feat: impl GetOrFunc to allow fallback code in closure

Author: steverusso

cli.go

@@ -188,7 +188,7 @@ func Get[T any](c *Command, id string) T {
 
 // GetOr looks for a parsed input value with the given id in the given Command and
 // converts the value to the given type T through an untested type assertion (so this
-// will panic if the value is found and can't be converted to type T). If the value
+// will panic if the value is found but can't be converted to type T). If the value
 // isn't found, the given fallback value will be returned. To check whether the value
 // is found instead of using a fallback value, see [Lookup].
 func GetOr[T any](c *Command, id string, fallback T) T {
@@ -198,6 +198,19 @@ func GetOr[T any](c *Command, id string, fallback T) T {
 	return fallback
 }
 
+// GetOrFunc is like [GetOr] in that it looks for a parsed input value with the given id
+// in the given Command and converts the value to the given type T through an untested
+// type assertion (so this will panic if the value is found but can't be converted to
+// type T). However, if the value isn't found, it will run the provided function fn in
+// order to create and return the fallback value. To check whether the value is found
+// instead of using a fallback, see [Lookup].
+func GetOrFunc[T any](c *Command, id string, fn func() T) T {
+	if v, ok := Lookup[T](c, id); ok {
+		return v
+	}
+	return fn()
+}
+
 // ParseOrExit will parse input based on this CommandInfo. If help was requested, it
 // will print the help message and exit the program successfully (status code 0). If
 // there is any other error, it will print the error and exit the program with failure

examples_test.go

@@ -439,6 +439,24 @@ func ExampleGetOr_positionlArgs() {
 	// b: "world"
 }
 
+func ExampleGetOrFunc() {
+	c := cli.NewCmd("example").
+		Opt(cli.NewOpt("a")).
+		Opt(cli.NewOpt("b")).
+		ParseOrExit("-a", "hello")
+
+	a := cli.GetOr(c, "a", "")
+	b := cli.GetOrFunc(c, "b", func() string {
+		return "world"
+	})
+
+	fmt.Printf("a: %q\n", a)
+	fmt.Printf("b: %q\n", b)
+	// Output:
+	// a: "hello"
+	// b: "world"
+}
+
 func ExampleLookup_positionalArgs() {
 	c := cli.NewCmd("example").
 		Arg(cli.NewArg("a")).