[erlang-questions] lists:all/2 unexpected result for the empty list

[James Aimonetti]

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Was wondering the reasoning for lists:all/2 returning true when passed
an empty list as the second parameter?

lists:all(some_fun/1, []) => 'true'

The docs state "Returns true if Pred(Elem) returns true for all elements
Elem in List, otherwise false."[1]

The definition in lists.erl for all/2 shows why it returns true:

all(Pred, [Hd|Tail]) ->
    case Pred(Hd) of
	true -> all(Pred, Tail);
	false -> false
    end;
all(Pred, []) when is_function(Pred, 1) -> true.

To me, at least, since Pred(Elem) never returned true, the outcome
should be false. I might write it as:

all(_, []) -> false;
all(Pred, L) ->
all_(Pred, L).

all_(Pred, [Hd|Tail]) ->
  case Pred(Hd) of
    true -> all(Pred, Tail);
    false -> false
  end;
all_(Pred, []) when is_function(Pred, 1) -> true.

lists:any/2 returns false, as expected, for the empty list, but for
similar reasons as lists:all/2.

Anyway, minor quibble, but I was curious of the reasoning...

James

[1] http://erldocs.com/R14B/stdlib/lists.html?i=0&search=lists%20all#all/2
- -- 
James Aimonetti
Distributed Systems Engineer / DJ MC_

2600hz | http://2600hz.com
sip:james@REDACTED
tel: 415.886.7905
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