Depends on what you mean exactly with "file format".
If declarative functional programming falls under that, I think something like Nickel, the already mentioned Dhall or Nix. Though Nix more so for packaging and some kind of system management (NixOS?), it's not easily embeddable into a runtime (your app?). Nickel or Dhall is better for that, as they are built from ground up with that in mind, Nickel is maybe the successor of Nix as it is inspired by Dhall and Nix (one goal is to use Nickel as frontend).
The reason why I recommend a simple declarative language, is that they are IMHO much better composable as it lets the user hide boilerplate via functions. I almost always feel limited by static configuration formats like yaml, json etc..
Depends on what you mean exactly with "file format".
If declarative functional programming falls under that, I think something like Nickel, the already mentioned Dhall or Nix. Though Nix more so for packaging and some kind of system management (NixOS?), it's not easily embeddable into a runtime (your app?). Nickel or Dhall is better for that, as they are built from ground up with that in mind, Nickel is maybe the successor of Nix as it is inspired by Dhall and Nix (one goal is to use Nickel as frontend).
The reason why I recommend a simple declarative language, is that they are IMHO much better composable as it lets the user hide boilerplate via functions. I almost always feel limited by static configuration formats like yaml, json etc..