In reply to @tumble1999:matrix.org
Does custom type casting exist or is it just set to string with outPath attribute?

Generally speaking, nix doesn't have type casting. For interpolating values into strings ("${expr}" syntax), different types follow different rules:

1. String values are interpolated as-is
2. Path values are copied-to-store, and the resulting store path is interpolated (with context)
3. Attrsets with a __toString function call that function and interpolate the result
4. Attrsets with an outPath attribute interpolate that string.
5. Other types typically raise an error

Examples in the wild: nixos options have a __toString function, which uses lib.showOption opt.loc internally; deprivations have an outPath, which is the store path the derivation will be installed to, once built.

In reply to @phozzy:tchncs.de
Are you looking for https://nickel-lang.org/ maybe?