lists:partition/2

[Luke Gorrie]

Raimo Niskanen <raimo@REDACTED> writes:

> Hello,
> 
> I am missing function in the module lists, one that using a predicate
> partitions a list into two; one with all elements for which the
> predicate was true, and one for the rest, while preserving the ordering
> of the elements.

Yes yes!

And here are a few other functions I've written lots of times:

  %% For 'lists'
  %% separate(X, L):
  %% Separate the elements of the list L by placing X between them.
  %% e.g.: lists:flatten(separate(", ", ["foo", "bar", "baz"]))
  %%         => "foo, bar, baz"
  separate(X, [])    -> [];
  separate(X, [H])   -> [H];
  separate(X, [H|T]) -> [H,X|separate(X, T)].

  %% For 'lists'
  %% first(Pred, List)
  %% Find the first element of List satisfying Pred.
  %% Returns: {value, Item} | false
  first(Pred, []) ->
      false;
  first(Pred, [H|T]) ->
      case Pred(H) of
          true ->
              {value, H};
          false ->
              first(Pred, T)
      end.

  %% For 'io_lib'
  %% Very simple, but it's amazing how often
  %% lists:flatten(io_lib:format(..)) makes me go over the 80-column
  %% boundry :)
  flatformat(Fmt, Args) ->
      lists:flatten(io_lib:format(Fmt, Args)).

I know that you can do the 'first' with 'dropwhile' and so on, but I'd
still like to have these :-)

P.S. it's very nice that the latest erlang's lists:sort can take a
predicate!

Cheers,
Luke