Moonbit Code Transform

MoonBit Code Transformations

Guide for building source-to-source code generation tools in MoonBit using moonbitlang/parser to parse source into AST and moonbitlang/parser/fmt to emit generated code. Uses golem_sdk_tools as the reference implementation.

Pipeline Overview

Every code transformation follows the same pattern:

Source (.mbt files)
  → Parse with @parser.parse_string()
  → Extract information from @syntax.Impl list
  → Construct new @syntax.Impl nodes (AST)
  → Format with @formatter.impls_to_string()
  → Write output file

Required Dependencies

In moon.mod.json:

{
  "deps": {
    "moonbitlang/parser": "0.3.18",
    "moonbitlang/lexer": "0.3.15"
  }
}

In the library package moon.pkg (where AST construction happens):

import {
  "moonbitlang/core/list",
  "moonbitlang/parser",
  "moonbitlang/parser/syntax",
  "moonbitlang/parser/basic",
}

import {
  "moonbitlang/parser/fmt" @formatter,
} for "test"

In the CLI/main package moon.pkg (where formatting and file I/O happen):

import {
  "moonbitlang/parser/fmt" @formatter,
  "moonbitlang/x/fs",
  "your/module/lib" @lib,
}

Key separation: The library package constructs @list.List[@syntax.Impl]. The CLI package calls @formatter.impls_to_string(impls) and writes the result. Tests in the library import moonbitlang/parser/fmt as @formatter via for "test" to verify output without depending on it at runtime.

Parsing Source Files

Use @parser.parse_string() to parse MoonBit source into a list of top-level AST items:

let (impls, _reports) = @parser.parse_string(content)
// impls : @list.List[@syntax.Impl]
// _reports : Array[...] — diagnostics, usually ignored for codegen

Walking the AST to Extract Information

Pattern-match on @syntax.Impl variants to find declarations:

impls.each(fn(impl_) {
  match impl_ {
    // Function declarations
    TopFuncDef(fun_decl~, ..) => {
      // fun_decl : @syntax.FunDecl
      let name = fun_decl.name.name          // String
      let vis = fun_decl.vis                  // @syntax.Visibility
      let params = fun_decl.decl_params       // Option[@list.List[@syntax.Parameter]]
      let ret = fun_decl.return_type          // Option[@syntax.Type]
      let doc = fun_decl.doc                  // @syntax.DocString
      let type_name = fun_decl.type_name      // Option[@syntax.TypeName] (for methods)
      ...
    }
    // Type definitions (struct, enum)
    TopTypeDef(type_decl) => {
      let name = type_decl.tycon              // String
      let attrs = type_decl.attrs             // @list.List[@syntax.Attribute]
      let doc = type_decl.doc                 // @syntax.DocString
      ...
    }
    _ => ()
  }
})

Checking Attributes (Annotations)

Attributes like #derive.agent are accessed via type_decl.attrs:

let mut is_agent = false
type_decl.attrs.each(fn(attr) {
  if attr.raw == "#derive.agent" {
    is_agent = true
  }
})

Extracting Function Parameters

Parameters come as @list.List[@syntax.Parameter]:

match fun_decl.decl_params {
  Some(ps) =>
    ps.each(fn(p) {
      match p {
        Positional(binder~, ty=Some(ty)) | Labelled(binder~, ty=Some(ty)) => {
          let name = binder.name    // String
          // ty : @syntax.Type — process recursively
        }
        _ => ()
      }
    })
  None => ()
}

Processing Types Recursively

@syntax.Type is an enum with variants like Name, Option, Tuple, etc.:

fn process_type(ty : @syntax.Type) -> MyTypeRepr {
  match ty {
    Option(ty~, ..) => MyOptional(process_type(ty))
    Name(constr_id~, tys~, ..) =>
      match constr_id.id {
        Ident(name~) =>
          if name == "Array" {
            match tys {
              More(inner, ..) => MyList(process_type(inner))
              Empty => ... // error
            }
          } else if tys.is_empty() {
            MySimple(name)
          } else { ... }
        Dot(pkg~, id~) => MyQualified(pkg, id)
      }
    _ => ... // unsupported
  }
}

Constructing AST Nodes

All AST nodes require a loc field. For generated code, use dummy locations:

let dummy_pos : @basic.Position = { fname: "", lnum: 0, bol: 0, cnum: 0 }
let dummy_loc : @basic.Location = { start: dummy_pos, end: dummy_pos }

The @list.List Pattern

The AST uses @list.List[T] (immutable linked lists), not Array[T]. Build with Array first, then convert:

fn[T] to_list(arr : Array[T]) -> @list.List[T] {
  @list.List::from_array(arr)
}

DSL Helper Functions

Create a set of small helper functions to make AST construction readable. Organize by category. The reference implementation is in golem_sdk_tools/lib/ast_helpers.mbt.

Primitives

fn make_binder(name : String) -> @syntax.Binder {
  @syntax.Binder::{ name, loc: dummy_loc }
}

fn make_label(name : String) -> @syntax.Label {
  @syntax.Label::{ name, loc: dummy_loc }
}

Type Constructors

// Simple type: String, Int, MyStruct, etc.
fn make_type(name : String) -> @syntax.Type {
  @syntax.Type::Name(
    constr_id=@syntax.ConstrId::{
      id: @syntax.LongIdent::Ident(name~),
      loc: dummy_loc,
    },
    tys=to_list([]),
    loc=dummy_loc,
  )
}

// Generic type: Array[T], Result[T, E], etc.
fn make_parameterized_type(
  name : String,
  type_args : Array[@syntax.Type],
) -> @syntax.Type {
  @syntax.Type::Name(
    constr_id=@syntax.ConstrId::{
      id: @syntax.LongIdent::Ident(name~),
      loc: dummy_loc,
    },
    tys=to_list(type_args),
    loc=dummy_loc,
  )
}

// Option type: T?
fn make_option_type(inner : @syntax.Type) -> @syntax.Type {
  @syntax.Type::Option(ty=inner, loc=dummy_loc, question_loc=dummy_loc)
}

Literal Expressions

fn make_string_expr(s : String) -> @syntax.Expr {
  @syntax.Expr::Constant(c=@syntax.Constant::String(s), loc=dummy_loc)
}

fn make_int_expr(n : Int) -> @syntax.Expr {
  @syntax.Expr::Constant(c=@syntax.Constant::Int(n.to_string()), loc=dummy_loc)
}

fn make_bool_expr(b : Bool) -> @syntax.Expr {
  @syntax.Expr::Constant(c=@syntax.Constant::Bool(b), loc=dummy_loc)
}

Other numeric types use their respective @syntax.Constant variants: UInt("0"), Int64("0"), UInt64("0"), Float("0.0"), Double("0.0"), Byte("\\x00"), Char('a').

Identifier Expressions

// Simple identifier: foo
fn make_ident_expr(name : String) -> @syntax.Expr {
  @syntax.Expr::Ident(
    id=@syntax.Var::{ name: @syntax.LongIdent::Ident(name~), loc: dummy_loc },
    loc=dummy_loc,
  )
}

// Qualified identifier: @pkg.foo
fn make_qualified_expr(pkg : String, id : String) -> @syntax.Expr {
  @syntax.Expr::Ident(
    id=@syntax.Var::{ name: @syntax.LongIdent::Dot(pkg~, id~), loc: dummy_loc },
    loc=dummy_loc,
  )
}

// Method reference: TypeName::method_name
fn make_method_ref(type_name : String, method_name : String) -> @syntax.Expr {
  @syntax.Expr::Method(
    type_name=@syntax.TypeName::{
      name: @syntax.LongIdent::Ident(name=type_name),
      is_object: false,
      loc: dummy_loc,
    },
    method_name=make_label(method_name),
    loc=dummy_loc,
  )
}

Constructor Expressions

// Enum variant without args: None, Ok, Err, etc.
fn make_constr_no_args(name : String) -> @syntax.Expr {
  @syntax.Expr::Constr(
    constr=@syntax.Constructor::{
      name: @syntax.ConstrName::{ name, loc: dummy_loc },
      extra_info: @syntax.ConstructorExtraInfo::NoExtraInfo,
      loc: dummy_loc,
    },
    loc=dummy_loc,
  )
}

// Qualified enum variant: @pkg.Type::Variant
fn make_qualified_constr(
  pkg : String,
  type_name : String,
  variant : String,
) -> @syntax.Expr {
  @syntax.Expr::Constr(
    constr=@syntax.Constructor::{
      name: @syntax.ConstrName::{ name: variant, loc: dummy_loc },
      extra_info: @syntax.ConstructorExtraInfo::TypeName(@syntax.TypeName::{
        name: @syntax.LongIdent::Dot(pkg~, id=type_name),
        is_object: false,
        loc: dummy_loc,
      }),
      loc: dummy_loc,
    },
    loc=dummy_loc,
  )
}

Function Application

fn make_positional_arg(expr : @syntax.Expr) -> @syntax.Argument {
  @syntax.Argument::{ value: expr, kind: @syntax.ArgumentKind::Positional }
}

// f(arg1, arg2, ...)
fn make_apply(func : @syntax.Expr, args : Array[@syntax.Expr]) -> @syntax.Expr {
  @syntax.Expr::Apply(
    func~,
    args=to_list(args.map(make_positional_arg)),
    attr=@syntax.ApplyAttr::NoAttr,
    loc=dummy_loc,
  )
}

// self.method(arg1, arg2, ...)
fn make_dot_apply(
  self_ : @syntax.Expr,
  method_name : String,
  args : Array[@syntax.Expr],
) -> @syntax.Expr {
  @syntax.Expr::DotApply(
    self=self_,
    method_name=make_label(method_name),
    args=to_list(args.map(make_positional_arg)),
    return_self=false,
    attr=@syntax.ApplyAttr::NoAttr,
    loc=dummy_loc,
  )
}

// lhs op rhs (e.g., a + b, x == y)
fn make_infix(
  op : String,
  lhs : @syntax.Expr,
  rhs : @syntax.Expr,
) -> @syntax.Expr {
  @syntax.Expr::Infix(
    op=@syntax.Var::{ name: @syntax.LongIdent::Ident(name=op), loc: dummy_loc },
    lhs~,
    rhs~,
    loc=dummy_loc,
  )
}

Collections and Composites

// Record literal: { field1: expr1, field2: expr2, ... }
fn make_record(fields : Array[@syntax.FieldDef]) -> @syntax.Expr {
  @syntax.Expr::Record(
    type_name=None,
    fields=to_list(fields),
    trailing=@syntax.TrailingMark::Comma,
    loc=dummy_loc,
  )
}

fn make_field(name : String, expr : @syntax.Expr) -> @syntax.FieldDef {
  @syntax.FieldDef::{
    label: make_label(name),
    expr,
    is_pun: false,
    loc: dummy_loc,
  }
}

// Array literal: [elem1, elem2, ...]
fn make_array(elems : Array[@syntax.Expr]) -> @syntax.Expr {
  @syntax.Expr::Array(exprs=to_list(elems), loc=dummy_loc)
}

// Tuple: (a, b, c)
fn make_tuple(elems : Array[@syntax.Expr]) -> @syntax.Expr {
  @syntax.Expr::Tuple(exprs=to_list(elems), loc=dummy_loc)
}

// Array indexing: arr[idx]
fn make_array_get(array : @syntax.Expr, index : @syntax.Expr) -> @syntax.Expr {
  @syntax.Expr::ArrayGet(array~, index~, loc=dummy_loc)
}

// Type constraint: (expr : Type)
fn make_constraint_expr(expr : @syntax.Expr, ty : @syntax.Type) -> @syntax.Expr {
  @syntax.Expr::Constraint(expr~, ty~, loc=dummy_loc)
}

Control Flow

// let name = expr; body
fn make_let(
  name : String,
  expr : @syntax.Expr,
  body : @syntax.Expr,
) -> @syntax.Expr {
  @syntax.Expr::Let(
    pattern=@syntax.Pattern::Var(make_binder(name)),
    expr~,
    body~,
    loc=dummy_loc,
  )
}

// let name : Type = expr; body
fn make_let_typed(
  name : String,
  ty : @syntax.Type,
  expr : @syntax.Expr,
  body : @syntax.Expr,
) -> @syntax.Expr {
  @syntax.Expr::Let(
    pattern=@syntax.Pattern::Constraint(
      pat=@syntax.Pattern::Var(make_binder(name)),
      ty~,
      loc=dummy_loc,
    ),
    expr~,
    body~,
    loc=dummy_loc,
  )
}

// raise expr
fn make_raise(err_value : @syntax.Expr) -> @syntax.Expr {
  @syntax.Expr::Raise(err_value~, loc=dummy_loc)
}

// guard cond else { otherwise }; body
fn make_guard_else(
  cond : @syntax.Expr,
  otherwise : @syntax.Expr,
  body : @syntax.Expr,
) -> @syntax.Expr {
  @syntax.Expr::Guard(cond~, otherwise=Some(otherwise), body~, loc=dummy_loc)
}

// try { body } catch { cases... }
fn make_try_catch(
  body : @syntax.Expr,
  catch_cases : Array[@syntax.Case],
) -> @syntax.Expr {
  @syntax.Expr::Try(
    body~,
    catch_=to_list(catch_cases),
    catch_all=false,
    try_else=None,
    has_try=true,
    try_loc=dummy_loc,
    catch_loc=dummy_loc,
    else_loc=dummy_loc,
    loc=dummy_loc,
  )
}

fn make_case(pattern : @syntax.Pattern, body : @syntax.Expr) -> @syntax.Case {
  @syntax.Case::{ pattern, body, guard_: None }
}

// fn(p1, p2) { body }
fn make_lambda(
  param_names : Array[String],
  body : @syntax.Expr,
) -> @syntax.Expr {
  let parameters = param_names.map(fn(name) {
    @syntax.Parameter::Positional(binder=make_binder(name), ty=None)
  })
  @syntax.Expr::Function(
    func=@syntax.Func::{
      body,
      error_type: @syntax.ErrorType::NoErrorType,
      has_error: None,
      is_async: None,
      kind: @syntax.FnKind::Lambda,
      loc: dummy_loc,
      parameters: to_list(parameters),
      params_loc: dummy_loc,
      return_type: None,
    },
    loc=dummy_loc,
  )
}

Sequences and Blocks

// Handles both single expression and multi-expression sequences
fn make_sequence(exprs : Array[@syntax.Expr]) -> @syntax.Expr {
  if exprs.length() == 1 {
    exprs[0]
  } else {
    let n = exprs.length()
    let init : Array[@syntax.Expr] = []
    for i in 0..<(n - 1) {
      init.push(exprs[i])
    }
    @syntax.Expr::Sequence(
      exprs=to_list(init),
      last_expr=exprs[n - 1],
      loc=dummy_loc,
    )
  }
}

Important: @syntax.Expr::Sequence separates the last expression from the rest. For a single expression, just return it directly — no Sequence wrapper.

Top-Level Nodes

// fn init { body }
fn make_init_block(body : @syntax.Expr) -> @syntax.Impl {
  @syntax.Impl::TopExpr(
    expr=body,
    is_main=false,
    local_types=to_list([]),
    is_async=None,
    loc=dummy_loc,
  )
}

// Typed parameter for fn declarations
fn make_param(name : String, type_name : String) -> @syntax.Parameter {
  @syntax.Parameter::Positional(
    binder=make_binder(name),
    ty=Some(make_type(type_name)),
  )
}

// pub fn name(params...) -> ReturnType { body }
fn make_top_fn(
  name : String,
  params : Array[@syntax.Parameter],
  return_type : @syntax.Type,
  body : @syntax.Expr,
) -> @syntax.Impl {
  @syntax.Impl::TopFuncDef(
    fun_decl=@syntax.FunDecl::{
      type_name: None,
      name: make_binder(name),
      has_error: None,
      is_async: None,
      decl_params: Some(to_list(params)),
      params_loc: dummy_loc,
      quantifiers: to_list([]),
      return_type: Some(return_type),
      error_type: @syntax.ErrorType::NoErrorType,
      vis: @syntax.Visibility::Pub(attr=None, loc=dummy_loc),
      attrs: to_list([]),
      doc: @syntax.DocString::empty(),
    },
    decl_body=@syntax.DeclBody::DeclBody(
      local_types=to_list([]),
      expr=body,
    ),
    loc=dummy_loc,
  )
}

Formatting Output

The final step converts AST nodes back to source code:

let impls : @list.List[@syntax.Impl] = ... // constructed AST
let formatted : String = @formatter.impls_to_string(impls)
let output = "// Generated by my-tool — DO NOT EDIT!\n\n" + formatted

@formatter.impls_to_string() produces properly formatted MoonBit source with ///| block separators.

Real-World Examples

Example 1: Generating Function Re-exports

Goal: Parse .mbt source files, find pub fn declarations, generate wrapper functions that delegate to the original package.

Input: Source file with pub fn wasmExportSave() -> Int { 42 }

Output:

///|
pub fn wasmExportSave() -> Int {
  @gen.wasmExportSave()
}

Implementation (from golem_sdk_tools/lib/reexports.mbt):

pub fn generate_reexports(
  fns : Array[FnSignature],
  gen_pkg : String,
) -> @list.List[@syntax.Impl] {
  let impls : Array[@syntax.Impl] = []
  for fn_ in fns {
    let params : Array[@syntax.Parameter] = []
    let args : Array[@syntax.Argument] = []
    for j, param_type in fn_.params {
      let name = "p\{j}"
      params.push(make_param(name, param_type))
      args.push(make_positional_arg(make_ident_expr(name)))
    }
    let call_expr = make_apply(
      make_qualified_expr(gen_pkg, fn_.gen_name),
      args.map(fn(a) { a.value }),
    )
    let fun_decl : @syntax.FunDecl = { ... }  // see source for full details
    impls.push(@syntax.Impl::TopFuncDef(fun_decl~, ...))
  }
  to_list(impls)
}

Example 2: Generating Agent Registration Code

Goal: Find structs annotated with #derive.agent, extract their ::new constructor signatures, and generate an fn init { ... } block with register_agent(...) calls.

Input: User-written agent code:

///| A counter agent
#derive.agent
pub(all) struct Counter {
  name : String
  mut value : UInt64
}

///| Creates a new counter
pub fn Counter::new(name : String) -> Counter {
  { name, value: 0 }
}

Output (generated golem_agents.mbt):

fn init {
  @agents.register_agent({
    name: "Counter",
    agent_type: { type_name: "Counter", description: "A counter agent", ... },
    construct: fn(input) {
      try {
        let elements = @extractor.extract_tuple(input)
        guard elements.length() == 1 else {
          raise @common.AgentError::InvalidInput(
            "Expected 1 elements, got " + elements.length().to_string(),
          )
        }
        let name : String = @schema.from_element_value_as(elements[0])
        Ok(Counter::new(name))
      } catch {
        e => Err(e)
      }
    },
  })
}

Implementation pattern (from golem_sdk_tools/lib/agents_emit.mbt):

The construct function body is built inside-out, starting with the innermost expression (Ok(Counter::new(args...))) and wrapping outward:

fn build_construct_fn(agent : AgentInfo) -> @syntax.Expr {
  // 1. Build innermost: Ok(AgentName::new(p1, p2, ...))
  let ok_call = make_apply(make_constr_no_args("Ok"), [
    make_apply(make_method_ref(agent.name, "new"), new_args),
  ])

  // 2. Wrap with let bindings (from last parameter to first)
  let mut body = ok_call
  for idx in 0..<param_count {
    let i = param_count - 1 - idx
    body = make_let_typed(name, ty, deserialize_expr, body)
  }

  // 3. Wrap with guard for parameter count check
  body = make_guard_else(length_check, raise_error, body)

  // 4. Wrap with let elements = extract_tuple(input)
  let try_body = make_let("elements", extract_call, body)

  // 5. Wrap in fn(input) { try { ... } catch { e => Err(e) } }
  make_lambda(["input"], make_try_catch(try_body, [catch_case]))
}

Key insight: Build AST expressions inside-out. make_let(name, expr, body) nests — the body of one let contains the next let. Start with the innermost expression and wrap outward.

Testing Code Transformations

Use snapshot tests with @formatter.impls_to_string() to verify output:

test "generate_reexports produces correct output" {
  let fns : Array[FnSignature] = [
    { gen_name: "wasmExportSave", user_name: "wasmExportSave",
      params: [], return_type: "Int" },
  ]
  let impls = generate_reexports(fns, "gen")
  let output = @formatter.impls_to_string(impls)
  inspect(
    output,
    content=(
      #|///|
      #|pub fn wasmExportSave() -> Int {
      #|  @gen.wasmExportSave()
      #|}
      #|
      #|
    ),
  )
}

Run moon test --update to auto-populate the content= parameter, then review the snapshot to confirm correctness.

For the parsing side, test extraction separately:

test "parse_agents finds simple agent struct" {
  let content =
    #|///| A counter agent
    #|#derive.agent
    #|pub(all) struct Counter {
    #|  name : String
    #|  mut value : UInt64
    #|}
    #|
    #|///| Creates a new counter
    #|pub fn Counter::new(name : String) -> Counter {
    #|  { name, value: 0 }
    #|}
  let agents = parse_agents([("counter.mbt", content)])
  inspect(agents[0].name, content="Counter")
  inspect(agents[0].constructor_params[0].1, content="Simple(\"String\")")
}

Key AST Types Reference

@syntax.Impl — Top-Level Items

@syntax.Expr — Expressions (Most Common Variants)

@syntax.Constant — Literal Variants

@syntax.LongIdent — Identifiers

@syntax.FunDecl — Function Declaration Fields

Common Patterns

Inside-Out Expression Building

When generating nested let bindings, build from the innermost expression outward. Each make_let wraps the previous body:

let mut body = final_expr
for i = params.length() - 1; i >= 0; i = i - 1 {
  body = make_let(params[i].name, params[i].init, body)
}
// body is now: let p0 = ...; let p1 = ...; final_expr

Multiple Statements in a Block

Use make_sequence to combine multiple expressions into a block body. For fn init { stmt1; stmt2; ... }:

let calls = agents.map(fn(a) { build_register_call(a) })
make_init_block(make_sequence(calls))

Generated File Convention

Always prefix generated files with a comment and write to a well-known filename:

let output = "// Generated by my-tool — DO NOT EDIT!\n\n" + formatted
@fs.write_string_to_file("\{target_dir}/my_generated.mbt", output)

CLI Entry Point Pattern

fn main {
  let args = @env.args()
  if args.length() < 2 { println("Usage: ..."); return }
  if args[1] == "my-command" {
    run_my_command(args[2]) catch { e => println("Error: \{e}") }
  }
}

fn run_my_command(dir : String) -> Unit raise {
  // 1. Read source files
  let entries = @fs.read_dir(dir)
  let files : Array[(String, String)] = []
  for entry in entries {
    if entry.has_suffix(".mbt") && entry != "my_generated.mbt" {
      files.push((entry, @fs.read_file_to_string("\{dir}/\{entry}")))
    }
  }
  // 2. Parse & extract
  let info = @lib.parse_things(files)
  // 3. Generate AST
  let impls = @lib.generate_things(info)
  // 4. Format & write
  let formatted = @formatter.impls_to_string(impls)
  let output = "// Generated by my-tool — DO NOT EDIT!\n\n" + formatted
  @fs.write_string_to_file("\{dir}/my_generated.mbt", output)
}