Comment by danieldk
3 days ago
If import-from-derivation is enabled
I have never looked at the implementation of IFD, but I assume that the evaluation and instantiation are still separated (and Nix will do multiple passes).
Even then functions like "readFile" are considered to be pure in nix, but not in haskell.
I am pretty sure that, unless you use --impure, all files that are read are required to be in the store. Since the store is read-only, it does not break purity.
At any rate, I agree that there will be some hoops to jump through. But I think it would be possible to make a Haskell DSL to define derivations similar to Nix. But I don't know why one would want to.
> I am pretty sure that, unless you use --impure, all files that are read are required to be in the store. Since the store is read-only, it does not break purity.
Right, but even then the logical type for readFile would be something like "string -> string" (because from nix's perspective it is pure), but in haskell it would have to be "string -> IO string" (because from haskell's perspective it is not).
Maybe this is fine, i just suspect it would make things messier than expected.
Alternatively this could be worked around using unsafePerformIO or the FFI, but that feels a bit far away from the idea of just making a DSL? Unclear...
> But I don't know why one would want to.
Same, I just think it's an interesting discussion.