code: func interpolationFuncBasename() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return filepath.Base(args[0].(string)), nil
  		},
  	}
  }
docs: // interpolationFuncBasename implements the "basename" function.