Builtin functions
Some functions are builtin like println. Here is the complete list:
fn print(s string) // prints anything on stdout
fn println(s string) // prints anything and a newline on stdout
fn eprint(s string) // same as print(), but uses stderr
fn eprintln(s string) // same as println(), but uses stderr
fn exit(code int) // terminates the program with a custom error code
fn panic(s string) // prints a message and backtraces on stderr, and terminates the program with error code 1
fn print_backtrace() // prints backtraces on stderr
[!NOTE] Although the
There is also a special built-in function called dump.
println
println is a simple yet powerful builtin function, that can print anything:
strings, numbers, arrays, maps, structs.
struct User {
name string
age int
}
println(1) // "1"
println('hi') // "hi"
println([1, 2, 3]) // "[1, 2, 3]"
println(User{ name: 'Bob', age: 20 }) // "User{name:'Bob', age:20}"
See also String interpolation.
Printing custom types
If you want to define a custom print value for your type, simply define a
str() string method:
struct Color {
r int
g int
b int
}
pub fn (c Color) str() string {
return '{${c.r}, ${c.g}, ${c.b}}'
}
red := Color{
r: 255
g: 0
b: 0
}
println(red)
Dumping expressions at runtime
You can dump/trace the value of any V expression using dump(expr).
For example, save this code sample as factorial.v, then run it with
v run factorial.v:
fn factorial(n u32) u32 {
if dump(n <= 1) {
return dump(1)
}
return dump(n * factorial(n - 1))
}
fn main() {
println(factorial(5))
}
You will get:
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: true
[factorial.v:3] 1: 1
[factorial.v:5] n * factorial(n - 1): 2
[factorial.v:5] n * factorial(n - 1): 6
[factorial.v:5] n * factorial(n - 1): 24
[factorial.v:5] n * factorial(n - 1): 120
120
Note that dump(expr) will trace both the source location,
the expression itself, and the expression value.
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