COWSEL (COntrolled Working SpacE Language) is a programming language designed between 1964 and 1966 by Robin Popplestone. It was based on an reverse Polish notation (RPN) form of the language Lisp, combined with some ideas from Combined Programming Language (CPL).

COWSEL was initially implemented on a Ferranti Pegasus computer at the University of Leeds and on a Stantec Zebra at the Bradford Institute of Technology. Later, Rod Burstall implemented it on an Elliot 4120 at the University of Edinburgh.

COWSEL was renamed POP-1 in 1966, during summer, and development continued under that name from then on.

Example code

function member
lambda x y
comment Is x a member of list y;
define      y atom then *0 end
            y hd x equal then *1 end
            y tl -> y repeat up

Reserved words (keywords) were also underlined in the original printouts. Popplestone performed syntax highlighting by using underscoring on a Friden Flexowriter.