Prelude functions

API summary (generated from the Beans source by npm run coverage): 8 package functions.

The prelude is the set of names available in every file without an import. Besides the builtin types, it gives you a few free functions. This page covers them. For the printing functions, which live in the io module, see the note below and the standard library reference.

panic

panic(message: string) stops the program for an error you cannot recover from. It reports the call location and your message, exits with status 3, and never returns. It does not run defers.

panic("index out of range")

Use panic only for bugs that should never happen. For errors you can handle, use Option and Result instead.

Compile-time layout functions

These three return a compile-time constant for the target you are building for. They take a type, not a value, and give the layout of that type on the selected target.

Function	Returns	Meaning
size_of(Type)	int	how many bytes a value of Type takes
align_of(Type)	int	the alignment of Type
offset_of(Type, field)	int	the byte offset of field inside Type

let s: int = size_of(i32)
let a: int = align_of(f64)
let o: int = offset_of(Point, x)

Rules:

• These take a type in a contextual form, not a runtime value.
• They are rejected on type parameters, and on Option, Result, and user enums, because those have no single layout.
• offset_of needs a struct or union type and a real field name.

See compile-time for more on constants known at build time.

Printing

Printing is done with the io module, so you import it and call io.println:

import std.io

fn main() {
    io.println("hello")
}

The four printing functions are:

• io.println: print a line to standard output
• io.print: print without a newline
• io.eprintln: print a line to standard error
• io.eprint: print to standard error without a newline

What can print:

• numbers, bools, and strings
• enums, shown as variant or variant(payload)
• lists of printable things, shown as [a, b, c]

What cannot print directly:

• Map values and class instances do not print; give them a string form of your own.
• Result is not printable; match on it instead.

See also

• Option, Result, and Error, panic alongside error handling.
• Compile-time, build-time constants.
• The standard library reference, the io module and other std.* functions.