lib.modules.evalOptionValue

The following is the current implementation of this function.

evalOptionValue = loc: opt: defs:
    let
      # Add in the default value for this option, if any.
      defs' =
          (optional (opt ? default)
            { file = head opt.declarations; value = mkOptionDefault opt.default; }) ++ defs;

      # Handle properties, check types, and merge everything together.
      res =
        if opt.readOnly or false && length defs' > 1 then
          let
            # For a better error message, evaluate all readOnly definitions as
            # if they were the only definition.
            separateDefs = map (def: def // {
              value = (mergeDefinitions loc opt.type [ def ]).mergedValue;
            }) defs';
          in throw "The option `${showOption loc}' is read-only, but it's set multiple times. Definition values:${showDefs separateDefs}"
        else
          mergeDefinitions loc opt.type defs';

      # Apply the 'apply' function to the merged value. This allows options to
      # yield a value computed from the definitions
      value = if opt ? apply then opt.apply res.mergedValue else res.mergedValue;

      warnDeprecation =
        warnIf (opt.type.deprecationMessage != null)
          "The type `types.${opt.type.name}' of option `${showOption loc}' defined in ${showFiles opt.declarations} is deprecated. ${opt.type.deprecationMessage}";

    in warnDeprecation opt //
      { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
        inherit (res.defsFinal') highestPrio;
        definitions = map (def: def.value) res.defsFinal;
        files = map (def: def.file) res.defsFinal;
        definitionsWithLocations = res.defsFinal;
        inherit (res) isDefined;
        # This allows options to be correctly displayed using `${options.path.to.it}`
        __toString = _: showOption loc;
      };