[Nix-dev] Creating dependency trees.

[stewart mackenzie]

On Fri, Jan 22, 2016 at 4:48 PM, aszlig <aszlig at redmoonstudios.org> wrote:
> import "${shinyDerivation}/nix-support/shiny.nix"

So from this subnet implementation: [4]

in => a nand (maths_boolean_nand) out => out
in => b nand ()

a /tmp/nix-build-maths_boolean_not.drv-*/lib/subnetDeps.nix should be
generated round about here: [5] (not yet implemented in the link)
That subnetDeps.nix should be generated on the fly then ${include
"$(pwd)/lib/subnetDeps.nix"} (<- this doesn't work) during one of the
phases of [3]. Is this what you suggested?

subnetDeps.nix should contain expressions to import all the
derivations such as "maths_boolean_nand" and what ever else is used in
a lib.subnet (anything between the paretheses in lib.subnet).

Alternatively, the developer needs to explicitly describe the
dependencies such as what I've done with components which have
capnproto contract dependencies here: [1]
Here is the interface of such an endeavour: [2]

It's probably better to go with the alternative way. It goes with the
nix design flow, but it'll cause bugs down the road, if the developer
forgets to properly write the `filteredSubnets` (for reference see [2]
the equivalent of `filteredContracts` in [1]) then the build-system
won't catch missing subnet derivations.

Why will it cause bugs down the line?: Later when `fvm` (the
fractalide virtual machine [6]) downloads and queries hydra's
generated MANIFEST file it'll download partial dependencies for the
subnet (because the subnet developer forgot to properly write the
subnet deps in `filteredSubnets`) which means a partial dep set will
be downloaded from hydra's cache and thus the subnet won't execute
properly!

By far the best way would be to extract the derivation names directly
from the [4] lib.subnet file, write a
/tmp/nix-build-maths_boolean_not.drv-*/lib/subnetDeps.nix then import
that during a buildFractalideSubnet.nix phase, that way if the
developer made a spelling mistake in the subnet say "...
nand(maths_boooolean_nand) out ...", the fractalide build would
immediately fail as it can't find the "maths_boooolean_nand"
derivation. The alternative way also handles misspelling of subnet
derivations but does not handle the case of forgotten dependencies.

I'm really sorry if my explanation is too vague or too complex.

[1] https://github.com/fractalide/fractalide/blob/master/build-support/buildFractalideComponent.nix#L91-L93
[2] https://github.com/fractalide/fractalide/blob/master/components/maths/boolean/nand/default.nix#L6
[3] https://github.com/fractalide/fractalide/blob/master/build-support/buildFractalideSubnet.nix
[4] https://github.com/fractalide/fractalide/blob/master/components/maths/boolean/not/lib.subnet
[5] https://github.com/fractalide/fractalide/blob/master/build-support/buildFractalideSubnet.nix#L16
[6] https://github.com/fractalide/fractalide/tree/master/fvm