CSC153 2004S, Class 17: Higher Order Procedures (2)

Admin:
* Exam 1 due.  How long did it take?
* No reading for tomorrow.
* HW 4 assigned.
* Cool biocomputing talk tomorrow at 4:30.  Extra credit for attending and reflecting.

Overview:
* Some more fun with higher-order procedures:
  * Operator sections
  * Combining boolean operations
  * Procedure definition, reconsidered
* Lab

More about Higher-Order Procedures
* "Real" Schemers tend to use higher-order procedures to define new procedures
(define disjunction
  (lambda (pred1? pred2?)
    (lambda (x)
      (or (pred1? x) (pred2? x)))))
OLD STYLE
(define symbol-or-string?
    (lambda (x)
      (or (symbol? x) (string? x)))))
NEW STYLE
(define symbol-or-string? (disjunction symbol? string?))
(define symbol-or-string? (or symbol? string?))
=> symbol?
* "Real" Schemers (e.g., Mr. Stone) therefore often write procedures that are useful for defining other procedures
(define left-section
  (lambda (binary-proc arg1)
    (lambda (arg2)
       (binary-proc arg1 arg2))))

* "and" is not a procedure.  It is instead a syntactic construct.
* "and" is not a procedure because it has a different order of operations than normal procedure application
(define silly (lambda (x) (display x) (newline) x))

TRY THE LAB! (Updated during class.)