Editing POP-2 (section)

===Arrays and doublet functions===
There are no special language constructs to create arrays or record structures as they are commonly understood: instead, these are created with the aid of special builtin functions, e.g., {{code|newarray}}<ref>{{Cite web |url=http://www.cs.bham.ac.uk/research/projects/poplog/doc/pophelp/newarray |title=Help Newarray |last1=Rubinstein |first1=Mark |last2=Sloman |first2=A. |date=October 1985 – April 1989 |website=University of Birmingham |access-date=22 March 2024}}</ref> (for arrays that can contain any type of item) and {{code|newanyarray}}<ref>{{Cite web |url=http://www.cs.bham.ac.uk/research/projects/poplog/doc/pophelp/newanyarray |title=Help Newanyarray |last1=Hardy |first1=Steven |last2=Williams |first2=John |last3=Sloman |first3=A. |date=January 1978 – April 1986 |website=University of Birmingham |access-date=22 March 2024}}</ref> to create restricted types of items.

Thus, array element and record field accessors are simply special cases of a ''doublet function'': this is a function that had another function attached as its ''updater'',<ref>{{Cite web |url=http://www.cs.bham.ac.uk/research/projects/poplog/doc/pophelp/updater |title=Help Updater |last=Sloman |first=A. |date=April 1985 |website=University of Birmingham |access-date=21 March 2024}}</ref> which is called on the receiving side of an assignment. Thus, if the variable {{code|a}} contains an array, then

 3 -> a(4);

is equivalent to

 updater(a)(3, 4);

the builtin function {{code|updater}} returning the updater of the doublet. Of course, {{code|updater}} is a doublet and can be used to change the updater component of a doublet.