Re: Mysterious error while refactoring guix/scripts/system.scm

[ludo@gnu.org (Ludovic Courtès)]

Hello,

Chris Marusich <cmmarusich@gmail.com> skribis:

> The reason I wanted to perform this refactoring in the first place is
> because I'd like to add a new procedure to guix/scripts/system.scm
> called 'switch-to-system-generation'.  Because this new procedure calls
> other procedures which seem to require access to the store, I thought I
> would need to call 'switch-to-system-generation' via 'run-with-store'.
> Am I just confused?

Essentially, to write a procedure that needs to access the store, you
need to write a “monadic procedure”—i.e., a procedure that returns a
value in the “store monad” instead of a “normal value.”

To do that, you would write:

  (define (switch-to-system-generation …)
    (with-monad %store-monad
      (return 'some-value)))    ;return this symbol as a monadic value

or:

  (define (switch-to-system-generation …)
     (some-monadic-procedure x y z))  ;tail-call a monadic procedure

The caller of a monadic procedure must be prepared to “unpack” its
value, using the monadic “bind” operator.  The ‘mlet’ form does exactly
that:

  (define (the-caller …)
    (mlet %store-monad ((result (switch-to-system-generation …)))
      …))

At the bottom, there must be somewhere a call to ‘run-with-store’ to
“run” the monadic value in the monad (info "(guix) The Store Monad").
This call is already in ‘process-action’ in (guix scripts system).

> In particular, 'switch-to-system-generation' will eventually call the
> existing procedure 'grub-configuration-file' (defined in
> gnu/system/grub.scm).  As I understand it, 'grub-configuration-file'
> returns a derivation that builds a GRUB configuration file.  This
> existing 'grub-configuration-file' procedure does a lot with the store
> and gexps.  I thought that if I didn't use 'run-with-store' to run
> 'switch-to-system-generation', it wouldn't work because
> 'grub-configuration-file' wouldn't work.

‘grub-configuration-file’ is a monadic procedure.  Thus, to “unpack” its
return value, you need to bind it, for instance with ‘mlet’:

  (mlet %store-monad ((file (grub-configuration-file …)))
    …)

or:

  (with-monad %store-monad
    (>>= (grub-configuration-file …)
         (lambda (file)
           …)))

HTH!

Ludo’.