(list-head lst k) with k longer than (length lst)

(list-head lst k) with k longer than (length lst)

[Jan Synáček]

Hello,

I'm getting:

scheme@(guile-user)> (list-head '(1 2 3) 5)
ERROR: In procedure list-head:
ERROR: In procedure list-head: Wrong type argument in position 1
(expecting pair): ()

This looks pretty much like a bug to me. Shouldn't list-head return
the entire list when the 'k' is bigger than its length? If that is not
the case, at least the error is really confusing. I'm using Guile
2.0.11.

Cheers,
-- 
Jan Synáček

Re: (list-head lst k) with k longer than (length lst)

[Chris Vine]

On Tue, 17 Nov 2015 11:53:05 +0100
Jan Synáček <jan.synacek@gmail.com> wrote:
> Hello,
> 
> I'm getting:
> 
> scheme@(guile-user)> (list-head '(1 2 3) 5)
> ERROR: In procedure list-head:
> ERROR: In procedure list-head: Wrong type argument in position 1
> (expecting pair): ()
> 
> This looks pretty much like a bug to me. Shouldn't list-head return
> the entire list when the 'k' is bigger than its length? If that is not
> the case, at least the error is really confusing. I'm using Guile
> 2.0.11.

The error message is confusing, but I guess the behaviour of list-head
mirrors R5RS list-tail: instead of mandating the return of an empty
list, R5RS states that "It is an error if list has fewer than k
elements".

Chris

Re: (list-head lst k) with k longer than (length lst)

[Mark H Weaver]

Chris Vine <chris@cvine.freeserve.co.uk> writes:

> On Tue, 17 Nov 2015 11:53:05 +0100
> Jan Synáček <jan.synacek@gmail.com> wrote:
>> Hello,
>> 
>> I'm getting:
>> 
>> scheme@(guile-user)> (list-head '(1 2 3) 5)
>> ERROR: In procedure list-head:
>> ERROR: In procedure list-head: Wrong type argument in position 1
>> (expecting pair): ()
>> 
>> This looks pretty much like a bug to me. Shouldn't list-head return
>> the entire list when the 'k' is bigger than its length? If that is not
>> the case, at least the error is really confusing. I'm using Guile
>> 2.0.11.
>
> The error message is confusing, but I guess the behaviour of list-head
> mirrors R5RS list-tail: instead of mandating the return of an empty
> list, R5RS states that "It is an error if list has fewer than k
> elements".

Exactly right, but I agree that the error message is confusing.

Unless otherwise specified, Scheme and Guile tend to be strict in cases
like this.  For example, if a procedure is documented to "Copy the first
K elements from LST", and says nothing about what happens for lists with
fewer than K elements, you should assume that it's an error.

     Mark

Re: (list-head lst k) with k longer than (length lst)

[Jan Synáček]

On Tue, Nov 17, 2015 at 10:27 PM, Mark H Weaver <mhw@netris.org> wrote:
> Chris Vine <chris@cvine.freeserve.co.uk> writes:
>
>> On Tue, 17 Nov 2015 11:53:05 +0100
>> Jan Synáček <jan.synacek@gmail.com> wrote:
>>> Hello,
>>>
>>> I'm getting:
>>>
>>> scheme@(guile-user)> (list-head '(1 2 3) 5)
>>> ERROR: In procedure list-head:
>>> ERROR: In procedure list-head: Wrong type argument in position 1
>>> (expecting pair): ()
>>>
>>> This looks pretty much like a bug to me. Shouldn't list-head return
>>> the entire list when the 'k' is bigger than its length? If that is not
>>> the case, at least the error is really confusing. I'm using Guile
>>> 2.0.11.
>>
>> The error message is confusing, but I guess the behaviour of list-head
>> mirrors R5RS list-tail: instead of mandating the return of an empty
>> list, R5RS states that "It is an error if list has fewer than k
>> elements".
>
> Exactly right, but I agree that the error message is confusing.
>
> Unless otherwise specified, Scheme and Guile tend to be strict in cases
> like this.  For example, if a procedure is documented to "Copy the first
> K elements from LST", and says nothing about what happens for lists with
> fewer than K elements, you should assume that it's an error.
>
>      Mark

Thank you for the explanation.

-- 
Jan Synáček