 In the 1980s, Friedman turned to the study of the Scheme programming language.He explored the use of macros for defining programming languages; with Eugene Kohlbecker, Matthias Felleisen, and Bruce Duba, he co-introduced the notion of hygienic macros in a 1986 LFP paper that is still widely cited today.With Christopher T. Haynes and Mitchell Wand, he simultaneously studied the nature of continuation objects, their uses, and the possibilities of constraining them.