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

Anthony Ramine <>
Tue Oct 22 12:04:43 CEST 2013


Forwarding to list.

-- 
Anthony Ramine

Le 22 oct. 2013 à 11:56, Anthony Ramine <> a écrit :

> Maybe we could just have dropuntil(F, L) -> dropwhile(fun (X) -> not F(X) end, L).
> 
> Of course, the function would be written less naively to avoid constructing a new fun.
> 
> -- 
> Anthony Ramine
> 
> Le 14 oct. 2013 à 15:52, Sean Cribbs <> a écrit :
> 
>> #1: Would tagging the return value address like Masklinn mentions address your concern? {ok, Term} | error (instead of a default value)?
>> 
>> #2: Also, I considered dropwhile but it involves introducing another fun, whereas the version I wrote is direct.
>> 
>> 
>> On Mon, Oct 14, 2013 at 5:28 AM, 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
>> 
>> 
>> 
>> 
>> -- 
>> Sean Cribbs <>
>> Software Engineer
>> Basho Technologies, Inc.
>> http://basho.com/
> 



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