Package Configuration

Package Configuration#

moon uses the moon.pkg.json file to identify and describe a package. For full JSON schema, please check moon's repository.

Name#

The package name is not configurable; it is determined by the directory name of the package.

is-main#

The is-main field is used to specify whether a package needs to be linked into an executable file.

The output of the linking process depends on the backend. When this field is set to true:

  • For the Wasm and wasm-gc backends, a standalone WebAssembly module will be generated.

  • For the js backend, a standalone JavaScript file will be generated.

Importing dependencies#

import#

The import field is used to specify other packages that a package depends on.

For example, the following imports moonbitlang/quickcheck and moonbitlang/x/encoding, aliasing the latter to lib and importing the function encode from the latter. User can write @lib.encode instead of encode.

{
  "import": [
    "moonbitlang/quickcheck",
    { "path" : "moonbitlang/x/encoding", "alias": "lib", "value": ["encode"] }
  ]
}

test-import#

The test-import field is used to specify other packages that the black-box test package of this package depends on, with the same format as import.

The test-import-all field is used to specify whether all public definitions from the package being tested should be imported (true) by default.

wbtest-import#

The wbtest-import field is used to specify other packages that the white-box test package of this package depends on, with the same format as import.

Conditional Compilation#

The smallest unit of conditional compilation is a file.

In a conditional compilation expression, three logical operators are supported: and, or, and not, where the or operator can be omitted.

For example, ["or", "wasm", "wasm-gc"] can be simplified to ["wasm", "wasm-gc"].

Conditions in the expression can be categorized into backends and optimization levels:

  • Backend conditions: "wasm", "wasm-gc", and "js"

  • Optimization level conditions: "debug" and "release"

Conditional expressions support nesting.

If a file is not listed in "targets", it will be compiled under all conditions by default.

Example:

{
  "targets": {
    "only_js.mbt": ["js"],
    "only_wasm.mbt": ["wasm"],
    "only_wasm_gc.mbt": ["wasm-gc"],
    "all_wasm.mbt": ["wasm", "wasm-gc"],
    "not_js.mbt": ["not", "js"],
    "only_debug.mbt": ["debug"],
    "js_and_release.mbt": ["and", ["js"], ["release"]],
    "js_only_test.mbt": ["js"],
    "js_or_wasm.mbt": ["js", "wasm"],
    "wasm_release_or_js_debug.mbt": ["or", ["and", "wasm", "release"], ["and", "js", "debug"]]
  }
}

Pre-build#

The "pre-build" field is used to specify pre-build commands, which will be executed before build commands such as moon check|build|test.

"pre-build" is an array, where each element is an object containing input, output, and command fields. The input and output fields can be strings or arrays of strings, while the command field is a string. In the command, you can use any shell commands, as well as the $input and $output variables, which represent the input and output files, respectively. If these fields are arrays, they will be joined with spaces by default.

Currently, there is a built-in special command :embed, which converts files into MoonBit source code. The --text parameter is used to embed text files, and --binary is used for binary files. --text is the default and can be omitted. The --name parameter is used to specify the generated variable name, with resource being the default. The command is executed in the directory where the moon.pkg.json file is located.

{
  "pre-build": [
    {
      "input": "a.txt",
      "output": "a.mbt",
      "command": ":embed -i $input -o $output"
    }
  ]
}

If the content of a.txt in the current package directory is:

hello,
world

After running moon build, the following a.mbt file will be generated in the directory where the moon.pkg.json is located:

let resource : String =
  #|hello,
  #|world
  #|

Warning List#

This is used to disable specific preset compiler warning numbers.

For example, in the following configuration, -2 disables the warning number 2 (Unused variable).

{
  "warn-list": "-2"
}

You can use moonc build-package -warn-help to see the list of preset compiler warning numbers.

$ moonc -v                      
v0.1.20250318+35770a65e

$ moonc build-package -warn-help
Available warnings: 
  1 Unused function.
  2 Unused variable.
  3 Unused type declaration.
  4 Unused abstract type.
  5 Unused type variable.
  6 Unused constructor.
  7 Unused field or constructor argument.
  8 Redundant modifier.
  9 Unused function declaration.
 10 Struct never constructed.
 11 Partial pattern matching.
 12 Unreachable code.
 13 Unresolved type variable.
 14 Lowercase type name.
 15 Unused mutability.
 16 Parser inconsistency.
 18 Useless loop expression.
 19 Top-level declaration is not left aligned.
 20 Invalid pragma
 21 Some arguments of constructor are omitted in pattern.
 22 Ambiguous block.
 23 Useless try expression.
 24 Useless error type.
 26 Useless catch all.
 27 Deprecated syntax.
 28 Todo
 29 Unused package.
 30 Empty package alias.
 31 Optional argument never supplied.
 32 Default value of optional argument never used.
 33 Unused import value
 35 Reserved keyword.
 36 Loop label shadows another label.
 37 Unused loop label.
 38 Useless guard.
 39 Duplicated method.
 40 Call a qualified method using regular call syntax.
 41 Closed map pattern.
 42 Invalid attribute.
 43 Unused attribute.
 44 Invalid inline-wasm.
 45 Type implements trait with regular methods.
  A all warnings

Alert List#

Disable user preset alerts.

{
  "alert-list": "-alert_1-alert_2"
}
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