> Pipes, conduits, and most of the stuff by Kmett appear to be libraries implementing other abstractions that themselves would have be justified by use in a practical program.
I'm not sure I understand what you mean by "practical programming".
This may just be a cultural difference but one of the big ideas of Haskell is using a library whose "core calculus" is provably correct and then combining them in ways that preserve that correctness. So I would argue that the justification of the libraries is not in the practical ( if I understand your usage of the word?) usage but in the thoroughness and compositional semantics that they provide. That's where the category theory inspired patterns ( zippers, monads, arrows, etc ) become important.
Okay, let's define "practical programming" as creating programs that will be of use to someone who doesn't know the programming language that the program was written in. So in this case, this means writing Haskell programs that will be used by people who don't know Haskell.
This isn't the only reason to write a program of course. You can do it just for fun or to explore the mathematics. But I'm mostly interested in abstractions that might be useful for solving problems outside mathematics.
None of the libraries or patterns I mentioned above are designed for the purpose of exploring category theory. Though some people do do that, but they tend to live in their own world.
A lot of Haskellers just happen to exploit bits of CT it because its a great framework for writing and reasoning about code. Some people do use Haskell as a way to explore category theory but for example Tekmo isn't writing pipes as a means to explore the theory of free monads, they've been well studied since the 80s. I believe he just happens to find that they are an excellent way of chaining together bits of control flow.
At that heart of the problem he's tackling is very practical problem that's tied to data flow and resource management. For example, I've written a library against pipes which handles resource management of ZeroMQ sockets for a large distributed system and find that my code under pipes is much more manageable.
Okay, sure, resource management is what I'd call a practical problem, and pipelines are familiar from Unix (and Go). It looks like this pipe library has a nice practical example: enforcing the category laws means that it handles terminating pipelines consistently:
I'm not sure I understand what you mean by "practical programming".
This may just be a cultural difference but one of the big ideas of Haskell is using a library whose "core calculus" is provably correct and then combining them in ways that preserve that correctness. So I would argue that the justification of the libraries is not in the practical ( if I understand your usage of the word?) usage but in the thoroughness and compositional semantics that they provide. That's where the category theory inspired patterns ( zippers, monads, arrows, etc ) become important.