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I tried to practice haskell a bit but I didn't get the following code to work:

rems :: Int -> [Int] -> [Int]
rems _ [] = []
rems k (x:xs)
  | k == x = rems k xs
  | otherwise [x] ++ rems k xs



main = print $
  rems 3 [5, 3, 2]

This function removes every k from a list x if you call it rems k x. I know that it should be working because we wrote it down in university.

I'm using this IDE: https://repl.it/languages/haskell

Will Ness
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Philipp Niedergesäß
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    No you forgot a `=` after the `otherwise`, it is `otherwise = [x] ++ rems k xs`. You can improve it (slightly), by writing `otherwise = (x: rems k xs)`. – Willem Van Onsem Jun 22 '19 at 17:29

1 Answers1

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It might be useful to know that otherwise isn't a keyword or special symbol like =, it's actually simply a boolean value defined in the prelude as True. I.e., a guard with otherwise works syntactically the same as any other guard, just the condition is trivial. You might also write it

rems _ [] = []
rems k (x:xs)
  | k == x  = rems k xs
  | True    = [x] ++ rems k xs

The alignment I chose above (all = aligned and at least two† spaces away from the guard-condition) is completely a matter of taste, but I think it helps avoiding confusion like the one you've found yourself in.

BTW, [x] ++ ... can be shortened to x : .... The preferred form of writing the function is

rems _ [] = []
rems k (x:xs)
  | k==x       = rems k xs
  | otherwise  = x : rems k xs

†Most people align the =s but use only one space. That's ok, but IMO k == x = res looks deceptive with all those equals-characters, and k and x further away from each other than from the result. k==x = res seems tidier.

leftaroundabout
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    an additional extra space to the right of `=` as well is even clearer (and symmetry is beautiful). another terrible-looking type of code is with `let` on the RHS. `foo x | x == y = let z = bar y x in ...`. brr. – Will Ness Jun 22 '19 at 18:39
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    @WillNess well, it's _not_ symmetric – the RHS is really part of an `a = b` like in other definitions, but the LHS is something completely different. That's also a reason why I like to have extra space on the left. – leftaroundabout Jun 22 '19 at 21:08
  • it's the *space* around the `=` sign that ought to be symmetric IMO, guard or no guard. in any case it's aesthetically pleasing *to me* that way; YMMV. – Will Ness Jun 23 '19 at 07:40