Re: Why (substring "abc" 0 4) does not return "abc" instead of an error?

[Bastien]

Hi Tassilo,

Tassilo Horn <address@hidden> writes:

> Bastien <address@hidden> writes:
>
>>> We have general functionality when you want to ignore some errors,
>>> such as condition-case.
>>
>> Also, I'm fine with
>>
>>   (substring "abc" -1 1)
>>     => #ERROR
>
> I don't see why that justifies an error and (substring "abc" 0 4) does
> not.  -1 is a valid FROM index meaning the length of the string minus
> one.  Its just that the TO index is smaller than FROM here, but IMO
> that's the same class of errors as a too large TO index.

I would distinguish problems caused by only one indice from those 
caused by the relationship between two indices.  Just nit-picking.

>> so using ̀condition-case' would not help me distinguish
>> between the case above and (substring "abc" 0 4), which
>> is what I want.
>
> A condition-case handler has access to the args given to the erroring
> form, so the cases could be distinguished although both signal
> args-of-range.  Well, not that it would help you much here.  

Fair enough.

>> I see the benefit of having 
>>
>>   (substring "abc" 0 4)
>>     => "abc"
>>
>> in terms of simplifying Elisp writing -- and I still fail
>> to see the harm (but maybe Pascal will tell me where he has
>> been bitten by this.)
>
> In my experiences, out-of-range indices into strings or arrays are
> almost always programming errors.  I'm not even able to come up with
> some concrete use-case where I'd like to have the suggested behavior.
> Either I know exactly what I'm operating on and use indices, or I have
> only some general assumptions and then use more fuzzy things like
> splitting by regular expressions.

I get your point about programming errors.  And I think it's just
part of my brain that cannot help thinking of the third argument as
a _length_ instead of an _index_.  I now see how allowing to fail 
quietly would make this misconception more common.

How about these two defun-future-defsubst candidates?

(defun string-head (string n)
  "Return N characters starting from the beginning of STRING.
If N is larger than the length of STRING, return it."
  (substring string 0 (min n (length string))))

(defun string-tail (string n)
  "Return N characters starting from the end of STRING.
If N is larger than the length of STRING, return it."
  (let* ((l (length string))
         (s (- l n))
         (d (if (> s 0) s 0)))
    (substring string d l)))

-- 
 Bastien