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# Description
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Relative: #8248
After this pr, user can define const variable inside a module.
And user can export const variables, the following screenshot shows how
it works (it follows
https://github.com/nushell/nushell/issues/8248#issuecomment-1637442612):
## About the change
1. To make module support const, we need to change `parse_module_block`
to support `const` keyword.
2. To suport export `const`, we need to make module tracking variables,
so we add `variables` attribute to `Module`
3. During eval, the const variable may not exists in `stack`, because we
don't eval `const` when we define a module, so we need to find variables
which are already registered in `engine_state`
## One more thing to note about the const value.
Consider the following code
```
module foo { const b = 3; export def bar [] { $b } }
use foo bar
const b = 4;
bar
```
The result will be 3 (which is defined in module) rather than 4. I think
it's expected behavior.
It's something like [dynamic
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding-Tips.html)
vs [lexical
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Lexical-Binding.html)
in lisp like language, and lexical binding should be right behavior
which generates more predicable result, and it doesn't introduce really
subtle bugs in nushell code.
What if user want dynamic-binding?(For example: the example code returns
`4`)
There is no way to do this, user should consider passing the value as
argument to custom command rather than const.
## TODO
- [X] adding tests for the feature.
- [X] support export const out of module to use.
# User-Facing Changes
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# Tests + Formatting
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# After Submitting
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