> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/sidef-lang/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/sidef-lang/programming_tasks/e/enforced_immutability.md).

# Enforced immutability

```ruby
define PI = 3.14159;            # compile-time defined constant
const MSG = "Hello world!";     # run-time defined constant
```