bug#47302: 27.1; calc math-format-number formatting for floats without decimals is unusual

[Stefan Kangas <stefan@marxist.se>]

Jelle Licht <jlicht@fsfe.org> writes:

> Hey folks,
>
> I run into the following behaviour when using calc in Emacs 27.1 (using
> `emacs -Q'):
>
> (calc-eval "123.0") ; => "123."
> (calc-eval "0.0") ; => "0."
>
> I would expect this output to be either "123.0" or "123" and "0.0" or
> "0" respectively.
>
> I have attached a super simple patch that fixes this issue for me, but I
> am not actually certain of the following:
>
> - Was this formatting quirk perhaps there with a purpose?
> - Does my 'fix' have unintended consequences?

Copying in Mattias Engdegård in the hope that he can provide some
insight here.

I also put David Gillespie on Cc.

>
> Thanks,
>  Jelle
>
>>From 88e52b073510c86b582487a65fa8ddf52a48e7dd Mon Sep 17 00:00:00 2001
> From: Jelle Licht <jlicht@fsfe.org>
> Date: Sun, 21 Mar 2021 22:39:41 +0100
> Subject: [PATCH] (calc): Fix formatting of floats without decimals
>
> * lisp/calc/calc.el (math-format-number): Fix formatting for floats
>   without decimals.
> ---
>  lisp/calc/calc.el | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/lisp/calc/calc.el b/lisp/calc/calc.el
> index ec09abb34c..fac9f70915 100644
> --- a/lisp/calc/calc.el
> +++ b/lisp/calc/calc.el
> @@ -3193,8 +3193,9 @@ math-format-number
>  		   ((= dpos 0)
>  		    (setq str (concat "0" point str)))
>  		   ((and (<= exp 0) (> dpos 0))
> -		    (setq str (concat (substring str 0 dpos) point
> -				      (substring str dpos))))
> +                    (when (> len dpos)
> +		      (setq str (concat (substring str 0 dpos) point
> +				        (substring str dpos)))))
>  		   ((> exp 0)
>  		    (setq str (concat str (make-string exp ?0) point)))
>  		   (t   ; (< dpos 0)