Thank you Efraim, this is exactly what I needed.  I've applied the trick
used by GHC (simpler in my opinion), it works perfectly.

As Mathieu pointed out, this is a bit unsettling at first.  Is this
(thunked) behaviour documented anywhere?  If not, shall we documented
it?  Where?

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/