
someobject.name
Later, you decide that the name really needs to be dependent on what’s in the object. So it should really be a function. Oops! Even with no arguments, that means all code that accesses it now has to be changed to
someobject.name()
But if you have a lot of instances of this call, the chances of nothing going wrong with that is pretty low – somewhere this change won’t be made, the program will crash when you get to that point, etc.
Properties in C#/Python allow you to substitute a function in for what used to be an attribute, while still allowing it to be accessed like an attribute i.e. with no change. You don’t even have to have planned ahead.
If your language doesn’t have that, you have to plan ahead, but you can still sort of support it the way people did before by making it a function from the beginning that doesn’t do anything. In the beginning, it won’t do anything interesting, and look like it should be removed, and many functions of this type are never changed. But later, if you realize that really whenever you call Widget.wtr you need to strip some special characters out, it’s no big deal – it’s already a function, you just add that in and you’re done.
TL;DR This could just be a wise programmer planning ahead for future changes.
3
It’s not useless, actually it can be extremely useful, because it makes your code more uniform and flexible.
Say, for example, that you have a listing of people and a dropdown for the user to select if we wants to see “all” or just those that are “male” or “female”.
You can either handle “all” as a special case, in which case you’ll need an extra if and to check for this case explicitly, or you can have a filter function that just returns its parameter (e.g allows everything to pass), assigned to be used for the “all” case.
4