# Description
This PR adds lazy closure evaluation to the `default` command (closes
#14160).
- For non-closure values and without providing a column name, `default`
acts the same as before
- The user can now provide multiple column names to populate if empty
- If the user provides a column name, the input must be a record or
list, otherwise an error is created.
- The user can now provide a closure as a default value
- This closure is run without any arguments or input
- The closure is never evaluated if the value isn't needed
- Even when column names are supplied, the closure is only run once (and
cached to prevent re-calling it)
For example:
```nushell
> default { 1 + 2 } # => 3
> null | default 3 a # => previously `null`, now errors
> 1 | default { sleep 5sec; 3 } # => `1`, without waiting 5 seconds
> let optional_var = null; $optional_var | default { input 'Enter value: ' } # => Returns user input
> 5 | default { input 'Enter value: ' } # => `5`, without prompting user
> ls | default { sleep 5sec; 'N/A' } name # => No-op since `name` column is never empty
> ls | default { sleep 5sec; 'N/A' } foo bar # => creates columns `foo` and `bar`; only takes 5 seconds since closure result is cached
# Old behavior is the same
> [] | default 'foo' # => []
> [] | default --empty 'foo' # => 'foo'
> default 5 # => 5
```
# User-Facing Changes
- Users can add default values to multiple columns now.
- Users can now use closures as the default value passed to `default`.
- To return a closure, the user must wrap the closure they want to
return inside another closure, which will be run (`default { $my_closure
}`).
# Tests + Formatting
All tests pass.
# After Submitting
---------
Co-authored-by: 132ikl <132@ikl.sh>
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