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

Anthony Ramine <>
Mon Oct 14 12:28:33 CEST 2013


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/
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> 
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