[Nix-dev] "live builds" proposal

[Marc Weber]

Excerpts from Yury G. Kudryashov's message of Sun Aug 01 01:20:28 +0200 2010:
> Hi!
> 
> I propose the following extension to the nix language that will allow native 
> support for "live scm builds" etc.
> 
> If nix is called with --enable-exec, the new function builtins.exec is 
> added. The proposed syntax is like the following:
> 
> src =
>   if builtins ? exec then
>     # fetch to ~/.nix-checkouts/name
>     builtins.exec [ "nix-prefetch-svn" .... ];
>     builtins.filterSource "~/.nix-checkouts/name"
>   else if pathExists "~/.nix-checkouts/name" then
>     builtins.filterSource ...
>   else
>     fetchsvn { # url and hash for some specific revision
>     };
> 
> Of course, all this logic should be hidden in new fetchsvn/fetchgit etc. 
> functions.
> 
> The main drawback of this solution is that the result of a nix expression 
> will depend on some things outside of the nix expression. That's why I 
> propose to enable this builtin only if nix is called with --enable-exec: if 
> you want to rebuild your system from the configuration written a year ago, 
> just do not add --enable-exec.
> 

I think its worth talking about this because most of us want this in
some or another way.

Yury: Think about having two repos:

nixpkgs-checkout-A
nixpkgs-checkout-B

nix-env --exec -f nixpkgs-chekcout-A -q \* # will this checkout / update everything?

Should this have an effect at

nix-env --exec -f nixpkgs-chekcout-B -i INSTALL_SOMETHING

?

The issue is that your state is located in ~/.nix-checkouts.

My state is located in the .nix file. Each time I run
nix-repository-manager . --update NAME the .nix file is changed telling
it exactly which snapshot to use.

So working on repo A never affects repo B.

I dislike this kind of impurity that repo-A affects repo-B. So I will
always prefer my solution for that reason.

How do you think about this? do you want to rm -fr ~/.nix-checkouts in
order to reproduce some compilations ?
Do you use a different system account on your machine then?

I'm willing to support a nixier solution. But I dislike this particular
design decision. Can you comment whether you were aware of it and how
you feel about it?

Can you think about how a DSL could look like? Something like this?

  src = upstream {
    type = "svn";
    svnUrl = "...";
    lastRevKnownToWork =  724;
  }

Its paying of thinking about it soon.

I'm willing to help finding a nixier solution if benefits / amount of
work ratio pays off.

Marc Weber