Fundamentals

Values, variables, and control flow. Sure has four basic types, two kinds of variable, and one opinion about ambiguity.

Variables

let makes an immutable binding; var makes a mutable one. Types are inferred from the value, or declared if you prefer. Statements end at the end of the line — no semicolons required, though they are accepted without comment.

func main() -> Int {
    let answer = 42          // Int, forever 42
    var count: Int = 0       // Int, changeable
    count = count + answer
    print(count)
    return 0
}

Assigning to a let is an error, and the message tells you the fix: 'x' is a let; use var if it needs to change. A name can be declared once per function; shadowing is not a feature Sure offers, on the grounds that one name should mean one thing.

The basic types

Arithmetic never mixes types. 1 + 2.0 is an error, not a conversion; Sure will not guess which precision you meant. Convert explicitly with to_float(n), and come back down with trunc, round, floor, or ceil. % works on Int only.

func main() -> Int {
    let ratio = to_float(7) / 2.0    // 3.5
    print(round(ratio))              // 4
    print(7 % 2)                     // 1
    return 0
}

Choosing

if takes a Bool — not an integer, not a hopeful expression. Chain with else if.

func describe(n: Int) -> String {
    if n < 0 {
        return "negative"
    } else if n == 0 {
        return "zero"
    } else {
        return "positive"
    }
}

func main() -> Int {
    print(describe(-3))
    return 0
}

When a choice produces a value, use an if expression. The else is mandatory — an expression must always be worth something:

func main() -> Int {
    let n = 12
    let label = if n % 2 == 0 { "even" } else { "odd" }
    print(label)
    return 0
}

Repeating

while loops on a condition. for iterates a range (end exclusive), an array, or — as you will see in chapter 9 — a channel. break and continue do what they say.

func main() -> Int {
    var total = 0
    for i in 1..11 {
        if i % 3 == 0 {
            continue
        }
        total = total + i
    }
    print(total)    // 1..10, skipping multiples of 3: 37
    return 0
}

Printing

print(x) takes an Int, Float, Bool, or String and adds a newline. To build a line out of mixed pieces, convert with str(x) and concatenate with +:

func main() -> Int {
    let sides = 6
    print("a die has " + str(sides) + " sides")
    return 0
}