[erlang-questions] lists:seq/3 strange behaviour

Pascal Chapier <>
Tue Jun 8 17:53:30 CEST 2010


OOps, I answer too fast,

I think the the code should be

seq(First,
 Last, Inc) 
    when is_integer(First), is_integer(Last), 
is_integer(Inc) -> 
    if
        Inc > 0, First =< 
Last;
        Inc < 0, First >= 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.

In this case we get:
    
 an exeption for both for lists:seq(2, 1, 10) and lists:seq(20, 1, 10).
    
 a list of one element if inc > (last-first) positive case

at 
least the behavior is symetric for positive and negative increment.

I
 know, the "let it crash" is more in conformance with the Erlang usage 
:o)
As the good behavior depends on the the application use case, may
 be you are right, and the application will have to include a try/catch 
if the case is likely to appear.

Pascal



> Date:
 Tue, 8 Jun 2010 15:58:07 +0200
> From: 

> To: 
>
 CC: 
> Subject: Re: [erlang-questions]
 lists:seq/3 strange behaviour
> 
> Piotr wrote:
> 
> What was the reason for lists:seq function to be implemented in a 
way that
> if you call:
> > lists:seq(2, 1, 10)
> 
> it works as expected returning an an empty list, while when beeing 
called
> like:
> > lists:seq(20, 1, 10)
> > it 
throws an exception?
> 
> Pascal wrote:
> > Or, a 
solution that I prefer, add a default condition which return an
> 
empty list as you suggest.
> 
> In earlier versions (for 
example R12B-5) lists:seq/3 on the examples above
> throws a 
function-clause exception which makes more sense than a missing
> 
true-branch in an if-clause. I would prefer stronger guards and a
>
 function-clause exception over a solution which returns an empty list 
simply
> because the function is undefined for those cases.
>
 
> /Jesper Pettersson
 		 	   		   		 	   		  
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