[erlang-questions] Q: Differenciating between List and List of Lists
Thu Oct 16 00:34:04 CEST 2008
On 15 Oct 2008, at 6:46 pm, jm wrote:
> What is the best way to tell the difference between a list and a
> list of
> lists (possible in a guard)?
It's impossible: an empty list and an empty list of lists
are the same thing.
You can tell the difference between a non-empty list of
non-empty lists and any other list thus:
of [[_|_]|_] -> non-empty list of lists
; X when is_list(X) -> any other list
; _ -> not a list at all
If you are specifically interested in distinguishing
between strings and lists of strings,
if is_integer(hd(Thingy)) -> non-empty string
; is_list( hd(Thingy)) -> non-empty list of strings
; Thingy =:=  -> empty so could be either
; true -> not a list at all
There are more variations on both of these than I have time
to write or you to read.
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