Flow Control

Janet 0.4.0 Documentation
Special Forms
String Library
Data Structures
Object Oriented Programming
Abstract Machine

Janet has only two built in primitives to change flow while inside a function. The first is the if special form, which behaves as expected in most functional languages. It takes two or three parameters: a condition, an expression to evaluate to if the condition is true (not nil or false), and an optional condition to evaluate to when the condition is nil or false. If the optional parameter is omitted, the if form evaluates to nil.

(if (> 4 3)
  "4 is greater than 3"
  "4 is not greater then three") # Evaluates to the first statement

(if true
  (print "Hey")) # Will print

(if false
  (print "Oy!")) # Will not print

The second primitive control flow construct is the while loop. The while behaves much the same as in many other programming languages, including C, Java, and Python. The while loop takes two or more parameters: the first is a condition (like in the if statement), that is checked before every iteration of the loop. If it is nil or false, the while loop ends and evaluates to nil. Otherwise, the rest of the parameters will be evaluated sequentially and then the program will return to the beginning of the loop.

# Loop from 100 down to 1 and print each time
(var i 100)
(while (pos? i)
  (print "the number is " i)
  (-- i))

# Print ... until a random number in range [0, 1) is >= 0.9
# (math/random evaluates to a value between 0 and 1)
(while (> 0.9 (math/random))
  (print "..."))

Besides these special forms, Janet has many macros for both conditional testing and looping that are much better for the majority of cases. For conditional testing, the cond, switch, and when macros can be used to great effect. cond can be used for making an if-else chain, where using just raw if forms would result in many parentheses. case For looping, the loop, seq, and generate implement janet's form of list comprehension, as in Python or Clojure.