Nullables

In Fa, every value can be optional, meaning it can have the value null.

-- create an optional undefined dog
let dog: Dog?

-- create an optional defined dog
let dog = Dog?()

Nullable primitives

Strings, booleans, and numbers can also be optionals.

• An optional integer will be falsy when equal to null or zero.
• An optional string will be falsy when equal to null or empty.
• An optional boolean will be falsy when equal to null or false.

mutable sum: Integer?
console.log(sum) -- will print 'null'

if no sum {
  sum = 0
}
console.log(sum) -- will print '0'

if sum is 0 {
  sum = 12
}
console.log(sum) -- will print '12'

If you want to check if an optional integer is strictly equal to null, you must use the equality operator:

let sum: Integer? = 0

assert(sum is 0)
assert(sum is not null)
assert(sum?) -- because sum is not null
assert(!sum)