bat: support boolean flags in config

Previously, users cannot enable boolean flags like `--show-all` in bat's
config since all options were expected to be either a string, or a list
of strings. With this commit boolean flags are simply appended to the
end of the config if they are set to `true`, and discarded otherwise.

For example, the config

    {
      theme = "TwoDark";
      show-all = true;
      lessopen = false;
    }

would produce a config file that looks like

    --theme='TwoDark'
    --show-all

Fixes #4657

modules/programs/bat.nix

@@ -8,10 +8,20 @@ let
 
   package = pkgs.bat;
 
-  toConfigFile = generators.toKeyValue {
-    mkKeyValue = k: v: "--${k}=${lib.escapeShellArg v}";
-    listsAsDuplicateKeys = true;
-  };
+  toConfigFile = attrs:
+    let
+      inherit (builtins) isBool attrNames;
+      nonBoolFlags = filterAttrs (_: v: !(isBool v)) attrs;
+      enabledBoolFlags = filterAttrs (_: v: isBool v && v) attrs;
+
+      keyValuePairs = generators.toKeyValue {
+        mkKeyValue = k: v: "--${k}=${lib.escapeShellArg v}";
+        listsAsDuplicateKeys = true;
+      } nonBoolFlags;
+      switches = concatMapStrings (k: ''
+        --${k}
+      '') (attrNames enabledBoolFlags);
+    in keyValuePairs + switches;
 
 in {
   meta.maintainers = [ ];
@@ -20,7 +30,7 @@ in {
     enable = mkEnableOption "bat, a cat clone with wings";
 
     config = mkOption {
-      type = with types; attrsOf (either str (listOf str));
+      type = with types; attrsOf (oneOf [ str (listOf str) bool ]);
       default = { };
       example = {
         theme = "TwoDark";

tests/modules/programs/bat/bat.nix

@@ -11,6 +11,10 @@ with lib;
         theme = "TwoDark";
         pager = "less -FR";
         map-syntax = [ "*.jenkinsfile:Groovy" "*.props:Java Properties" ];
+        show-all = true;
+
+        # False boolean options should not appear in the config
+        lessopen = false;
       };
 
       themes.testtheme.src = pkgs.writeText "testtheme.tmTheme" ''
@@ -32,6 +36,7 @@ with lib;
           --map-syntax='*.props:Java Properties'
           --pager='less -FR'
           --theme='TwoDark'
+          --show-all
         ''
       }