(&) = flip ($) in Haskell

F# obviously approves |>, but in my software project that has been continually maintained for ten years, we introduced -->. Now re-factoring our project we use |>. It kind of messes with my sight against pattern matching in OCaml though.

gizmo6862• 13 years ago

Bikesheddin at its finest.

When I was first learning haskell, I thought I was missunderstanding how to use the language because I would define (|>) a f = f a in every project I do. I thought that such an operator seemed so obvious that it couldn't possibly not be standard. The fact that it wasn't seemed to suggest that I was doing Haskell badly, and that there was some deep reason I should only work right to left using operators such as ($), which I still do use a fair amount.

tel2• 13 years ago

This is included in the popular Control.Lens package since it sometimes has natural right-to-left composition. It's still unnatural for 99% of Haskell code you'll see, but (&) = flip ($) seems to be cemented as the "correct" representation now.

It's a wontfix on the language spec itself, though.

vimes656• 13 years ago

In Control.Lens package also comes with <&> which is the flipped version of <$> with the right infix priority, which I find it even handier than $.

AFAIK, there is very little sample code out there with these flipped operators, but to a programmer used to OOP, using them may be more natural.

PeterisP• 13 years ago

Many current elements of modern languages have been wontfix'es in them for many years.

gizmo6862• 13 years ago

It was closed with the message: "I'm closing this ticket while the discussion is going on on the libraries list. Please reopen, or open a new ticket, if a positive decision is reached" [1]

The discussion itself [2] is basically bikeshedding at this point.

[1] http://hackage.haskell.org/trac/ghc/ticket/7434 [2] http://www.haskell.org/pipermail/libraries/2012-November/018...

I took both of these links directly from lelf's comment in this thread.

arianvanp• 13 years ago

right-to-left application, not composition

Tyr42• 13 years ago

Diagrams defines # to be reverse function application.

Evbn2• 13 years ago

You'll flip your id

($) = id Except that id works on values and functions, but $ only works on functions.

(&) = flip id

$ is just an less-expressive alias for id that looks prettier (and has different precedence/associativity when used infix)

tel2• 13 years ago

No, not strictly true! ($) has a compiler hack to let it handle impredicativity.

    import Control.Monad.ST

    infix 0 %
    (%) = id

    runST $ return () -- :: ()
    runST % return () -- Type error!

    Couldn't match expected type `forall s. ST s a0'
                with actual type `m0 a1'
    In the return type of a call of `return'
    In the second argument of `id', namely `(return ())'
    In the expression: runST `id` (return ())

kenko2• 13 years ago

I used ¢ as flip ($).

kenko2• 13 years ago

I guess this is too heavyweight, after all:

    Prelude GOA> (return [1..]) >>= return . map (^ 2) >>= return . takeWhile (< 64)
    [1,4,9,16,25,36,49]

taejo• 13 years ago

Diagrams uses (#) for this.

(&) = flip ($) in Haskell

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