Standard library

std/fmt

Formatting and printing.

Formatting and printing. The functions here are built into the compiler.

print, println, eprint and eprintln (to standard error) need no import. They, fmt.sprint and fmt.sprintln (which return the text) take any number of values of any type but functions, separated by spaces:

println("x =", 1, 2.5, true)          // x = 1 2.5 true
println(P{x: 1, y: "q"}, [1, 2])      // P{x: 1, y: "q"} [1, 2]
let s = fmt.sprint("id", 7)           // "id 7"

Strings print as they are at the top level and quoted inside other values; floats print in the shortest form that reads back the same; ?T values print as none or some(x); maps keep their insertion order. A rune is an i32 and prints as a number: s.pushRune(r) adds it to a string.

fmt.format returns a string, and fmt.printf prints it, from a template with {} placeholders:

fmt.format("{} has {} items", name, n)
fmt.printf("{:>8}|{:<4}|{:^7}|\n", "r", 1, "mid")   //        r|1   |  mid  |
fmt.format("{:08.3}", -3.14159)                      // -003.142
fmt.format("{:x} {:X} {:o} {:b}", 255, 255, 8, 5)    // ff FF 10 101

{}        the value, as `print` shows it
{:8}      at least 8 characters wide (numbers right-aligned, the rest left)
{:<8} {:>8} {:^8}   left, right, centered;   {:*^8}   with `*` as the fill
{:08}     numbers padded with zeros after the sign
{:.2}     a float with 2 digits after the point
{:x} {:X} {:o} {:b}   an integer in hex, octal or binary
{{ }}     literal braces

The template must be a string literal: the compiler checks the number of arguments and that each specification fits its argument's type.