defaults

Overview ¶

Package defaults lets a Go library type declare schema defaults for its inputs in a type-safe, string-free way. A type lists them from a Defaults method, referring to its own fields directly:

func (f File) Defaults() []defaults.Default {
	return []defaults.Default{
		defaults.Value(f.Mode, 0o644),
		defaults.Optional(f.CreateDirectory),
	}
}

Value declares that a field left out of a body takes the given value, which the runtime fills in before the type's code runs and the plan renders as the real input. Optional declares that a field may be left out with no declared value; the type's code handles the zero value the decoder produces. A field with neither marker, and not declared as a pointer, is required.

Because the fields are real struct fields, the Go compiler checks that they exist, that a rename updates them, and that a Value default matches its field's type. unobin reads these declarations from source at compile time (goschema); the methods are never called, so a returned value identifies the rule but not its field, which the compiler supplies from source.