This post originated from an RSS feed registered with Java Buzz by Elliotte Rusty Harold. Original Post: Haskell: Concatenation vs. Prepending Feed Title: Mokka mit Schlag Feed URL: http://www.elharo.com/blog/feed/atom/? Feed Description: Ranting and Raving Latest Java Buzz Posts Latest Java Buzz Posts by Elliotte Rusty Harold Latest Posts From Mokka mit Schlag

This confused me a little until I grokked it. In Haskell, an item is not the same as a list containing the one item. (Obvious, I know, but some languages like XQuery take the opposite view).

The : operator prepends an item to the beginning of a list. It can only be used to insert an item at the beginning of a list. It cannot be used to append an item to the end of a list. Thus x:xs is correct but xs:x is a syntax error. (Haskell’s design seems stuck with assumptions going all the way back to Lisp and 1950s era processors including that the singly linked list is a good implementation for the list abstract data type. This has other consequences. For instance, length is an O(n) operation. And now that I look for it, insert into the middle of the list also seems to be missing.)

Lists are concatenated with the ++ operator, not :.

So, to sum up:

• Use : to insert an item at the beginning of a list.
• Use ++ to join two lists together.
• To append an item to a list, insert the item into an empty list, and then concatenate the two lists together.
• More generally, to insert an item into a list at any position other than the first:
  1. Split the list into two sublists at the insertion point with splitAt.
  2. Use : to prepend the new item to the second list.
  3. Concatenate the two lists back together with ++

Am I missing something obvious here?