44

A quick question, is there an operator in Haskell that works like the dollar sign but gives precedence to the left hand side. I.E. instead of

f (x 1)

being written as

f $ x 1

I'd like to write it as

x 1 $ f

This is purely a stylistic thing. I'm running a sequence of functions in order and it would be nice if I could write them left to right to match that I read left to right. If there an operator for this?

[update] A couple of people have asked if I can't define my own. In answer, I wanted to check there wasn't an existing operator before I reinvented the wheel.

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5
  • 6
    This is like F#'s pipeline composition operator. Related discussion (on why it's not built into Haskell): stackoverflow.com/questions/1457140/… Commented Nov 3, 2010 at 18:20
  • It looks like the answer to my question is "no". Thank you :) Commented Nov 3, 2010 at 18:26
  • 2
    You realize you're combining both left-to-right and right-to-left application in the same expression, don't you? Most people I think would baulk at this. Reverse application is the so called T-combinator. It was often denoted with (#) a few years ago, but now # is a special symbol in GHC, so it is no longer used much. Commented Nov 3, 2010 at 18:30
  • you want: f (g 1). Why not just f . g $ 1? Commented Oct 31, 2011 at 15:13
  • @demi I believe OP is looking to express f (g 1) in a left-to-right pipeline. f . g $ 1 is the opposite of that Commented May 19, 2020 at 15:44

6 Answers 6

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48

As of GHC 7.10 (base 4.8.0.0), & is in Data.Function: https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Function.html

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36

In Haskell you can use flip to change arguments' order of any binary function or operator:

ghci> let (|>) = flip ($)
ghci> 3 |> (+4) |> (*6)
42
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Does it keep the same operator precedence?
21

I do not know, whether there is an standart operator, but what prevents you from writing your own? This works in ghci:

Prelude> let a $> b = b a
Prelude> 1 $> (+2)
3
Prelude> sum [1, 2] $> (+2)
5
Prelude> map (+2) [1, 2] $> map (+3)
[6,7]

UPDATE: searching on hoogle for a -> (a -> b) -> b (it is the type of this operator) found nothing useful.

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5 Comments

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Looks I'll be using this, thank you. An £ appeals to me for the operator.
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@Benjamin Confino: Puns are always enjoyable, but for real-world use I'd suggest considering |>. As noted above, that operator in F# does exactly what you're looking for, so using the same name will help people understand your code more easily.
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It is one of Haskell's real strengths that you can write your own control flow operators. Haskell doesn't have a "loop" construct? No problem! Just write your own.
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|> is used by Data.Sequence. # sounds like a better idea.
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Link: Hoogle search for a -> (a -> b) -> b. The only results with that exact type are ($) and ($!).
10

This combinator is defined (tongue in cheek) in the data-aviary package:

Prelude Data.Aviary.BirdsInter> 1 `thrush` (+2)
Loading package data-aviary-0.2.3 ... linking ... done.
3

Although actually using that package is a rather silly thing to do, reading the source is fun, and reveals that this combinator is formed via the magic incantation of flip id (or, in ornithological parlance, cardinal idiot).

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caridinalIdiot is a magnificent combinator name!
4

I am not aware of any standard version, but I've seen (#) used for that purpose in a couple places. The one in particular that comes to mind is HOC, which uses it in an idiom like:

someObject # someMessage param1 param2

I seem to recall seeing other "object-oriented" libraries using the # operator in the same way, but cannot remember how many or which ones.

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Wash uses (#). Erik Meijer, Daan Leijen and James Hook used it to script MS Agent with Active Haskell (a Haskell-COM bridge). Possibly the initial Haskell / ObjectiveC bridge also used (#).
-1

Can't you just redefine $.

let ($) x f = f x

Or just choose a different operator, like $$

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