[T-R]

I think those single-letter names like "S combinator" are a holdover from Math notation, where it was a practical consideration, but in practice in functional programming, those names aren't used (I certainly haven't memorized them); If you look at the Haskell column on the page, you can see that they're actually named for readability (though it might not initially seem like it if you don't do a lot of coding in Haskell):

- K is "const" and C is "flip"; they're generally used to for arguments to a higher order function: "map (const 5) myList", "foldl (flip f) 0 myList"

- Psi is "on", since it's regularly used to construct a new function "h = f `on` g"

- S is a specific type of application, called infix as "f `ap` x", but also has an operator to intentionally make it look more like just line noise: "f < * > x". (Had to add spaces to this operator so HN wouldn't interpret it as italics)

The operators might seem opaque, but the idea is to make it more visually apparent that it's a pattern, not some application-specific business logic. There are lots of concepts of "apply" - there's pure application ($), Applicative application (< * >), Monadic application (=<<), etc. - writing them out would distracting to read. Using operators makes it easier to skim and get the general idea of how the code works without worrying about the underlying structural details, while still being precise about them.