# `fun` statement The `fun` keyword is used to define & undefine [functions](../../functions/index.md). A function can be defined as follows: ```typeql define fun () -> : ``` A function can be undefined using: undefine fun ; A function’s signature and implementation can be re-defined as follows: ```typeql redefine fun () -> : ``` [Edit on GitHub](https://github.com/typedb/typedb-docs/edit/3.x-development/typeql-reference/modules/ROOT/pages/statements/fun.adoc) Edit this page on GitHub.