Control Flow

Overview

Conditions and loops are expressions like every other construct: an if with both branches has a value, and a repeat loop answers the operand of the break that ends it. A condition needs no parentheses; a body of one statement needs no braces, a longer one is a block in braces that begins on a line of its own. if cond stmt without an else runs the statement when the condition is true, and control continues after it.

Conditions are written as and/or chains. A null test proves its name for what follows it in the chain: in p <> null and p.x = 7 the right side reads p.x only where p is there.

If Expressions

If expressions evaluate a condition and return one of two values.

let x 5
if x > 3 : x * 2 else x

let a 7
let b 3
if a > b : a else b

function clamp(dmin: int) returns int
{
    var adj 7
    if dmin > 100
        adj := 0
    else if dmin > 50
        adj := dmin + 1
    adj
}
clamp(200) * 10000 + clamp(60) * 100 + clamp(10)

While Loops

While loops repeat as long as a condition is true.

var x 0
while x < 3 : x := x + 1
x

var x 10
while x < 5 : x := x + 1
x

var sum 0
var i 1
while i <= 3 : { sum := sum + i: i := i + 1 }
sum

var result 1
var n 5
while n > 0 : { result := result * n: n := n - 1 }
result

For Loops

For loops iterate over a range of integers.

var count 0
for i in 3 : count := count + 1
count

var x 42
for i in 0 : x := 0
x

var sum 0
for i in 5 : sum := sum + i
sum

var prod 1
for i in 4 : prod := prod * (i + 1)
prod

Break and Continue

Break exits a loop early, continue skips to the next iteration.

var sum 0
for i in 10 : { if i = 3 : break else sum := sum + i }
sum

var x 0
while x < 100 : { x := x + 1: if x = 5 : break }
x

var sum 0
for i in 6 : { if i % 2 = 1 : continue else sum := sum + i }
sum

var sum 0
var i 0
while i < 10 : { let current i: i := i + 1: if current % 2 = 1 : continue else sum := sum + current }
sum

var count 0
for i in 3 : { for j in 10 : { if j = 2 : break else count := count + 1 } }
count

var count 0
for i in 3 : { for j in 5 : { if j % 2 = 1 : continue else count := count + 1 } }
count

var count 0
for i in 3 : label outer { for j in 5 : { if j = 2 : break loop outer else count := count + 1 } }
count

var count 0
for i in 3 : label outer { for j in 5 : { if j = 0 : continue loop outer else count := count + 100 } }
count

Repeat Loops

A repeat loop runs its body forever and exits only through break (or return). Because break is the only way out, a break carrying a value (break v) makes the repeat an expression that yields that value.

var i 0
let answer repeat { if i = 5 : break i + 100 else i := i + 1 }
answer

var i 0
repeat { if i = 3 : break else i := i + 1 }
i

Mutable Variables

A binding introduced with var may be assigned a new value; one introduced with let may not. The assignment is written target := source, in the Algol tradition, and is a statement of its own: it introduces no name and is never part of a condition. The equals sign always compares.

var x 5
x

var x 5
x := 10
x

var x 3
x := x + 1
x

var a 2
var b 3
a := a * b
a

Match

match compares one value against a list of case alternatives and takes the first that is equal; one case may name several values, and else takes the rest. It works on numbers, characters and strings, which it compares by content:

function kind(n: int) returns int
    match n
        case 1: 10
        case 2 case 3: 20
        else 0

let word "b"
match word
    case "a": print "A"
    case "b": print "B"
    else print "?"

A union is matched by its variants with choose, described under Union Types.