| Many thanks for taking the time to explain this, I can see that you have a point. >Or you can focus on the meaning of the words, then you need just a few types, see how those meanings compound, and go from there. >I agree, FP is bad at extending things... but it is not FP fault that you see the world as things that need to be extended. I should probably give another look to FP and continue to study the subject, I have likely been approaching it with a OOP mindset as you suggest. One last question though, what about code duplication ? If you focus on the meaning of words and compounding these meanings to create new things, wouldn't it most of the time create lot of duplicate code ? For example in the case of the local shop and the supermarket, wouldn't you have two functions (or a switch case) containing exactly the same code to calculate the quantity multiplied by the price of the groceries ? One would take a local shop as parameter, and the other one would take a supermarket. Splitting the sale processing function in two parts - one summing up the groceries and the other one calculating the taxes - so that you can reuse the first function implies knowing in advance the existence of the supermarket structure (which you will only know many years after putting your product in production). Not doing it in advance either means duplicate code or refactoring every time you add an operation. The extension mindset initially comes from the need to avoid code duplication which is known to make maintenance difficult. Now maybe the strongly-typed experience of FP and its compile-time checks are so convenient in practice that they make the maintenance of this kind of code easily manageable ? Maybe it's not even really duplicate code since what would be a basic number in OOP would be two different types here in FP ? Or maybe it's still thinking about FP from an OOP perspective ? |
What goes through the sale function pipe is not the store and its size, it really does not matter for the meaning of sale were the sale happened. What matter for the sale is the cart. The cart might be more or less sofisticated because the size of the store, but is still a cart.
So the sale function is a pipe that eats a cart and spits out a receipt.
Or if you want to put it in another way, a sale is something that might happen to a cart over time. Functions are the clock on the system. In a functional system, times does not pass until a function is executed (does it sounds like lazy evaluation?)
In functional programming, we avoid duplication of code by appliying functions to the data so it spits what the next function needs. If you have a function sale that can eat a simple cart, but now you have a complex cart, one solution is to create a function that converts the complex cart into the simple one that the sale function needs; another way is to make the complex cart having the same data as the simpler cart. What you choose to do is proportional to how different the meaning of cart is, or maybe the cart remains the same. In general terms, this type of thinking tend to produce less lines of code needed per similar acomplishment.
So in OOP your are using objects to put together the store, while in FP your are descriving what happens in the store by connecting pipes betweem the different data. It is not about what can happen to the data, but what makes the data travel. It is not about what steps makes it work, but the fluidity of the data life span.