2.4 Pairs, Lists, and Racket Syntax
The cons function actually accepts any two values, not just a list for the second argument. When the second argument is not empty and not itself produced by cons, the result prints in a special way. The two values joined with cons are printed between parentheses, but with a dot (i.e., a period surrounded by whitespace) in between:
| > (cons 1 2) |
'(1 . 2) |
| > (cons "banana" "split") |
'("banana" . "split") |
Thus, a value produced by cons is not always a list. In general, the result of cons is a pair. The more traditional name for the cons? function is pair?, and we’ll use the traditional name from now on.
The name rest also makes less sense for non-list pairs; the more traditional names for first and rest are car and cdr, respectively. (Granted, the traditional names are also nonsense. Just remember that “a” comes before “d,” and cdr is pronounced “could-er.”)
Examples: |
| > (car (cons 1 2)) |
1 |
| > (cdr (cons 1 2)) |
2 |
| > (pair? empty) |
#f |
| > (pair? (cons 1 2)) |
#t |
| > (pair? (list 1 2 3)) |
#t |
Racket’s pair datatype and its relation to lists is essentially a historical curiosity, along with the dot notation for printing and the funny names car and cdr. Pairs are deeply wired into to the culture, specification, and implementation of Racket, however, so they survive in the language.
You are perhaps most likely to encounter a non-list pair when making a mistake, such as accidentally reversing the arguments to cons:
| > (cons (list 2 3) 1) |
'((2 3) . 1) |
| > (cons 1 (list 2 3)) |
'(1 2 3) |
Non-list pairs are used intentionally, sometimes. For example, the make-hash function takes a list of pairs, where the car of each pair is a key and the cdr is an arbitrary value.
The only thing more confusing to new Racketeers than non-list pairs is the printing convention for pairs where the second element is a pair, but is not a list:
| > (cons 0 (cons 1 2)) |
'(0 1 . 2) |
In general, the rule for printing a pair is as follows: use the dot notation always, but if the dot is immediately followed by an open parenthesis, then remove the dot, the open parenthesis, and the matching close parenthesis. Thus, ’(0 . (1 . 2)) becomes '(0 1 . 2), and ’(1 . (2 . (3 . ()))) becomes '(1 2 3).
2.4.1 Quoting Pairs and Symbols with quote
A list prints with a quote mark before it, but if an element of a list is itself a list, then no quote mark is printed for the inner list:
| > (list (list 1) (list 2 3) (list 4)) |
'((1) (2 3) (4)) |
For nested lists, especially, the quote form lets you write a list as an expression in essentially the same way that the list it prints:
| > (quote ("red" "green" "blue")) |
'("red" "green" "blue") |
| > (quote ((1) (2 3) (4))) |
'((1) (2 4) (4)) |
| > (quote ()) |
'() |
The quote form works with the dot notation, too, whether the quoted form is normalized by the dot-parenthesis elimination rule or not:
| > (quote (1 . 2)) |
'(1 . 2) |
| > (quote (0 . (1 . 2))) |
'(0 1 . 2) |
Naturally, lists of any kind can be nested:
| > (list (list 1 2 3) 5 (list "a" "b" "c")) |
'((1 2 3) 5 ("a" "b" "c")) |
| > (quote ((1 2 3) 5 ("a" "b" "c"))) |
'((1 2 3) 5 ("a" "b" "c")) |
If you wrap an identifier with quote, then you get output that looks like an identifier, but with a ' prefix:
| > (quote jane-doe) |
'jane-doe |
A value that prints like a quoted identifier is a symbol. In the same way that parenthesized output should not be confused with expressions, a printed symbol should not be confused with an identifier. In particular, the symbol (quote map) has nothing to do with the map identifier or the predefined function that is bound to map, except that the symbol and the identifier happen to be made up of the same letters.
Indeed, the intrinsic value of a symbol is nothing more than its character content. In this sense, symbols and strings are almost the same thing, and the main difference is how they print. The functions symbol->string and string->symbol convert between them.
Examples: |
| > map |
#<procedure:map> |
| > (quote map) |
'map |
| > (symbol? (quote map)) |
#t |
| > (symbol? map) |
#f |
| > (procedure? map) |
#t |
| > (string->symbol "map") |
'map |
| > (symbol->string (quote map)) |
"map" |
In the same way that quote for a list automatically applies itself to nested lists, quote on a parenthesized sequence of identifiers automatically applies itself to the identifiers to create a list of symbols:
| > (car (quote (road map))) |
'road |
| > (symbol? (car (quote (road map)))) |
#t |
When a symbol is inside a list that is printed with ', the ' on the symbol is omitted, since ' is doing the job already:
| > (quote (road map)) |
'(road map) |
The quote form has no effect on a literal expression such as a number or string:
| > (quote 42) |
42 |
| > (quote "on the record") |
"on the record" |
2.4.2 Abbreviating quote with ’
As you may have guessed, you can abbreviate a use of quote by just putting ' in front of a form to quote:
| > '(1 2 3) |
'(1 2 3) |
| > 'road |
'road |
| > '((1 2 3) road ("a" "b" "c")) |
'((1 2 3) road ("a" "b" "c")) |
In the documentation, ' within an expression is printed in green along with the form after it, since the combination is an expression that is a constant. In DrRacket, only the ' is colored green. DrRacket is more precisely correct, because the meaning of quote can vary depending on the context of an expression. In the documentation, however, we routinely assume that standard bindings are in scope, and so we paint quoted forms in green for extra clarity.
A ' expands to a quote form in quite a literal way. You can see this if you put a ' in front of a form that has a ':
| > (car ''road) |
'quote |
| > (car '(quote road)) |
'quote |
The ' abbreviation works in output as well as input. The REPL’s printer recognizes the symbol 'quote as the first element of a two-element list when printing output, in which case it uses ’ to print the output:
| > (quote (quote road)) |
''road |
| > '(quote road) |
''road |
| > ''road |
''road |
2.4.3 Lists and Racket Syntax
Now that you know the truth about pairs and lists, and now that you’ve seen quote, you’re ready to understand the main way in which we have been simplifying Racket’s true syntax.
The syntax of Racket is not defined directly in terms of character streams. Instead, the syntax is determined by two layers:
a reader layer, which turns a sequence of characters into lists, symbols, and other constants; and
an expander layer, which processes the lists, symbols, and other constants to parse them as an expression.
The rules for printing and reading go together. For example, a list is printed with parentheses, and reading a pair of parentheses produces a list. Similarly, a non-list pair is printed with the dot notation, and a dot on input effectively runs the dot-notation rules in reverse to obtain a pair.
One consequence of the read layer for expressions is that you can use the dot notation in expressions that are not quoted forms:
| > (+ 1 . (2)) |
3 |
This works because (+ 1 . (2)) is just another way of writing (+ 1 2). It is practically never a good idea to write application expressions using this dot notation; it’s just a consequence of the way Racket’s syntax is defined.
Normally, . is allowed by the reader only with a parenthesized sequence, and only before the last element of the sequence. However, a pair of .s can also appear around a single element in a parenthesized sequence, as long as the element is not first or last. Such a pair triggers a reader conversion that moves the element between .s to the front of the list. The conversion enables a kind of general infix notation:
| > (1 . < . 2) |
#t |
| > '(1 . < . 2) |
'(< 1 2) |
This two-dot convention is non-traditional, and it has essentially nothing to do with the dot notation for non-list pairs. Racket programmers use the infix convention sparingly – mostly for asymmetric binary operators such as < and is-a?.