nix-derivation
The nix-derivation project provides Dhall types to
Nix derivations. On top of this, there is also a a Nix
"evaluator" that can take the the Nix encoding of a Dhall derivation and
transform it into something that nix-build can understand. To clarify, let's
consider the example for unpacking x86_64-linux bootstrap
tools.
In this .dhall file we are fetching three remote files - the busybox binary,
bootstrap-tools.tar.xz and unpack-bootstrap-tools.sh (these are all files
used to bootstrap nixpkgs - thanks!). All these fetchurl invocations are
Derivations. We can inspect just busybox for example, and see that it's
normal form is:
λ ( Mk-Derivation
: { args :
List Text
, builder :
< Builtin : < Fetch-Url : {} > | Exe : Text >
, environment :
List { name : Text, value : < `Bool` : Bool | `Text` : Text > }
, name :
Text
, output-hash :
Optional
{ algorithm :
< SHA256 : {} >
, hash :
Text
, mode :
< Flat : {} | Recursive : {} >
}
, outputs :
List Text
, system :
< builtin : {} | x86_64-linux : {} >
}
→ Text
)
→ Mk-Derivation
{ args =
[] : List Text
, builder =
< Builtin = < Fetch-Url = {=} > | Exe : Text >
, environment =
[ { name =
"url"
, value =
< `Text` =
"http://tarballs.nixos.org/stdenv-linux/i686/4907fc9e8d0d82b28b3c56e3a478a2882f1d700f/busybox"
| `Bool` :
Bool
>
}
, { name = "preferLocalBuild", value = < `Bool` = True | `Text` : Text > }
, { name = "executable", value = < `Bool` = True | `Text` : Text > }
]
, name =
"busybox"
, output-hash =
Some
{ algorithm =
< SHA256 = {=} >
, hash =
"ef4c1be6c7ae57e4f654efd90ae2d2e204d6769364c46469fa9ff3761195cba1"
, mode =
< Recursive = {=} | Flat : {} >
}
, outputs =
[ "out" ]
, system =
< builtin = {=} | x86_64-linux : {} >
}As you can see, there's quite a lot going on! Essentially, this is the "assembly level" view of a derivation - but you won't normally be working at quite this level.
Continuing with the example file, we see that these three fetched files are then
composed into a new derivation, using dhallix.derivation.
dhallix.derivation gives you a special function that lets you look at the
store-path of a derivation (the location of it's output when built), and you can
use that to build a new derivation. In this case, we look at the store-path of
the files we've downloaded to essentially invoke busybox ash unpack-bootstrap-tools.sh, with the environment
tarball=bootstrap-tools.tar.xz. When we realise this derivation, Nix will
produce a new store path with the contents of this tar file unpacked. Neat!