4.19 Undefined
| (require racket/undefined) | package： base |
The bindings documented in this section are provided by the racket/undefined library, not racket/base or racket.
The constant undefined can be used as a placeholder value for a value to be installed later, especially for cases where premature access of the value is either difficult or impossible to detect or prevent.
The undefined value is always eq? to itself.
添加于package base的6.0.0.6版本。
The “undefined” constant.