Pre-defined infix operator list in Haskell?-CodePudding

Basically, I need to define infix operator for function composition, flipped g . f manner.

(#) :: (a -> b) -> (b -> c) -> a -> c
(#) = flip (.)

infixl 9 #

Here, temporarily, I just have chosen the symbol: #, but I'm not certain this choice is not problematic in terms of collision to other definitions.

The weird thing is somehow I could not find a Pre-defined infix operator list in Haskell.

I want the composition operator as concise as possible, hopefully, a single character like ., and if possible, I want to make it &. Is it OK?

Is there any guidance or best pracice tutorial? Please advise.

CodePudding user response：

You don't see a list of Haskell built-in operators for the same reason you don't see a list of all built-in functions. They're everywhere. Some are in Prelude, some are in Control.Monad, etc., etc. Operators aren't special in Haskell; they're ordinary functions with neat syntax. And Haskellers are generally pretty operator-happy in general. Spend any time inside your favorite lens library and you'll find plenty of amusing-looking operators.

In terms of (#), it might be best to avoid. I don't know of any built-in operators called that, but # can be a bit special when it comes to parsing. Specifically, a compiler extension enables # at the end of ordinary identifiers, and GHC defines a lot of built-in (primitive) types following this practice. For instance, Int is the usual (boxed) integer type, whereas Int# is a primitive integer. Most people don't need to interface with this directly, but it is a use that # has in Haskell that would be confused slightly by the addition of your operator.

Your (#) operator is called (>>>) in Haskell, and it works on all Category instances, including functions. Its companion is (<<<), the generalization of (.) to all Category instances. If three characters is too long for you, I've seen it called (|>) in some other languages, but Haskell already uses that operator for something else. If you're not using Data.Sequence, you could use that operator. But personally, I'd just go with (>>>). Any Haskeller will recognize it pretty quickly.