retroforth/doc/book/techniques/lexical-scope

# Lexical Scope

RETRO has a single dictionary, but does provide a means of using
lexical scope to keep this dictionary clean.

## Example

```
{{
  'A var
  :++A  &A v:inc ;
---reveal---
  :B ++A ++A @A n:put nl ;
}}
```

In this example, the lexical namespace is created with `{{`. A
variable (`A`) and word (`++A`) are defined. Then a marker is
set with `---reveal---`. Another word (`B`) is defined, and the
lexical area is closed with `}}`.

The headers between `{{` and `---reveal---` are then hidden from
the dictionary, leaving only the headers between `---reveal---`
and `}}` exposed.

If you wish to hide all headers in a `{{` ... `}}` block, leave
out the `---reveal---`.

```
{{
  :a #3 ;
  :b a dup * ;
}}
```

## Notes

This only affects word visibility within the scoped area. As an
example:

```
:a #1 ;

{{
  :a #2 ;
---reveal---
  :b 'a s:evaluate n:put ;
}}
```

In this, after `}}` closes the area, the `:a #2 ;` is hidden and
the `s:evaluate` will find the `:a #1 ;` when `b` is run.

## A Word of Warning

Use of these words can result in a corrupt dictionary and system
crashes. Specifically, use of `---reveal---` with an empty private
or public section will result in dictionary corruption.

If you don't need private words, don't put them in a scope. And if
you don't need public words, don't include the `---reveal---`.