Hi everyone,

Actually, let me add a big erratum, since it appears I misread the
intention of gexp->derivation (and realized 2 minutes after writing the
previous email).

Josselin Poiret <dev@jpoiret.xyz> writes:
> --8<---------------cut here---------------start------------->8---
> (mlet* %store-monad ( ;; The following binding forces '%current-system' and
>                      ;; '%current-target-system' to be looked up at >>=
>                      ;; time.
>                      (graft?    (set-grafting graft?))
>
>                      (system -> (or system (%current-system)))
>                      (target -> (if (eq? target 'current)
>                                     (%current-target-system)
>                                     target))
>                      ...)
>   ...)
> --8<---------------cut here---------------end--------------->8---
>
> Well, the issue here is that such an mlet starts by translating the
> graft? binding into a >>= call, which ends up putting the rest of the
> translation into a function call that will *not* be called until the
> monadic value is run.  That means that the system and target bindings
> afterwards are *not* looked up at call time but at monadic run time!

This is actually what the comment above hints at, I misunderstood its
meaning.  It seems that this piece of code used to be (before 2015)

--8<---------------cut here---------------start------------->8---
(mlet* %store-monad ( ;; The following binding forces '%current-system' and
                     ;; '%current-target-system' to be looked up at >>=
                     ;; time.
                     (unused    (return #f)

                     (system -> (or system (%current-system)))
                     (target -> (if (eq? target 'current)
                                    (%current-target-system)
                                    target))
                     ...)
  ...)
--8<---------------cut here---------------end--------------->8---

probably at a time when (current-system) didn't exist.  In turn, this
means that gexp->derivation intentionally delays getting the current
system to monadic run time.  Thus, we probably need to pass an optional
#:system argument to the hooks that they can forward to
gexp->derivation to fix this “properly”

> IMO, this is way too complicated to keep in mind at all times, and there
> are bugs lurking under the surface absolutely everywhere, waiting for a
> corner case to be uncovered.

My comment still stands.

Best,
-- 
Josselin Poiret