Re: Why should build phases not return unspecified values?

[Danny Milosavljevic]

Hi Mark,

I agree that exceptions are the way to go here.

Btw, Ludo and I put "invoke" into ./guix/build/utils.scm somewhen in June (now 
in master).

(define (invoke program . args)
  "Invoke PROGRAM with the given ARGS.  Raise an error if the exit
code is non-zero; otherwise return #t."
  (let ((status (apply system* program args)))
    (unless (zero? status)
      (error (format #f "program ~s exited with non-zero code" program)
             status))
    #t))

So we could actually use "invoke" in phases where we use "system*" now.  

There are 1331 lines with "system*" in gnu/packages - that would take some time 
to port.  Or we could substitute (zero? (system* ...)) by (invoke ...) with the 
editor - I don't see much risk with doing the latter, in core-updates-next or 
something.

In the far future, phase API could change to not examine the result of a phase 
and "invoke" would still work fine.