Re: How to read an integer from the minibuffer

[Marcin Borkowski]

On 2021-11-12, at 08:21, Yuri Khan <yuri.v.khan@gmail.com> wrote:

> On Thu, 11 Nov 2021 at 14:19, Marcin Borkowski <mbork@mbork.pl> wrote:
>
>> >> How do I do that?  One way would be to use `read-from-minibuffer' with
>> >> a suitable keymap, but that seems slightly low-level.  If that is the
>> >> way to go, is there a good way to set up a keymap so that nothing except
>> >> some specified characters are self-inserting?  IOW, is `suppress-keymap'
>> >> the way to go or is there some other way?
>> >
>> > Perhaps you're looking for `read-number'?
>>
>> Thanks, that is nice, but not very general.  Another value I want to
>> read is a time in the hh:mm format, for example.
>
> There are lots of examples out there of form controls trying to
> prevent the user from entering invalid data, and they demonstrate many
> things you can get wrong while implementing one.
>
> A form control for integer numbers only permits typing digits (as you
> initially asked). Now the user cannot enter -42.
>
> Okay, after a bug report and a fix, the control now permits the minus
> sign. But the control validates the entry after every keypress, so, as
> soon as the user starts typing, they are presented with an error
> message, “‘-’ is not a valid integer”. Well, duh, I just started
> typing, let me finish it!
>
> My bank’s money transfer web form’s amount field filters keypresses to
> only allow digits. One, I cannot paste a number I copied from an
> invoice opened elsewhere. Two, I cannot press Ctrl+L to activate my
> browser’s address bar, or switch to another tab with Ctrl+PgUp,
> Ctrl+PgDn, or Alt+[1…9]. Three, I can’t move my cursor if I
> inadvertently skip or mistype a digit, e.g., to correct a 1000 to
> 15000, I cannot type ‘← ← ← 5’, I have to type ‘backspace backspace
> backspace 5 0 0 0’.
>
> A license activation input box in a proprietary software installer
> wants a string of 20 semi-random letters. Since no one can reliably
> enter that in one go, it splits those 20 letters into 4 groups of 5
> and presents them as separate input boxes. As a courtesy to the user,
> as soon as 5 letters are typed in the first box, the cursor
> automatically moves to the second box. However, if you mistype the
> last of those 5 letters, Backspace does not delete it, because
> technically you’re in the second box and it’s technically empty,
> there’s nothing to delete. Also, you can’t paste the whole 20-letter
> code in one go, because the first box is limited to 5 letters.
>
> A bank money transfer is confirmed with a 4-digit code that is sent in
> an SMS message. As soon as you type the 4th digit, it goes on to
> verify the code, without you having to press Enter. However, this
> means you cannot immediately correct a mistyped 4th digit; you get a
> “verification failed” message first.
>
>
> Guideline: Perform input validation only after the user has indicated
> they are finished with data entry.

Very good points, thanks.  I am now convinced that I should just
`read-string' - it's also simpler from the coding perspective.

Thanks,

-- 
Marcin Borkowski
http://mbork.pl