|
|
# Quoting: `quote` and `'`
|
|
|
|
|
|
> +\[missing\] in \[missing\] also documents `quote`.
|
|
|
|
|
|
The `quote` form produces a constant:
|
|
|
|
|
|
```racket
|
|
|
(quote datum)
|
|
|
```
|
|
|
|
|
|
The syntax of a `datum` is technically specified as anything that the
|
|
|
`read` function parses as a single element. The value of the `quote`
|
|
|
form is the same value that `read` would produce given `datum`.
|
|
|
|
|
|
The `datum` can be a symbol, a boolean, a number, a \(character or
|
|
|
byte\) string, a character, a keyword, an empty list, a pair \(or list\)
|
|
|
containing more such values, a vector containing more such values, a
|
|
|
hash table containing more such values, or a box containing another such
|
|
|
value.
|
|
|
|
|
|
Examples:
|
|
|
|
|
|
```racket
|
|
|
> (quote apple)
|
|
|
'apple
|
|
|
> (quote #t)
|
|
|
#t
|
|
|
> (quote 42)
|
|
|
42
|
|
|
> (quote "hello")
|
|
|
"hello"
|
|
|
> (quote ())
|
|
|
'()
|
|
|
> (quote ((1 2 3) #("z" x) . the-end))
|
|
|
'((1 2 3) #("z" x) . the-end)
|
|
|
> (quote (1 2 . (3)))
|
|
|
'(1 2 3)
|
|
|
```
|
|
|
|
|
|
As the last example above shows, the `datum` does not have to match the
|
|
|
normalized printed form of a value. A `datum` cannot be a printed
|
|
|
representation that starts with `#<`, so it cannot be `#<void>`,
|
|
|
`#<undefined>`, or a procedure.
|
|
|
|
|
|
The `quote` form is rarely used for a `datum` that is a boolean, number,
|
|
|
or string by itself, since the printed forms of those values can already
|
|
|
be used as constants. The `quote` form is more typically used for
|
|
|
symbols and lists, which have other meanings \(identifiers, function
|
|
|
calls, etc.\) when not quoted.
|
|
|
|
|
|
An expression
|
|
|
|
|
|
```racket
|
|
|
'datum
|
|
|
```
|
|
|
|
|
|
is a shorthand for
|
|
|
|
|
|
`(quote` `datum)`
|
|
|
|
|
|
and this shorthand is almost always used instead of `quote`. The
|
|
|
shorthand applies even within the `datum`, so it can produce a list
|
|
|
containing `quote`.
|
|
|
|
|
|
> +\[missing\] in \[missing\] provides more on the `'` shorthand.
|
|
|
|
|
|
Examples:
|
|
|
|
|
|
```racket
|
|
|
> 'apple
|
|
|
'apple
|
|
|
> '"hello"
|
|
|
"hello"
|
|
|
> '(1 2 3)
|
|
|
'(1 2 3)
|
|
|
> (display '(you can 'me))
|
|
|
(you can (quote me))
|
|
|
```
|