Racket function that checks if an instance of a struct consists of certain values in DrRacket - struct

Question

Hi I'm trying to write a function in Racket (like Scheme/Lisp) to check if an instance of a struct based on time consists of only am/AM and not pm/PM.
Here are my attempts at checking to see if an instance of AM/PM is just "AM" or just "am" :
(define (isAM? AM/PM-meridiem)
(if (or(equal? AM/PM-meridiem "AM")
(equal? AM/PM-meridiem "am"))
(#t))
(else (#f)))
(define (time) (AM/PM 2 20 "am"))
(define (isAM?2 time)
(or (equal? (AM/PM-meridiem time) "AM")
(equal? (AM/PM-meridiem time) "am")))
(define (isAM?3 time)
(or (equal? AM/PM-meridiem "AM")
(equal? AM/PM-meridiem "am")))
Here's the two struct functions the isAM? function uses that I wrote:
#lang slideshow
(struct 24HourClock (hours minutes)
#:transparent
#:guard (lambda (hours minutes err_name)
(if (not (and (>= hours 0) (< hours 24)))
(error err_name "Bad hours")
(if (not (and (>= minutes 0) (< minutes 60)))
(error err_name "Bad minutes")
(values hours minutes)))))
(struct AM/PM 24HourClock (meridiem)
#:transparent
#:guard (lambda (hours minutes meridiem err_name)
(if (not(and(and(number? hours)(> hours 0)(<= hours 12)(number? minutes))))
(error err_name "Hours must be between 1 and 12 : 0")
(if [not (and(string? meridiem)(or
(equal? meridiem "am")
(equal? meridiem "AM")
(equal? meridiem "pm")
(equal? meridiem "PM")))]
(error err_name "Invalid value for meridiem")
(values hours minutes meridiem)))))

Answer 1

It looks like you're struggling with the fundamentals of syntax.
(#t) tries to use #t as a procedure, which it isn't.
(Parentheses are not block-structuring punctuation in the manner of curly braces in some other languages.)
And, as you seem to already know, a conditional does not have an "else" - the syntax is (if condition true-expression false-expression), as you wrote in the guards.
And third, the parameter should be an instance of your struct - AM/PM-meridiem is the selector procedure you need to apply to it.
You also seem to have a habit of writing unnecessary conditionals with boolean values.
You should drop that habit in those other languages you are familiar with, as well.
Here is a corrected version:
(define (is-AM? time)
(or (equal? (AM/PM-meridiem time) "AM")
(equal? (AM/PM-meridiem time) "am")))
As an aside, the equivalent in C++ (or Java) would look something like this:
bool is_AM(AMPM time)
{
return time.meridiem == "AM" || time.meridiem == "am";
}
and in Python,
def is_AM(time):
{
return time.meridiem == "AM" or time.meridiem == "am";
}