Cond

Cascading alternate conditions using if becomes a bit clumsy for which there is a powerful solution, cond.

cond extends the syntax by allowing the use of (condition consequent) tuples and accepts a final clause where the condition is the keyword else.

The result of a cond expression is that of the last expression evaluated or #<void> if no clauses match.

As these are usually going to be on multiple lines for readability the whole expression is normally wrapped in parentheses:

(cond
 (cond1 conseq1)
 (cond2 conseq2)
 (cond3 conseq3)
 (else  conseq4))

simple-cond.idio

define (foo a) {

  (cond
   ((a gt 0) {
     printf "a is positive\n"
   })
   ((a lt 0) {
     printf "a is negative\n"
   })
   (else {
     printf "a is zero\n"
   }))
}

foo 10
foo -10
foo 0

$ idio simple-cond
a is positive
a is negative
a is zero

=>

An advanced use in cond is the => form where the tuple becomes (condition => f) where f is a function that accepts one argument.

Here, the condition is evaluated and if the result is “true” then f is invoked with the result.

It is a form of anaphoric if expression roughly equivalent to:

tmp := condition
if tmp {
  f tmp
}

cond.idio

ht := #{ }
ht.'a = "apple"
ht.'b = "banana"

define (foo k) {

  ;; hash-ref takes an optional "default" argument to return if the
  ;; key doesn't exist (otherwise it raises a condition)
  ;;
  ;; If k does exist in ht then the function will be called with
  ;; whatever hash-ref returns.  If k doesn't exist in ht then
  ;; hash-ref returns #f and we try the next clause.
  (cond
   ((hash-ref ht k #f) => (function (v) {
     printf "%s is %s\n" k v
   }))
   (else {
     printf "%s is unknown\n" k
   }))
}

foo 'b
foo 24

$ idio cond
b is banana
24 is unknown

Last built at 2024-05-21T06:11:39Z+0000 from 77077af (dev) for Idio 0.3