Lecture 6: HOPs, Types, Nimrod
and and or are not procedures, but special forms. They have a common feature of “short-
circuiting” at a value if their task is completed early, and then they do not need to
evaluate all of their arguments.
Scheme
1. Special Forms
(a) and – (and arg1 arg2 …) Evaluates arguments from left to right, stopping
at the first one that evaluates to false and returning false. Should all arguments
evaluate “true-ishly”, returns the values of the last argument.
Example:
(and (not null? lst)
(= (car lst) 3))
If the list lst is not null, then the procedure continues to check if the first
element of the list is a three. If it is, the return is true; if not the return is false.
However, if the list lst is null, the entire thing has a return of false even though
the second argument was not evaluated.
(b) or – (or arg1 arg2 …) Evaluates arguments from left to right, stopping at
the first one that evaluates to “true-ish” and returns that value. Should all the
arguments evaluate to false, returns false.
Example:
(or (= (car lst) 3)
(check-3 (cdr lst)))
If the first element of the list is three, procedure returns true. If not, it
continues to the next, and performs the same check. If true, it returns true; if
false, it returns false.
As an aside and reminder, a procedure takes in inputs and returns outputs.
Higher Order Procedures
The idea of programming is to capture patterns, as in the summation of n, and n – 1, or
the squares of n, and n-1, and use those patterns to form procedures for evaluation.
(The following are de-sugared to show the multiple lambdas.)
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