Why is the <derivation> scheme record needed?

Why is the <derivation> scheme record needed?

[jgart]

Hi,

Why is the <derivation> scheme record needed?

How does it function in Guix?

all best,

jgart

Re: Why is the <derivation> scheme record needed?

[Ricardo Wurmus]

jgart <jgart@dismail.de> writes:

> Why is the <derivation> scheme record needed?

It is a Scheme representation of a derivation, which also has an on-disk
representation.

> How does it function in Guix?

Everything that the daemon is asked to build must be compiled to a
derivation on disk first.  From the perspective of the daemon there are
no first-class packages, only derivations.  Much of what Guix as a
library does is to turn package values into derivation files and
builders to feed them into the daemon.

-- 
Ricardo

Re: Why is the <derivation> scheme record needed?

[jgart]

On Sat, 17 Sep 2022 20:41:43 +0200 Ricardo Wurmus <rekado@elephly.net> wrote:

Hi rekado,

Thanks for the explanations! Much appreciated. Here are some more questions:

> It is a Scheme representation of a derivation, which also has an on-disk
> representation.

Why was the scheme representation of a derivation needed?

Was it primarily so that we can inspect it at the REPL like a scheme
object before it gets sent to disk or is there some additional reason?

> Everything that the daemon is asked to build must be compiled to a
> derivation on disk first.  From the perspective of the daemon there are
> no first-class packages, only derivations.  Much of what Guix as a
> library does is to turn package values into derivation files and
> builders to feed them into the daemon.

Why are the bags needed? What do the bags solve in the system?

all best,

jgart

Re: Why is the <derivation> scheme record needed?

[Ricardo Wurmus]

jgart <jgart@dismail.de> writes:

>> It is a Scheme representation of a derivation, which also has an on-disk
>> representation.
>
> Why was the scheme representation of a derivation needed?

You can search the “guix” directory in the source tree for “derivation-”
to see the many ways that we’re accessing parts of the Scheme
representation of derivation files.  We’re accessing the expected output
location, the list of inputs, we using its fields to build narinfos,
etc.

The only alternative is to hit the disk every time we want to know
something about a derivation and parse the file repeatedly.  That would
be very unwise.

>> Everything that the daemon is asked to build must be compiled to a
>> derivation on disk first.  From the perspective of the daemon there are
>> no first-class packages, only derivations.  Much of what Guix as a
>> library does is to turn package values into derivation files and
>> builders to feed them into the daemon.
>
> Why are the bags needed? What do the bags solve in the system?

The manual says:

--8<---------------cut here---------------start------------->8---
   Under the hood, build systems first compile package objects to
“bags”.  A “bag” is like a package, but with less ornamentation—in other
words, a bag is a lower-level representation of a package, which
includes all the inputs of that package, including some that were
implicitly added by the build system.  This intermediate representation
is then compiled to a derivation (*note Derivations::).
--8<---------------cut here---------------end--------------->8---

Bags are an implementation detail.

-- 
Ricardo