my_module
├── moon.mod.json             # Module metadata, source field (optional) specifies the source directory of the module
├── moon.pkg             # Package metadata (each directory is a package like Golang)
├── README.mbt.md             # Markdown with tested code blocks (`test "..." { ... }`)
├── README.md -> README.mbt.md
├── cmd                       # Command line directory
│   └── main
│       ├── main.mbt
│       └── moon.pkg     # executable package with `options("is-main": true)`
├── liba/                     # Library packages
│   └── moon.pkg         # Referenced by other packages as `@username/my_module/liba`
│   └── libb/                 # Library packages
│       └── moon.pkg     # Referenced by other packages as `@username/my_module/liba/libb`
├── user_pkg.mbt              # Root packages, referenced by other packages as `@username/my_module`
├── user_pkg_wbtest.mbt       # White-box tests (only needed for testing internal private members, similar to Golang's package mypackage)
└── user_pkg_test.mbt         # Black-box tests
└── ...                       # More package files, symbols visible to current package (like Golang)

///|
/// Get the largest element of a non-empty `Array`.
///
/// # Example
/// ```mbt check
/// test {
///   inspect(sum_array([1, 2, 3, 4, 5, 6]), content="21")
/// }
/// ```
///
/// # Panics
/// Panics if the `xs` is empty.
pub fn sum_array(xs : Array[Int]) -> Int {
  xs.fold(init=0, (a, b) => a + b)
}

///|
declare pub type Yaml

///|
declare pub fn Yaml::to_string(y : Yaml) -> String raise

///|
declare pub impl Eq for Yaml

///|
declare pub fn parse_yaml(s : String) -> Yaml raise

undefined

**Best practice**: Treat this section as command reference; execution order is defined in `Agent Workflow`.

$ moon ide rename compute_sum calculate_sum --loc math_utils.mbt:2

*** Begin Patch
*** Update File: cmd/main/main.mbt
@@
 ///|
 fn main {
-  println(@math_utils.compute_sum(1, 2))
+  println(@math_utils.calculate_sum(1, 2))
 }
*** Update File: math_utils.mbt
@@
 ///|
-pub fn compute_sum(a: Int, b: Int) -> Int {
+pub fn calculate_sum(a: Int, b: Int) -> Int {
   a + b
 }
*** Update File: math_utils_test.mbt
@@
 ///|
 test {
-  inspect(@math_utils.compute_sum(1, 2))
+  inspect(@math_utils.calculate_sum(1, 2))
 }
*** End Patch

$ moon ide hover  filter --loc hover.mbt:14
test {
  let a: Array[Int] = [1]
  inspect(a.filter((x) => {x > 1}))
            ^^^^^^
            ```moonbit
            fn[T] Array::filter(self : Array[T], f : (T) -> Bool raise?) -> Array[T] raise?
            ```
            ---

             Creates a new array containing all elements from the input array that satisfy
             ... omitted ...
}

L45:|///|
L46:|fn Parser::read_u32_leb128(self : Parser) -> UInt raise ParseError {
L47:|  ...
...:| }

$ moon ide peek-def Parser --loc src/parse.mbt:46:4
Definition found at file src/parse.mbt
  | ///|
2 | priv struct Parser {
  |             ^^^^^^
  |   bytes : Bytes
  |   mut pos : Int
  | }
  |

$ moon ide peek-def String::rev
Found 1 symbols matching 'String::rev':

`pub fn String::rev` in package moonbitlang/core/builtin at /Users/usrname/.moon/lib/core/builtin/string_methods.mbt:1039-1044
1039 | ///|
     | /// Returns a new string with the characters in reverse order. It respects
     | /// Unicode characters and surrogate pairs but not grapheme clusters.
     | pub fn String::rev(self : String) -> String {
     |   self[:].rev()
     | }

$ moon ide outline .
spec.mbt:
 L003 | pub(all) enum CStandard {
        ...
 L013 | pub(all) struct Position {
        ...

$ moon ide find-references TranslationUnit

moon add moonbitlang/x        # Add latest version
moon add moonbitlang/x@0.4.6  # Add specific version

moon update                   # Update package index

{
  "name": "username/hello", // Required format for published modules
  "version": "0.1.0",
  "source": ".", // Source directory(optional, default: ".")
  "repository": "", // Git repository URL
  "keywords": [], // Search keywords
  "description": "...", // Module description
  "deps": {
    // Dependencies from mooncakes.io, using`moon add` to add dependencies
    "moonbitlang/x": "0.4.6"
  }
}

import {
  "username/hello/liba",
  "moonbitlang/x/encoding" @libb
}
import {...} for "test"
import {...} for "wbtest"
options("is-main" : true) // other options

{
  "is_main": true,                 // Creates executable when true
  "import": [                      // Package dependencies
    "username/hello/liba",         // Simple import, use @liba.foo() to call functions
    {
      "path": "moonbitlang/x/encoding",
      "alias": "libb"              // Custom alias, use @libb.encode() to call functions
    }
  ],
  "test-import": [...],            // Imports for black-box tests, similar to import
  "wbtest-import": [...]           // Imports for white-box tests, similar to import (rarely used)
}

///|
/// In main.mbt after importing "username/hello/liba" in `moon.pkg`
fn main {
  println(@liba.hello()) // Calls hello() from liba package
}

///|
/// Declare error types with 'suberror'
suberror ValueError {
  ValueError(String)
}

///|
/// Tuple struct to hold position info
struct Position(Int, Int) derive(ToJson, Show, Eq)

///|
/// ParseError is subtype of Error
pub(all) suberror ParseError {
  InvalidChar(pos~ : Position, Char) // pos is labeled
  InvalidEof(pos~ : Position)
  InvalidNumber(pos~ : Position, String)
  InvalidIdentEscape(pos~ : Position)
} derive(Eq, ToJson, Show)

///|
/// Functions declare what they can throw
fn parse_int(s : String, position~ : Position) -> Int raise ParseError {
  // 'raise' throws an error
  if s is "" {
    raise ParseError::InvalidEof(pos=position)
  }
  ... // parsing logic
}

///|
/// Just declare `raise` to not track specific error types
fn div(x : Int, y : Int) -> Int raise {
  if y is 0 {
    fail("Division by zero")
  }
  x / y
}

///|
test "inspect raise function" {
  let result : Result[Int, Error] = try? div(1, 0)
  guard result is Err(Failure(msg)) && msg.contains("Division by zero") else {
    fail("Expected error")
  }
}

// Three ways to handle errors:

///|
/// Propagate automatically
fn use_parse(position~ : Position) -> Int raise ParseError {
  let x = parse_int("123", position~) // label punning, equivalent to position=position
  // Error auto-propagates by default.
  // Unlike Swift, you do not need to mark `try` for functions that can raise
  // errors; the compiler infers it automatically. This keeps error handling
  // explicit but concise.
  x * 2
}

///|
/// Mark `raise` for all possible errors, do not care which error it is.
/// For quick prototypes, `raise` is acceptable.
fn use_parse2(position~ : Position) -> Int raise {
  let x = parse_int("123", position~) // label punning
  x * 2
}

///|
/// Convert to Result with try?
fn safe_parse(s : String, position~ : Position) -> Result[Int, ParseError] {
  let val1 : Result[_] = try? parse_int(s, position~) // Returns Result[Int, ParseError]
  // try! is rarely used - it panics on error, similar to unwrap() in Rust
  // let val2 : Int = try! parse_int(s) // Returns Int otherwise crash

  // Alternative explicit handling:
  let val3 = try parse_int(s, position~) catch {
    err => Err(err)
  } noraise { // noraise block is optional - handles the success case
    v => Ok(v)
  }
  ...
}

///|
/// Handle with try-catch
fn handle_parse(s : String, position~ : Position) -> Int {
  try parse_int(s, position~) catch {
    ParseError::InvalidEof => {
      println("Parse failed: InvalidEof")
      -1 // Default value
    }
    _ => 2
  }
}

///|
test "integer and char literal overloading disambiguation via type in the current context" {
  let a0 = 1 // a is Int by default
  let (int, uint, uint16, int64, byte) : (Int, UInt, UInt16, Int64, Byte) = (
    1, 1, 1, 1, 1,
  )
  assert_eq(int, uint16.to_int())
  let a1 : Int = 'b' // this also works, a5 will be the unicode value
  let a2 : Char = 'b'

}

///|
test "bytes literals overloading and indexing" {
  let b0 : Bytes = b"abcd"
  let b1 : Bytes = "abcd" // b" prefix is optional, when we know the type
  let b2 : Bytes = [0xff, 0x00, 0x01] // Array literal overloading
  guard b0 is [b'a', ..] && b0[1] is b'b' else {
    // Bytes can be pattern matched as BytesView and indexed
    fail("unexpected bytes content")
  }
}

///|
test "array literals overloading: disambiguation via type in the current context" {
  let a0 : Array[Int] = [1, 2, 3] // resizable
  let a1 : FixedArray[Int] = [1, 2, 3] // Fixed size
  let a2 : ReadOnlyArray[Int] = [1, 2, 3]
  let a3 : ArrayView[Int] = [1, 2, 3]

}

///|
test "string indexing and utf8 encode/decode" {
  let s = "hello world"
  let b0 : UInt16 = s[0]
  guard b0 is ('\n' | 'h' | 'b' | 'a'..='z') && s is [.. "hello", .. rest] else {
    fail("unexpected string content")
  }
  guard rest is " world"  // otherwise will crash (guard without else)

  // In check mode (expression with explicit type), ('\n' : UInt16) is valid.

  // Using get_char for Option handling
  let b1 : Char? = s.get_char(0)
  assert_true(b1 is Some('a'..='z'))

  // ⚠️ Important: Variables won't work with direct indexing
  let eq_char : Char = '='
  // s[0] == eq_char // ❌ Won't compile - eq_char is not a literal, lhs is UInt while rhs is Char
  // Use: s[0] == '=' or s.get_char(0) == Some(eq_char)
  let bytes = @utf8.encode("中文") // utf8 encode package is in stdlib
  assert_true(bytes is [0xe4, 0xb8, 0xad, 0xe6, 0x96, 0x87])
  let s2 : String = @utf8.decode(bytes) // decode utf8 bytes back to String
  assert_true(s2 is "中文")
  for c in "中文" {
    let _ : Char = c // unicode safe iteration
    println("char: \{c}") // iterate over chars
  }
}

///|
test "string interpolation basics" {
  let name : String = "Moon"
  let config = { "cache": 123 }
  let version = 1.0
  println("Hello \{name} v\{version}") // "Hello Moon v1.0"
  // ❌ Wrong - quotes inside interpolation not allowed:
  // println("  - Checking if 'cache' section exists: \{config["cache"]}")

  // ✅ Correct - extract to variable first:
  let has_key = config["cache"] // `"` not allowed in interpolation
  println("  - Checking if 'cache' section exists: \{has_key}")
  let sb = StringBuilder::new()
  sb
  ..write_char('[') // dotdot for imperative method chaining
  ..write_view([1, 2, 3].map(x => "\{x}").join(","))
  ..write_char(']')
  inspect(sb.to_string(), content="[1,2,3]")
}

///|
test "multi-line string literals" {
  let multi_line_string : String =
    #|Hello "world"
    #|World
    #|
  let multi_line_string_with_interp : String =
    $|Line 1 ""
    $|Line 2 \{1+2}
    $|
  // no escape in `#|`,
  // only escape '\{..}` in `$|`
  assert_eq(multi_line_string, "Hello \"world\"\nWorld\n")
  assert_eq(multi_line_string_with_interp, "Line 1 \"\"\nLine 2 3\n")
}

///|
test "map literals and common operations" {
  // Map literal syntax
  let map : Map[String, Int] = { "a": 1, "b": 2, "c": 3 }
  let empty : Map[String, Int] = {} // Empty map, preferred
  let also_empty : Map[String, Int] = Map::new()
  // From array of pairs
  let from_pairs : Map[String, Int] = Map::from_array([("x", 1), ("y", 2)])

  // Set/update value
  map["new-key"] = 3
  map["a"] = 10 // Updates existing key

  // Get value - returns Option[T]
  guard map is { "new-key": 3, "missing"? : None, .. } else {
    fail("unexpected map contents")
  }

  // Direct access (panics if key missing)
  let value : Int = map["a"] // value = 10

  // Iteration preserves insertion order
  for k, v in map {
    println("\{k}: \{v}") // Prints: a: 10, b: 2, c: 3, new-key: 3
  }

  // Other common operations
  map.remove("b")
  guard map is { "a": 10, "c": 3, "new-key": 3, .. } && map.length() == 3 else {
    // "b" is gone, only 3 elements left
    fail("unexpected map contents after removal")
  }
}

///|
enum Tree[T] {
  Leaf(T) // Unlike Rust, no comma here
  Node(left~ : Tree[T], T, right~ : Tree[T]) // enum can use labels
} derive(Show, ToJson) // derive traits for Tree

///|
pub fn Tree::sum(tree : Tree[Int]) -> Int {
  match tree {
    Leaf(x) => x
    // we don't need to write Tree::Leaf, when `tree` has a known type
    Node(left~, x, right~) => left.sum() + x + right.sum() // method invoked in dot notation
  }
}

///|
struct Point {
  x : Int
  y : Int
} derive(Show, ToJson) // derive traits for Point

///|
test "user defined types: enum and struct" {
  @json.inspect(Point::{ x: 10, y: 20 }, content={ "x": 10, "y": 20 })
}

///|
pub fn binary_search(arr : ArrayView[Int], value : Int) -> Result[Int, Int] {
  let len = arr.length()
  // functional for loop:
  // initial state ; [predicate] ; [post-update] {
  // loop body with `continue` to update state
  //} else { // exit block
  // }
  // predicate and post-update are optional
  for i = 0, j = len; i < j; {
    // post-update is omitted, we use `continue` to update state
    let h = i + (j - i) / 2
    if arr[h] < value {
      continue h + 1, j // functional update of loop state
    } else {
      continue i, h // functional update of loop state
    }
  } else { // exit of for loop
    if i < len && arr[i] == value {
      Ok(i)
    } else {
      Err(i)
    }
  } where {
    invariant: 0 <= i && i <= j && j <= len,
    invariant: i == 0 || arr[i - 1] < value,
    invariant: j == len || arr[j] >= value,
    reasoning: (
      #|For a sorted array, the boundary invariants are witnesses:
      #|  - `arr[i-1] < value` implies all arr[0..i) < value (by sortedness)
      #|  - `arr[j] >= value` implies all arr[j..len) >= value (by sortedness)
      #|
      #|Preservation proof:
      #|  - When arr[h] < value: new_i = h+1, and arr[new_i - 1] = arr[h] < value ✓
      #|  - When arr[h] >= value: new_j = h, and arr[new_j] = arr[h] >= value ✓
      #|
      #|Termination: j - i decreases each iteration (h is strictly between i and j)
      #|
      #|Correctness at exit (i == j):
      #|  - By invariants: arr[0..i) < value and arr[i..len) >= value
      #|  - So if value exists, it can only be at index i
      #|  - If arr[i] != value, then value is absent and i is the insertion point
      #|
    ),
  }
}

///|
test "functional for loop control flow" {
  let arr : Array[Int] = [1, 3, 5, 7, 9]
  inspect(binary_search(arr, 5), content="Ok(2)") // Array to ArrayView implicit conversion when passing as arguments
  inspect(binary_search(arr, 6), content="Err(3)")
  // for iteration is supported too
  for i, v in arr {
    println("\{i}: \{v}") // `i` is index, `v` is value
  }
}

for ... {
  ...
} where {
  invariant : <boolean_expr>,   // checked at runtime in debug builds
  invariant : <boolean_expr>,   // multiple invariants allowed
  reasoning : <string>         // documentation for proof sketch
}

///|
fn g(
  positional : Int,
  required~ : Int,
  optional? : Int, // no default => Option
  optional_with_default? : Int = 42, // default => plain Int
) -> String {
  // These are the inferred types inside the function body.
  let _ : Int = positional
  let _ : Int = required
  let _ : Int? = optional
  let _ : Int = optional_with_default
  "\{positional},\{required},\{optional},\{optional_with_default}"
}

///|
test {
  inspect(g(1, required=2), content="1,2,None,42")
  inspect(g(1, required=2, optional=3), content="1,2,Some(3),42")
  inspect(g(1, required=4, optional_with_default=100), content="1,4,None,100")
}

///|
fn with_config(a : Int?, b : Int?, c : Int) -> String {
  "\{a},\{b},\{c}"
}

///|
test {
  inspect(with_config(None, None, 1), content="None,None,1")
  inspect(with_config(Some(5), Some(5), 1), content="Some(5),Some(5),1")
}

///|
fn f_misuse(a? : Int?, b? : Int = 1) -> Unit {
  let _ : Int?? = a // rarely intended
  let _ : Int = b

}
// How to fix: declare `(a? : Int, b? : Int = 1)` directly.

///|
fn f_correct(a? : Int, b? : Int = 1) -> Unit {
  let _ : Int? = a
  let _ : Int = b

}

///|
test {
  f_misuse(b=3)
  f_misuse(a=Some(5), b=2) // works but confusing
  f_correct(b=2)
  f_correct(a=5)
}

///|
/// Do not use struct to group options.
struct APIOptions {
  width : Int?
  height : Int?
}

///|
fn not_idiomatic(opts : APIOptions, arg : Int) -> Unit {

}

///|
test {
  // Hard to use in call site
  not_idiomatic({ width: Some(5), height: None }, 10)
  not_idiomatic({ width: None, height: None }, 10)
}