I This book is currently out of print. Upon kind permission of the M.I.T.-Press, it is available on ftp.ens.fr/pub/dmi/users/longo/CategTypesStructures All references should be made to the published book. CATEGORIES TYPES AND STRUCTURES An Introduction to Category Theory for the working computer scientist Andrea Asperti Giuseppe Longo FOUNDATIONS OF COMPUTING SERIES M.I.T. PRESS, 1991 II INTRODUCT