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"Being clever" antipattern in Elm

aneb sometimes more is less

Disclaimer: I write this article based on my experience and "feels" - and thus the points might not be right and are open to discussion! In the post I say things like "Elm values X and Y" - and might be wrong. I believe I'm not though :) Definitely feel free to write a comment below!


I sometimes lurk on the Elm Slack (obligatory registration link!) and talk with people hanging there. We have the #beginners and #help channels for questions of any kind, however trivial.

Today a person wanted to optimize this expression:

List.partition ((==) 2) [1,2,3]

with regards to parentheses count (ie. get rid of them). A few solutions appeared:

equals2 =
    (==) 2
List.partition equals2 [1,2,3]

flip List.partition [1,2,3] <| (==) 2

and some judgement was made:

"personally, the parentheses before made it easier to read and like more normal elm code"

I understand the poster's question - he wants to see if there's a better way to write his code. I often feel this way in Haskell. Given how there are many (many many many) operators, there often is a way to write more succint code.

In Elm, though, there is usually One Good Way™ to do a particular thing, and it's a simple way at that.

In this particular example, the original code is probably good enough. But I would even go as far as to not use the ((==) 2) part, and go for maximum readability. For me, that means:

List.partition (\x -> x == 2) [1,2,3]

The thing is, ((==) 2) might not be too bad, but ((<) 2) would stop me for anywhere from 5 to 30 seconds before I was sure of what it does. ("Do I have the condition reversed in my head?")

Compare it to this version, where it's immediately obvious what the function does and that it's correct.

List.partition (\x -> x < 2) [1,2,3]

Some functions are commutative (a == b is the same as b == a) and some are not (a < b vs b < a), and I really really don't want to think about "do I have it right?" two weeks from now just because I used the partial function application a few minutes ago and feel clever about it.


The same goes for "point-free" style (ommiting arguments from function definitions):

sum : List number -> number
sum =
    List.foldl (+) 0

Again, this particular example isn't too bad but it makes me freeze for a few seconds: "Wait a minute, the type definition says something else than the arguments of the function!"

I guess going a bit more silly with this example would illustrate it better:

sumWithStartingValue : number -> List number -> number
sumWithStartingValue =
    List.foldl (+)

I'd much rather have:

sumWithStartingValue : number -> List number -> number
sumWithStartingValue startingValue numbers =
    List.foldl (+) startingValue numbers

because there is absolutely zero magic, zero cleverness, zero figuring out how the particular function works.


Haskell has this culture of "clever is better", and sure, it allows the programs be very, very terse. But I don't really see the value in that, and as far as I can tell, Elm doesn't either.

Elm doesn't value writing terse code, or clever code. Elm values readability. If you get thoughts like "This is so painfully explicit", like I do when seeing and writing code like (\x -> x == 2), don't let them make you refactor it into something more clever, but less readable.

Napište komentář

1. Franklin Chen
24. 4. 2017 23:20

My preference for the partition example is to use the piping operator:

[1, 2, 3] |> List.partition ((==) 2)

2. Martin Janiczek
25. 4. 2017 13:49

@Franklin Chen [1]:

Yeah! With newlines before |>, I'd use that too instead of x (y (z (...))) if there was more processing to chain after that. The readability is greater in that case :)