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.