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Re: Mysterious error while refactoring guix/scripts/system.scm
From: |
Ludovic Courtès |
Subject: |
Re: Mysterious error while refactoring guix/scripts/system.scm |
Date: |
Fri, 30 Sep 2016 22:36:29 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
Hello,
Chris Marusich <address@hidden> 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’.