Flipped / reversed fmap (<$>)?

Question

I found defining the following

(%)  = flip fmap

I can write code like this:

readFile "/etc/passwd" % lines % filter (not . null)

To me it makes more sense than the alternative:

filter (not . null) <$> lines <$> readFile "/etc/passwd"

Obviously, it's just a matter of order.

Does anyone else do this? Is there a valid reason not to write code like this?

The more common name for this is probably ```<&>``` following ```(&) = flip ($)``` by lens. — fread2281, Aug 30 '14 at 23:03

score 12 · Answer 1 · answered Dec 07 '19 at 21:08

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(<&>) :: Functor f => f a -> (a -> b) -> f b

Now available from Data.Functor in base.

https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Functor.html#v:-60--38--62-

answered Dec 07 '19 at 21:08

Chris Stryczynski

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Since `base` version 4.11.0.0 – MikaelF Jan 11 '20 at 03:48

score 10 · Answer 2 · edited Oct 27 '18 at 08:20

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Your operator (%) is exactly the operator (<&>) from the lens package.

It can be imported with:

import Control.Lens.Operators ((<&>))

edited Oct 27 '18 at 08:20

Chris Stryczynski

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answered Jul 01 '16 at 13:24

Bakuriu

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score 6 · Answer 3 · edited Jul 31 '19 at 10:03

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There is a similar function for the Applicative type class called <**>; it's a perfectly reasonable thing to want or use for Functor as well. Unfortunately, the semantics are a bit different for <**>, so it can't be directly widened to apply to Functor as well.

edited Jul 31 '19 at 10:03

Matthias Braun

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answered Nov 05 '09 at 03:14

bdonlan

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score 4 · Answer 4 · answered Nov 05 '09 at 05:31

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-- (.) is to (<$>) as flip (.) is to your (%).

I usually define (&) = flip (.) and it's just like your example, you can apply function composition backwords. Allows for easier to understand points-free code in my opinion.

answered Nov 05 '09 at 05:31

codebliss

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There's also the Arrow combinator (>>>) which is the same as (flip (.)) for functions – Will Nov 05 '09 at 22:23
Why thank you sir! Other than the import boilerplate, this will be useful so I can keep standard code. – codebliss Nov 07 '09 at 04:58
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Note that lens defines ```(&) = flip ($)``` – fread2281 Aug 30 '14 at 23:04

score 1 · Answer 5 · answered Nov 05 '09 at 08:46

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Personally I wouldn't use such an operators because then I have to learn two orders in which to read programs.

answered Nov 05 '09 at 08:46

Martijn

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Not sure I find this argument compelling enough, but it's an interesting point of view, so +1. Out of curiosity: do you prefer `=<<` over `>>=` for that same reason? – Stephan202 Nov 05 '09 at 08:52
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I never understood the previous statement. It seems to make sense using standard notation because then it's like unix piping? – codebliss Nov 05 '09 at 20:16
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I prefer =<< to >>= because the form has a clear interpretation in terms of the Kleisli category of your monad which can be inverted to give an understandable definition for comonads. – Edward Kmett Nov 05 '09 at 20:37
Stephan: that said your argument seems I'll posed. Preferring >>= to =<< would be more appropriate as you are given the former and the other is derived. I realize you were after the argument order though. – Edward Kmett Nov 05 '09 at 20:40
I disagree. One *already* has to reverse the narration flow in deep function compositions (point-free style most notably), to be able to construe the data flow of the code. At the same time, `>>=` does lead to code flow which matches data flow. These together can easily lead to code lines like this: `action1 >>= action3 . action2 >>= action4`, where the reader has to *reverse his reading order multiple times in a line* to be able to understand the data flow pipeline. Flipped function composition will be helpful in this case. – ulidtko Dec 24 '14 at 16:54

Flipped / reversed fmap (<$>)?

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