### Foreword to The Little Schemer

In 1967 I took an introductory course in photography. Most of the students (including me) came into that course hoping to learn how to be creative---to take pictures like the ones I admired by artists such as Edward Weston. On the first day the teacher patiently explained the long list of technical skills that he was going to teach us during the term. A key was Ansel Adams' Zone System'' for previsualizing the print values (blackness in the final print) in a photograph and how they derive from the light intensities in the scene. In support of this skill we had to learn the use of exposure meters to measure light intensities and the use of exposure time and development time to control the black level and the contrast in the image. This is in turn supported by even lower level skills such as loading film, developing and printing, and mixing chemicals. One must learn to ritualize the process of developing sensitive material so that one gets consistent results over many years of work. The first laboratory session was devoted to finding out that developer feels slippery and that fixer smells awful.

But what about creative composition? In order to be creative one must first gain control of the medium. One can not even begin to think about organizing a great photograph without having the skills to make it happen. In engineering, as in other creative arts, we must learn to do analysis to support our efforts in synthesis. One cannot build a beautiful and functional bridge without a knowledge of steel and dirt and considerable mathematical technique for using this knowledge to compute the properties of structures. Similarly, one cannot build a beautiful computer system without a deep understanding of how to previsualize'' the process generated by the procedures one writes.

Some photographers choose to use black-and-white 8\times/10 plates while others choose 35mm slides. Each has its advantages and disadvantages. Like photography, programming requires a choice of medium. Lisp is the medium of choice for people who enjoy free style and flexibility. Lisp was initially conceived as a theoretical vehicle for recursion theory and for symbolic algebra. It has developed into a uniquely powerful and flexible family of software development tools, providing wrap-around support for the rapid-prototyping of software systems. As with other languages, Lisp provides the glue for using a vast library of canned parts, produced by members of the user community. In Lisp, procedures are first-class data, to be passed as arguments, returned as values, and stored in data structures. This flexibility is valuable, but most importantly, it provides mechanisms for formalizing, naming, and saving the idioms---the common patterns of usage that are essential to engineering design. In addition, Lisp programs can easily manipulate the representations of Lisp programs---a feature that has encouraged the development of a vast structure of program synthesis and analysis tools, such as cross-referencers.

The Little LISPer is a unique approach to developing the skills underlying creative programming in Lisp. It painlessly packages, with considerable wit, much of the drill and practice that is necessary to learn the skills of constructing recursive processes and manipulating recursive data-structures. For the student of Lisp programming, The Little LISPer can perform the same service that Hanon's finger exercises or Czerny's piano studies perform for the student of piano.

Gerald J. Sussman
Cambridge, Massachusetts