# Description
This PR adds two new `ParseError` and `ShellError` cases for type errors
relating to operators.
- `OperatorUnsupportedType` is used when a type is not supported by an
operator in any way, shape, or form. E.g., `+` does not support `bool`.
- `OperatorIncompatibleTypes` is used when a operator is used with types
it supports, but the combination of types provided cannot be used
together. E.g., `filesize + duration` is not a valid combination.
The other preexisting error cases related to operators have been removed
and replaced with the new ones above. Namely:
- `ShellError::OperatorMismatch`
- `ShellError::UnsupportedOperator`
- `ParseError::UnsupportedOperationLHS`
- `ParseError::UnsupportedOperationRHS`
- `ParseError::UnsupportedOperationTernary`
# User-Facing Changes
- `help operators` now lists the precedence of `not` as 55 instead of 0
(above the other boolean operators). Fixes#13675.
- `math median` and `math mode` now ignore NaN values so that `[NaN NaN]
| math median` and `[NaN NaN] | math mode` no longer trigger a type
error. Instead, it's now an empty input error. Fixing this in earnest
can be left for a future PR.
- Comparisons with `nan` now return false instead of causing an error.
E.g., `1 == nan` is now `false`.
- All the operator type errors have been standardized and reworked. In
particular, they can now have a help message, which is currently used
for types errors relating to `++`.
```nu
[1] ++ 2
```
```
Error: nu::parser::operator_unsupported_type
× The '++' operator does not work on values of type 'int'.
╭─[entry #1:1:5]
1 │ [1] ++ 2
· ─┬ ┬
· │ ╰── int
· ╰── does not support 'int'
╰────
help: if you meant to append a value to a list or a record to a table, use the `append` command or wrap the value in a list. For example: `$list ++ $value` should be
`$list ++ [$value]` or `$list | append $value`.
```
# Description
The "append" operator currently serves as both the append operator and
the concatenation operator. This dual role creates ambiguity when
operating on nested lists.
```nu
[1 2] ++ 3 # appends a value to a list [1 2 3]
[1 2] ++ [3 4] # concatenates two lists [1 2 3 4]
[[1 2] [3 4]] ++ [5 6]
# does this give [[1 2] [3 4] [5 6]]
# or [[1 2] [3 4] 5 6]
```
Another problem is that `++=` can change the type of a variable:
```nu
mut str = 'hello '
$str ++= ['world']
($str | describe) == list<string>
```
Note that appending is only relevant for lists, but concatenation is
relevant for lists, strings, and binary values. Additionally, appending
can be expressed in terms of concatenation (see example below). So, this
PR changes the `++` operator to only perform concatenation.
# User-Facing Changes
Using the `++` operator with a list and a non-list value will now be a
compile time or runtime error.
```nu
mut list = []
$list ++= 1 # error
```
Instead, concatenate a list with one element:
```nu
$list ++= [1]
```
Or use `append`:
```nu
$list = $list | append 1
```
# After Submitting
Update book and docs.
---------
Co-authored-by: Douglas <32344964+NotTheDr01ds@users.noreply.github.com>
