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weekly 2023-12-18

· 2 min read

In December 2023, MoonBit underwent Alpha testing. This post aims to introduce recent updates to MoonBit’s language and toolchain, along with applications developed using MoonBit.


MoonBit Update

1. Added Debug Interface

interface Debug {
debug_write(Self, Buffer)

Added Debug Interface and the debug[X: Debug](X) function. The Debug interface has been implemented for built-in types and can be derived automatically for user-defined types. Compared to the previous Show interface, Debug has better performance, and would correctly add double quotes around values of type String. Users are encouraged to replace Show with Debug and println with debug for debugging purpose. Show and println may get deprecated or receive breaking change in the future.

Example usage of Debug:

struct T[X] {
s: String
x: X
} derive (Debug)

fn init {
let t = { s : "hello", x: "world" }
debug(t) // Output: {s: "hello", x: "world"}

2. Added iteration expression in while loops, to be executed after each iteration

var i = 0
while i < len, i = i + 1 {

The example above is roughly equivalent to:

var i = 0
while i < len {
i = i + 1

However, continuestatements will not skip the iteration expression, which makes it very handy to write for-like loops. For example, the following program will do something on all odd numbers smaller than 10, instead of looping forever:

var i = 0
while i < 10, i = i + 1 {
if i % 2 == 0 {

3. Removed id := expr syntax

Now, immutable variables can only be declared using let id = expr.

Before: before|378x176

After: After|690x150

4. Reimplemented high-performance to_string method

Reimplemented high-performance to_string method of Int/Int64 using the built-in Bytes type.

5. Added String::default

fn init {
let s = String::default()
debug(s) // Output: ""

IDE Updates

1. Native VS Code plugin now automatically runs moon check --watch without manual execution

2. Added hints for methods



3. Improved many type error messages.