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Cobalt pen (talk | contribs) Added the cons-cell.svg here to give it a use. The picture was intended to replace the articles "logo". But this remains to be seen. |
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===Conses and lists===
{{Main|Cons}}
[[File:Cons-
A Lisp list is implemented as a [[singly linked list]].<ref name="SebestaLanguages">{{cite book |last1=Sebesta |first1=Robert W. |title=Concepts of Programming Languages |chapter="2.4 Functional Programming: LISP";"6.9 List Types";"15.4 The First Functional Programming Language: LISP" |date=2012 |publisher=Addison-Wesley |___location=Boston, MA, US |isbn=978-0-13-139531-2 |pages=47–52;281–284;677–680 |edition=10th |url=https://www.pearson.com/us/higher-education/product/Sebesta-Concepts-of-Programming-Languages-10th-Edition/9780131395312.html |language=en |format=print}}</ref> Each cell of this list is called a ''cons'' (in Scheme, a ''pair'') and is composed of two [[pointer (computer programming)|pointers]], called the [[CAR and CDR|''car'' and ''cdr'']]. These are respectively equivalent to the {{Lisp2|data}} and {{Lisp2|next}} fields discussed in the article ''[[linked list]]''.
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====S-expressions represent lists====
[[File:Cons-cells.svg|thumb|right|300px|Box-and-[[pointer (computer programming)|pointer]] diagram for the list (42 69 613)]]
Parenthesized S-expressions represent linked list structures. There are several ways to represent the same list as an S-expression. A cons can be written in ''dotted-pair notation'' as {{Lisp2|(a . b)}}, where {{Lisp2|a}} is the car and {{Lisp2|b}} the cdr. A longer proper list might be written {{Lisp2|(a . (b . (c . (d . nil))))}} in dotted-pair notation. This is conventionally abbreviated as {{Lisp2|(a b c d)}} in ''list notation''. An improper list<ref name="r3sL3">NB: a so-called "dotted list" is only one kind of "improper list". The other kind is the "circular list" where the cons cells form a loop. Typically this is represented using #n=(...) to represent the target cons cell that will have multiple references, and #n# is used to refer to this cons. For instance, (#1=(a b) . #1#) would normally be printed as ((a b) a b) (without circular structure printing enabled), but makes the reuse of the cons cell clear. #1=(a . #1#) cannot normally be printed as it is circular, although (a...) is sometimes displayed, the CDR of the cons cell defined by #1= is itself.</ref> may be written in a combination of the two – as {{Lisp2|(a b c . d)}} for the list of three conses whose last cdr is {{Lisp2|d}} (i.e., the list {{Lisp2|(a . (b . (c . d)))}} in fully specified form).
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