bug#56114: Guix does not have a documented general and practical procedure for lowering a single lowerable object to the /gnu/store/... string.

[Maxime Devos <maximedevos@telenet.be>]

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Variants of this appear repeatedly on #guix every now and then,
and also sometimes unasked by me not posted anywhere.

How do I run compile a package, computed-file or more generally a
lowerable object from the REPL?  Let's look in the manual:

> Note that the ‘(guix monad-repl)’ module extends the Guile REPL with
> new “meta-commands” to make it easier to deal with monadic
> procedures: ‘run-in-store’, and ‘enter-store-monad’.  The former is
> used to “run” a single monadic value through the store:
> 
>      scheme@(guile-user)> ,run-in-store (package->derivation hello)
>      $1 = #<derivation /gnu/store/...-hello-2.9.drv => ...>
> 

Um, I want the actual store item, not the derivation, and I'm looking
into lowering a non-package, let's look elsewhere:

> As it turns out, the call to ‘package->derivation’ can even be
> omitted since it will take place implicitly, as we will see later
> (*note G-Expressions::):
> 
>      (define (sh-symlink)
>        (gexp->derivation "sh"
>                          #~(symlink (string-append #$bash
> "/bin/bash")
>                                     #$output)))
> 
>    Calling the monadic ‘sh-symlink’ has no effect.  As someone once
> said, “you exit a monad like you exit a building on fire: by
> running”. So, to exit the monad and get the desired effect, one must
> use ‘run-with-store’:
> 
>      (run-with-store (open-connection) (sh-symlink))
>      ⇒ /gnu/store/...-sh-symlink

Let's try this (using the REPL command):

scheme@(guix-user)> ,run-in-store (plain-file "foo" "bar")
While executing meta-command:
Wrong type to apply: #<<plain-file> name: "foo" content: "bar"
references: ()>

doesn't work, let's try the 'lower-object' procedure which I vaguely
recall to helped me out in the past but doesn't have any actual
examples in the manual:

scheme@(guix-user)> ,run-in-store (lower-object (plain-file "foo" "bar"))
$15 = "/gnu/store/798sxvdgr0680czdggbls7rd3sfwk2yx-foo"

This looks useful, let's try it for mixed-text-file:

scheme@(guix-user)> ,run-in-store (lower-object (mixed-text-file "foo"
"bar"))
$17 = #<derivation /gnu/store/ah3yfw0nvw6jaif56r121sgwjiygmvrk-foo.drv
=> /gnu/store/raihpcxdz6wmdpyj67bwd4bjk02xq0x3-foo 7f862e53f820>

nooo there's the #<derivation ...> again how do I get a compiled mixed-
text-file from the REPL?

(I mean I could use 'build-derivations' from (guix)The Store, but that
accepts a list, not an individual derivation, and it doesn't accept strings
according to the documentation so it cannot be used for the 'plain-file'
and aren't derivations a super-low-level thing for which G-exps and such
were intended to make the low level thing almost an implementation detail?)

Seems like we are missing a general & documented & simple to use
'lower-object-completely' (or maybe 'compile-object'?) procedure for
this ...  And maybe also a ,build-object REPL command?

Greetings,
Maxime.

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