Control flow

prepoly has the usual control-flow constructs: if/else, while, for, break, and continue. A distinctive point is that if (and match, covered in Pattern matching) are expressions — they yield a value.

if is an expression

fun grade(score) {
    let result = if score >= 60 { "pass" } else { "fail" }
    return result
}

println(grade(72))   // pass
println(grade(31))   // fail

else if chains work as you would expect:

fun size_of(n) {
    if n < 10 {
        return "small"
    } else if n < 100 {
        return "medium"
    } else {
        return "large"
    }
}

while

Here is the Collatz step counter — while runs as long as the condition holds:

fun collatz_steps(n) {
    let count = 0
    let x = n
    while x != 1 {
        if x % 2 == 0 {
            x = x / 2
        } else {
            x = 3 * x + 1
        }
        count += 1
    }
    return count
}

for n in [6, 7, 27] {
    println("collatz({n}) = {collatz_steps(n)} steps")
}

for over arrays and ranges

for x in xs iterates the elements of an array. The bracket form [lo..hi] builds the half-open integer range lo, lo+1, ..., hi-1, so counting loops look like this:

let sum = 0
for i in [1..11] {
    sum += i
}
println(sum)   // 55

break and continue

continue skips to the next iteration, break exits the loop:

let sum = 0
for n in [1, 2, 3, 4, 5, 6, 7, 8] {
    if n % 2 == 1 {
        continue
    }
    if n > 6 {
        break
    }
    sum += n
}
println("sum of evens up to 6 = {sum}")   // 12

There is no statement terminator: a newline ends a statement. A line continues onto the next when it ends with a binary operator or when the next line starts with . (a method chain) — see Syntax for the exact rules.