[erlang-patches] Add lists:find/2,3

Håkan Mattsson <>
Tue Oct 22 10:15:00 CEST 2013


Further, the function find/2  is inconsistent with the style in the lists
module. Other functions in the lists module returns 'false' instead of
'undefined' when an item not is found. See lists:keyfind/3 for example.

/Håkan

On Mon, Oct 14, 2013 at 12:28 PM, Anthony Ramine <> wrote:

> Hello Sean,
>
> I don't like this patch.
>
> 1/ You can't know whether an element was found or not if the list elements
> are arbitrary terms.
> 2/ I don't see why you would say people hack either foreach/2 or foldl/3
> to do that when they should just use dropwhile/2.
>
> A naive implementation of find/2 using dropwhile/2:
>
> find(F, L) ->
>         case dropwhile(fun (E) -> not F(E) end, L) of
>                 [] -> undefined;
>                 [E|_] -> E
>         end.
>
> Regards,
>
> --
> Anthony Ramine
>
> Le 14 oct. 2013 à 02:44, Sean Cribbs <> a écrit :
>
> > `lists:find/2,3` returns the first element of the passed list for which
> the predicate fun returns `true`. If no elements result in the predicate
> being true, `undefined` (/2) or the given default value (/3) is returned.
> >
> > ## Why this new feature?
> >
> > A common task is to select the first element from a list that matches a
> condition, but there is no existing lists function or language feature that
> avoids traversing the entire list, while still returning a "safe" value.
> `lists:find/2,3` codifies the pattern of a tail-recursive search for the
> matching item without resorting to exceptions (used to abort `foreach/2` or
> `foldl/3`) and always returns either the first matching item, or an
> otherwise safe value.
> >
> > ## Risks / uncertain artifacts
> >
> > It is unclear the desired order of arguments for the 3-arity version. I
> have made the default value the final argument which is consistent with
> `application:get_env/3` and `proplists:get_value/3`, but most functions in
> lists place the `List` argument last.
> >
> > ## How did you solve it?
> >
> > Following the patterns of other functions in the lists module,
> `lists:find/3` tests the predicate function against the head of the list,
> returning the head if the predicate passes, or recursing over the tail if
> it does not.
> >
> > https://github.com/erlang/otp/pull/102
> >
> > --
> > Sean Cribbs <>
> > Software Engineer
> > Basho Technologies, Inc.
> > http://basho.com/
> > _______________________________________________
> > erlang-patches mailing list
> > 
> > http://erlang.org/mailman/listinfo/erlang-patches
>
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