Temporal coupling happens between sequential method calls when they must stay in a particular order. This is inevitable in imperative programming, but we can reduce the negative effect of it just by turning those static procedures into functions. Take a look at this example.
Here is the code:
What do you think about that? I believe it's clear what
names() is doing—creating a list of names. In order to avoid duplication, there is a supplementary procedure,
append(), which converts an item to lowercase and adds it to the list.
This is poor design.
It is a procedural design, and there is temporal coupling between lines in method
Let me first show you a better (though not the best!) design, then I will try to explain its benefits:
An ideal design for method
with() would create a new instance of
List, populate it through
add(item) to it, and finally return. That would be perfectly immutable, but slow.
So, what is wrong with this:
It looks perfectly clean, doesn't it? Instantiate a list, append two items to it, and return it. Yes, it is clean—for now. Because we remember what
append() is doing. In a few months, we'll get back to this code, and it will look like this:
Is it so clear now that
append() is actually adding
list? What will happen if I remove that line? Will it affect the result being returned in the last line? I don't know. I need to check the body of method
append() to make sure.
Also, how about returning
list first and calling
append() afterwards? This is what possible "refactoring" may do to our code:
First of all, we return
list too early, when it is not ready. But did anyone tell me that these two calls to
append() must happen before
return list? Second, we changed the order of
append() calls. Again, did anyone tell me that it's important to call them in that particular order?
Nobody. Nowhere. This is called temporal coupling.
Our lines are coupled together. They must stay in this particular order, but the knowledge about that order is hidden. It's easy to destroy the order, and our compiler won't be able to catch us.
To the contrary, this design doesn't have any "order":
It just returns a list, which is constructed by a few calls to the
with() method. It is a single line instead of four.
As discussed before, an ideal method in OOP must have just a single statement, and this statement is
The same is true about validation. For example, this code is bad:
While this one is much better:
See the difference?
And, of course, an ideal approach would be to use composable decorators instead of these ugly static methods. But if it's not possible for some reason, just don't make those static methods look like procedures. Make sure they always return results, which become arguments to further calls.