Ok I think figured it out! Attached is the updated patch. I had to use
(target-guile-effective-version) instead of effective-version.

On 5/1/20 5:47 AM, Mathieu Othacehe wrote:
> 
> Hello Martin,
> 
> Thanks for this patch.
> 
>> +               (substitute* files
>> +                 (("\"libguile-ncurses\"")
>> +                  (format #f "\"~a/lib/guile/3.0/libguile-ncurses\""
>> +                          out)))
>> +               #t))))))))
> 
> I think you could modify guile-ncurses to use "effective-version"
> instead of hardcoded "2.2". This way, guile3.0-ncurses would just
> inherit from guile-ncurses this way:
> 
> --8<---------------cut here---------------start------------->8---
> (define-public guile3.0-ncurses
>   (package
>     (inherit guile-ncurses)
>     (name "guile3.0-ncurses")
>     (version "3.0")
>     (inputs `(("ncurses" ,ncurses)
>               ("guile" ,guile-3.0)))))
> --8<---------------cut here---------------end--------------->8---
> 
> WDYT?
> 
> Thanks,
> 
> Mathieu
>