Implementing Haskell's `union` Learn You a Haskell shows the union function:


  union also acts like a function on sets. It returns the union of two
  lists. It pretty much goes over every element in the second list and
  appends it to the first one if it isn't already in yet. Watch out
  though, duplicates are removed from the second list!


Example:

ghci&gt; [1..7] `union` [5..10]  
[1,2,3,4,5,6,7,8,9,10] 


Please critique my implementation.

import Data.List (nub)

union' :: (Eq a) =&gt; [a] -&gt; [a] -&gt; [a]
union' xs ys = nub (xs ++ ys)


This 1-line implementation seems easy to understand, however I'm concerned at its bad/mediocre performance. Consider that I'm appending xs to ys, and then performing nub on the whole list.
The formatting of your code is broken, so I've reproduced it here for clarity.

union' :: (Eq a) =&gt; [a] -&gt; [a] -&gt; [a]
union' xs ys = xs ++ union'' ys xs
    where union'' [] _         = []
          union'' (a:as) first = if (a `elem` first) then union'' as first else a : union'' as (a:first)


Your code is correct, but can be rewritten to exhibit better style.

module MyUnion where

import qualified Data.List as List (union) -- For testing with QuickCheck

union :: (Eq a) =&gt; [a] -&gt; [a] -&gt; [a]
union xs ys = xs ++ union' ys xs
    where union' []       skips                   = []
          union' (y':ys') skips | y' `elem` skips =      union' ys'     skips
                                | otherwise       = y' : union' ys' (y':skips)
                             -- ^ Guards instead of ifs
                                               -- ^ Aligned for readability

prop_equal xs ys = union xs ys == List.union xs ys -- QuickCheck-able property


Good Haskell style also involves using higher-level functions whenever other considerations don't require otherwise (e.g., performance, comprehensibility). Here's my version of union, note that it looks much more like your version from before the edits.

union xs ys = xs ++ filter (`notElem` xs) (nub ys)


For reference here is the definition of unionBy from base (union is defined as unionBy (==)).

unionBy                 :: (a -&gt; a -&gt; Bool) -&gt; [a] -&gt; [a] -&gt; [a]
unionBy eq xs ys        =  xs ++ foldl (flip (deleteBy eq)) (nubBy eq ys) xs