Hi Ludo’,

Thank you for getting back to me!

ludo@gnu.org (Ludovic Courtès) writes:

> Hi Chris, and sorry for the delay!
>
> Chris Marusich <cmmarusich@gmail.com> skribis:
>
>> Backtrace:
>> In ice-9/boot-9.scm:
>>  157: 15 [catch #t #<catch-closure 26304a0> ...]
>> In unknown file:
>>    ?: 14 [apply-smob/1 #<catch-closure 26304a0>]
>> In ice-9/boot-9.scm:
>>   63: 13 [call-with-prompt prompt0 ...]
>> In ice-9/eval.scm:
>>  432: 12 [eval # #]
>> In ice-9/boot-9.scm:
>> 2401: 11 [save-module-excursion #<procedure 264f940 at ice-9/boot-9.scm:4045:3 ()>]
>> 4050: 10 [#<procedure 264f940 at ice-9/boot-9.scm:4045:3 ()>]
>> 1724: 9 [%start-stack load-stack ...]
>> 1729: 8 [#<procedure 2668ea0 ()>]
>> In unknown file:
>>    ?: 7 [primitive-load "/root/guix/scripts/guix"]
>> In guix/ui.scm:
>> 1209: 6 [run-guix-command system "list-generations"]
>> In ice-9/boot-9.scm:
>>  157: 5 [catch srfi-34 #<procedure 52af180 at guix/ui.scm:425:2 ()> ...]
>>  157: 4 [catch system-error ...]
>> In guix/scripts/system.scm:
>>  884: 3 [#<procedure 52ae750 at guix/scripts/system.scm:876:2 ()>]
>>  818: 2 [process-command list-generations () ...]
>> In guix/store.scm:
>> 1182: 1 [run-with-store # ...]
>> In unknown file:
>>    ?: 0 [#<unspecified> #<build-daemon 256.15 52b8e80>]
>>
>> ERROR: In procedure #<unspecified>:
>> ERROR: Wrong type to apply: #<unspecified>
>
> This means that we’re trying to invoke #<unspecified>, but
> #<unspecified> is not a procedure.
>
> Note that this is from within ‘run-to-store’, which is the procedure to
> “run” a monadic value.  So in effect, what happened is equivalent to:
>
>
> scheme@(guile-user)> ,use(guix monads)
> scheme@(guile-user)> ,use(guix store)
> scheme@(guile-user)> (with-store s
> 		       (run-with-store s *unspecified*))
> ERROR: In procedure #<unspecified>:
> ERROR: Wrong type to apply: #<unspecified>
>
>
> The bug here is in fact a type error: ‘run-with-store’ expect a monadic
> value, but what we have here is a regular value.
>
> To “fix” it, we need:
>
>
> scheme@(guile-user)> (with-store s
> 		       (run-with-store s
> 			 (with-monad %store-monad (return *unspecified*))))
>
>
> … where ‘return’ procedure a monadic value from a normal value.
>
> In your case, ‘list-generations’ is not a monadic procedure (a procedure
> that returns a monadic value), so in this patch:
>
>   (run-with-store store
>     …
>     (list-generations))
>
> … triggers this very type error.

I see.  I've read the sections in the manual about gexps, and I've
peeked at the gexp code.  I thought that basically I could use
run-with-store to run any Guile code I want, but I seem to be missing
something.  I will try experimenting with monads and the store to learn
more and hopefully avoid surprises like this going forward.

> I imagine this may be more than you wanted to learn.  ;-)
> Monads in a dynamically typed setting are kinda annoying because of
> this.

I appreciate the info.  Hopefully I will fully understand it soon.

>>  (define (process-command command args opts)
>>    "Process COMMAND, one of the 'guix system' sub-commands.  ARGS is its
>>  argument list and OPTS is the option alist."
>> -  (case command
>> -    ((list-generations)
>> -     ;; List generations.  No need to connect to the daemon, etc.
>> -     (let ((pattern (match args
>> -                      (() "")
>> -                      ((pattern) pattern)
>> -                      (x (leave (_ "wrong number of arguments~%"))))))
>> -       (list-generations pattern)))
>> -    (else
>> -     (process-action command args opts))))
>> +  (with-store store
>> +    (set-build-options-from-command-line store opts)
>> +
>> +    (run-with-store store
>> +      (mbegin %store-monad
>> +        (set-guile-for-build (default-guile))
>> +        (case command
>> +          ((list-generations)
>> +           (let ((pattern (match args
>> +                            (() "")
>> +                            ((pattern) pattern)
>> +                            (x (leave (_ "wrong number of arguments~%"))))))
>> +             (list-generations pattern)))
>> +          (else
>> +           (process-action command args opts))))
>> +      #:system (assoc-ref opts 'system))))
>
> As the comment above suggests, the idea here was to avoid connecting to
> the daemon for operations that do not need it, such as
> ‘list-generations’.  I think we should preserve this property.

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?

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.

> Thanks for your work!  I’m sorry this is more painful than I thought.
> :-/
>
> Ludo’.

I'm happy to help.  I just wish I understood these monads better.  And I
wish I had more time to work on it!  Anyway, I'll keep at it.

-- 
Chris