Add lists:find/2,3 #102
Add lists:find/2,3 #102
Conversation
|
Patch has passed first testings and has been assigned to be reviewed |
|
Isn't |
| Default :: term(). | ||
|
|
||
| find(Pred, [], Default) when is_function(Pred, 1)-> Default; | ||
| find(Pred, [H|T], Default) -> |
There was a problem hiding this comment.
The guard for the predicate should be on the clause where you actually call the function :)
EDIT: Actually, you should probably remove the guard altogether. Without it, a Pred that is not a function will give a "bad function" error which is more informative than "function clause".
There was a problem hiding this comment.
Normally I would agree with you but all of the other functions in this module only guard on the empty-list case.
There was a problem hiding this comment.
Yes, but in the case where you pass an empty list, you won't get an error if Pred is an invalid function.
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, 'false' (/2) or the given default value (/3) is returned.
|
We find that lists:dropwhile/2 does this already |
|
I'm surprised this was closed so abruptly, since there hasn't been any discussion or feedback from OTP in the past three months.
|
This is essentially PR 102, erlang#102. The OTP Technical Board decided to change the name of the function to search/2.
…roper-lists try to fix non-proper lists in state
lists:find/2,3returns the first element of the passed list for which the predicate fun returnstrue, wrapped in a tuple{value, Elem}. If no elements result in the predicate being true,false(/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/2,3codifies the pattern of a tail-recursive search for the matching item without resorting to exceptions (used to abortforeach/2orfoldl/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/3andproplists:get_value/3, but most functions in lists place theListargument last.How did you solve it?
Following the patterns of other functions in the lists module,
lists:find/3tests 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.