[Nix-dev] Accessing files in derivations.

[Harald van Dijk]

On 19/04/16 18:51, Vladimír Čunát wrote:
> On 04/19/2016 04:01 PM, Layus wrote:
>> That operator would ensure that the path exists in the derivation, but
>> also look up through the outputs to find one containing that file.
>
> Unfortunately, during evaluation you can't access the results of builds
> to check whether some path exists. (Well, technically you can, but we
> avoid it, as it has nasty consequences.)

What should be possible is to refuse to build the derivation when, 
checking just before starting the builder, Nix finds that dependencies 
do not contain the required paths. The dependencies need to be built, 
sure, but avoiding the final build would already be an improvement.