note that this does not yet contain the 'type check', as this does not
function yet.
this could be added in `data-model.nix` like:
```nix
function-type-check = mkOption {
type = application.config.config-mapping.function-type;
readOnly = true;
default = input: {
inherit input;
output = application.config.config-mapping.implementation-type
input;
};
};
```
... or even to `function.nix` itself, like:
```nix
function-type-check = mkOption {
type = config.function-type;
readOnly = true;
default = input: {
inherit input;
output = config.implementation-type input;
};
};
```
