various fixes

lib/fields.nix

@@ -26,9 +26,8 @@
       internal ? null,
       # Whether the field shows up in the manual. Default: true. Use false to hide the field and any sub-options from submodules. Use "shallow" to hide only sub-options.
       visible ? null,
+      # Whether the field is omitted from the final record when undefined. Default false.
+      optional ? null,
     }@attrs:
-    assert lib.assertMsg (
-      (attrs.optional or false) -> (!attrs ? default)
-    ) "mkField: `optional` is true, but a `default` is provided";
     attrs // { _type = "field"; };
 }

lib/tests/modules.sh

@@ -119,7 +119,7 @@ checkConfigError 'define-record-mallory.nix.: "beginning of time"' config.people
 checkConfigOutput '^true$' config.people.bob.isCool ./declare-record.nix ./define-record-alice.nix ./define-record-bob.nix
 
 # record field bad default definition
-checkConfigError 'In .the default value of option people.mallory.: "yeah"' config.people.mallory.isCool ./declare-record-bad-default.nix ./define-record-mallory.nix
+checkConfigError 'In .the default value of field people.mallory.: "yeah"' config.people.mallory.isCool ./declare-record-bad-default.nix ./define-record-mallory.nix
 checkConfigError 'A definition for option .people.mallory.isCool. is not of type .boolean.. Definition values:' config.people.mallory.isCool ./declare-record-bad-default.nix ./define-record-mallory.nix
 
 # record field works in presence of wildcard
@@ -129,7 +129,7 @@ checkConfigOutput '^2016$' config.people.alice.nixerSince ./declare-record-wildc
 checkConfigOutput '^true$' config.people.alice.mechKeyboard ./declare-record-wildcard.nix ./define-record-alice-prefs.nix
 
 # record definition without corresponding field
-checkConfigError 'A definition for option .people.mike. has an unknown field' config.people.mike.age ./declare-record.nix ./define-record-mike.nix
+checkConfigError 'A definition for option .people.mike. has an unknown fields' config.people.mike.age ./declare-record.nix ./define-record-mike.nix
 # record optional field without definition
 checkConfigError "attribute 'age' in selection path 'config.people.alice.age' not found" config.people.alice.age ./declare-record-optional-field.nix ./define-record-alice.nix
 # record optional field with definition

lib/tests/modules/declare-record-bad-default.nix

@@ -1,13 +1,13 @@
 { lib, ... }:
 
 let
-  inherit (lib) mkOption types;
+  inherit (lib) mkField mkOption types;
 
   person = types.record {
     fields = {
-      nixerSince = mkOption { type = types.int; };
-      name = mkOption { type = types.str; };
-      isCool = mkOption {
+      nixerSince = mkField { type = types.int; };
+      name = mkField { type = types.str; };
+      isCool = mkField {
         type = types.bool;
         default = "yeah";
       };

lib/tests/modules/declare-record-optional-field.nix

@@ -1,17 +1,18 @@
 { lib, ... }:
 
 let
-  inherit (lib) mkOption types;
+  inherit (lib) mkField mkOption types;
 
   person = types.record {
     fields = {
-      nixerSince = mkOption { type = types.int; };
-      name = mkOption { type = types.str; };
+      nixerSince = mkField { type = types.int; };
+      name = mkField { type = types.str; };
+      age = mkField {
+        type = types.ints.unsigned;
+        optional = true;
+      };
     };
-    optionalFields = {
-      age = mkOption { type = types.ints.unsigned; };
-    };
-    wildcard = mkOption { type = types.bool; };
+    freeformType = types.bool;
   };
 
 in

lib/tests/modules/declare-record-wildcard.nix

@@ -1,14 +1,14 @@
 { lib, ... }:
 
 let
-  inherit (lib) mkOption types;
+  inherit (lib) mkField mkOption types;
 
   person = types.record {
     fields = {
-      nixerSince = mkOption { type = types.int; };
-      name = mkOption { type = types.str; };
+      nixerSince = mkField { type = types.int; };
+      name = mkField { type = types.str; };
     };
-    wildcard = mkOption { type = types.bool; };
+    freeformType = types.bool;
   };
 
 in

lib/tests/modules/declare-record.nix

@@ -1,13 +1,13 @@
 { lib, ... }:
 
 let
-  inherit (lib) mkOption types;
+  inherit (lib) mkField mkOption types;
 
   person = types.record {
     fields = {
-      nixerSince = mkOption { type = types.int; };
-      name = mkOption { type = types.str; };
-      isCool = mkOption {
+      nixerSince = mkField { type = types.int; };
+      name = mkField { type = types.str; };
+      isCool = mkField {
         type = types.bool;
         default = true;
       };

lib/types/record.nix

@@ -15,6 +15,14 @@ let
     isAttrs
     ;
 
+  inherit (lib.options)
+    showDefs
+    ;
+
+  inherit (lib.strings)
+    escapeNixIdentifier
+    ;
+
   record =
     {
       fields ? { },
@@ -24,7 +32,9 @@ let
       checkField =
         name: field:
         if field._type or null != "field" then
-          throw "Record field `${lib.escapeNixIdentifier name}` must be declared with `mkField`."
+          throw "Record field `${escapeNixIdentifier name}` must be declared with `mkField`."
+        else if (field.optional or false) && (field ? default) then
+          throw "Record field `${escapeNixIdentifier name}` is optional, but a `default` is provided."
         else
           field;
 
@@ -59,17 +69,18 @@ let
           fieldValues = concatMapAttrs (
             fieldName: field:
             let
-              fieldOption = mergeDefinitions (loc ++ [ fieldName ]) fieldOption.type (
+              mergedOption = mergeDefinitions (loc ++ [ fieldName ]) field.type (
                 data.${fieldName} or [ ]
                 ++ optional (field ? default) {
-                  value = lib.mkOptionDefault fieldOption.default;
+                  value = lib.mkOptionDefault field.default;
                   file = "the default value of field ${showOption loc}";
                 }
               );
+              isRequired = !field.optional or false;
             in
             builtins.addErrorContext "while evaluating the field `${fieldName}' of option `${showOption loc}'" (
-              optionalAttrs (!field.optional || fieldOption.isDefined) {
-                ${fieldName} = fieldOption.mergedValue;
+              optionalAttrs (isRequired || mergedOption.isDefined) {
+                ${fieldName} = mergedOption.mergedValue;
               }
             )
           ) checkedFields;
@@ -86,7 +97,7 @@ let
             else
               throw ''
                 A definition for option `${showOption loc}' has an unknown fields:
-                ${lib.concatMapAttrsStringSep "\n" (name: defs: "`${name}'${lib.showDefs defs}") extraData}'';
+                ${lib.concatMapAttrsStringSep "\n" (name: defs: "`${name}'${showDefs defs}") extraData}'';
         in
         if freeformType == null then checkedExtraDefs else fieldValues // extraValues;
       nestedTypes = lib.optionalAttrs (freeformType != null) {