CS 2135: Programming Language Concepts
Examples of Templates

Templates arise from data definitions. It's meaningless to talk about a template without a data definition. The idea of a template is to write a skeleton of a function that consumes a particular kind of data as input. The template captures everything we know about the structure of the program based on the structure of the data.

A Template for Structs

; A circle is a (make-circle posn number)

(define-struct circle (center radius))

would yield the template

(define (circ-func acirc)
   (circle-center acirc) ...
   (circle-radius acirc) ... )

For a structure, we simply pull out all of the pieces inside of the template.

A Template for Collections of Structs

; A circle is a (make-circle posn number)
(define-struct circle (center radius))

; A square is a (make-square posn number)
(define-struct square (top-left width))

; A shape is either
;  - a circle, or
;  - a square

Here, the template for shape has two cases (one for each option in the definition). In each case, we call the template for the corresponding shape.

(define (circ-func acirc)
   (circle-center acirc) ...
   (circle-radius acirc) ... )

(define (square-func asqr)
   (square-top-left asqr) ...
   (square-width asqr) ...)

(define (shape-func ashape)
  (cond [(circle? ashape) (circ-func ashape)]
        [(square? ashape) (square-func ashape)]))

A Template for Lists

; A list-of-symbols is
;  - empty, or
;  - (cons symbol list-of-symbols)

Now, the template breaks down the list inside the cons? case and includes a recursive call to capture the "arrow" from list-of-symbols back to itself.

(define (list-of-sym-func alos)
  (cond [(empty? alos) ...]
        [(cons? alos)
         ... (first alos)
         ... (list-of-sym-func (rest alos)) ...]))