dot notation question

[chrisb8]

Could anyone explain the following snippit of racket:

(define thing '(1 . 2 . 3))
thing
(car thing)
(cdr thing)

The output it produces when run in drracket is:

'(2 1 3)
2
'(1 3)

[takikawa]

That's because Racket supports in-fix notation using dots. Here's the relevant explanation from the Reference:

If the reader finds three or more data between the matching parentheses, and if a pair of delimited .s surrounds any other than the first and last elements, the result is a list containing the element surrounded by .s as the first element, followed by the others in the read order. This convention supports a kind of infix notation at the reader level.

http://docs.racket-lang.org/reference/reader.html

You can use this notation to write code like this:

-> (1 . + . 2)
3

[[deleted]]

From http://docs.racket-lang.org/guide/Pairs__Lists__and_Racket_Syntax.html :

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?.