Why is `\middle\mid` not working?

[limber laurel]

Consider the following example. $$\left{\bigcup\middle\mid\bigcup\right}$$ Notice the error.

[timid ledge]

xpple

Consider the following example. $$\left\{\bigcup\middle\mid\bigcup\right\}$$ Notice the error.
```Compilation error:```! Missing delimiter (. inserted).
<to be read again> 
                   \mid 
l.56 ...owing example. $$\left\{\bigcup\middle\mid
                                                  \bigcup\right\}$$ Notice t...
I was expecting to see something like `(' or `\{' or
`\}' here. If you typed, e.g., `{' instead of `\{', you
should probably delete the `{' by typing `1' now, so that
braces don't get unbalanced. Otherwise just proceed.
Acceptable delimiters are characters whose \delcode is
nonnegative, or you can use `\delimiter <delimiter code>'.```

[limber laurel]

Observe how $$\left{\bigcup\middle\vert\bigcup\right}$$ does in fact work, but doesn't have correct spacing.

[timid ledge]

xpple

[sharp hare]

\mid is not a delimiter

You must use \vert for a scalable bar

If you want spacing around the bar like \mid, just do \,\middle\vert\,.

[dense crescent]

isn't there something with \mathrel as well?

[timid ledge]

@dense crescent

\begin{gather*}
\left\{ \bigcup \mathrel{}\middle\vert\mathrel{} \bigcup \right\}
\end{gather*}

[dense crescent]

yeah that's it

[livid lintel]

that's what \mid does though

[livid lintel]

Technical pov (according to texdoc symbols p.129):

\mvert and \mid are identical and produce a relation. \vert is a synonym for | and both produce the same symbol, but should be used in the context of an ordinal, and should be used as an operator, not as a **delimiter **

[dense crescent]

I think this means that \vert should be used as an operator while | should be used as a delimiter?

[livid lintel]

No it means that \vert and | are the same

but yes, should be used as operators

but then again, both are defined as delimiters so when one intends to use \middle then |/\vert would be appropriate

what the symbols implied by suggesting to not use it as a delimiter is don't use it as |x|

[dense crescent]

then the word "but" is unnecessary. that usage of "but" makes it seem as though they meant "although they produce the same symbol, they have different uses"

unless they meant "produce the same symbol[ as \mvert and \mid]"

[limber laurel]

I see