# lists:seq/3 strange behaviour

Pascal Chapier pascalchapier@REDACTED
Tue Jun 8 14:08:51 CEST 2010

```In fact it does exactly what it is written in the documentation:

seq(From, To, Incr) -> Seq

Types:

>From = To = Incr = int()

Seq = [int()]

Returns a sequence of integers which starts with From
and contains the successive results of adding Incr to
the previous element, until To has
been reached or
passed (in the latter case, To is
not an element of
the sequence). Incr defaults to 1.

Failure: If To<From-Incr and Incr
is positive, or if To>From-Incr
and Incr is
negative, or if Incr==0 and From/=To.

and implemented in the code:

seq(First, Last, Inc)
when is_integer(First), is_integer(Last), is_integer(Inc) ->
if
Inc > 0, First - Inc =< Last;
Inc < 0, First - Inc >= Last ->
N = (Last - First + Inc) div Inc,
seq_loop(N, Inc*(N-1)+First, Inc, []);
Inc =:= 0, First =:= Last ->
seq_loop(1, First, Inc, [])
end.

But you are right, the purpose of the If test is to evaluate the length of the list, and as there is no default contition which could produce an empty list, I guess that the intent was to generate an exception when parameters cannot produce finite non empty list. So I think it is a bug and tha the code should be

seq(First, Last, Inc)

when is_integer(First), is_integer(Last), is_integer(Inc) ->

if

Inc > 0, First + Inc =< Last;

Inc < 0, First - Inc >= Last ->

N = (Last - First + Inc) div Inc,

seq_loop(N, Inc*(N-1)+First, Inc, []);

Inc =:= 0, First =:= Last ->

seq_loop(1, First, Inc, [])

end.

and the doc: Failure: If To<From+Incr and Incr
is positive, or if To>From-Incr
and Incr is
negative, or if Incr==0 and From/=To.

Or, a solution that I prefer, add a default condition which return an empty list as you suggest.

Pascal.

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