Package Configuration

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

New format (moon.pkg)

The new format is a concise DSL. You can generate or reformat it from an existing moon.pkg.json with:

moon fmt -C <module_dir>

Example:

import {
  "moonbit-community/language/packages/virtual",
}

options(
  "is-main": true,
  overrides: [ "moonbit-community/language/packages/implement" ],
)

In moon.pkg, dependencies are declared in an import { ... } block. Use @alias to set a custom alias:

import {
  "moonbit-community/language/packages/pkgA",
  "moonbit-community/language/packages/pkgC" @c,
  "moonbitlang/core/builtin",
}

All other fields from moon.pkg.json move into a single options(...) block. The key names and value shapes are unchanged; the legacy keys that contain - must be quoted.

options(
  "virtual": { "has-default": true },
)

The moon.pkg format allows comments //....

Full syntax of moon.pkg is as follows:

moon_pkg ::= statement*
statement ::= import | assign | apply

import ::= "import" "{" (import_item ",")* import_item? "}" import_kind?
import_item ::= STRING ("@" PKG_NAME)?
import_kind ::= "for" STRING

assign ::= LIDENT "=" expr

apply ::= LIDENT "(" (argument ",")* argument? ")"
argument ::= LIDENT ":" expr | STRING ":" expr  

expr ::= array | object | apply | STRING | INT | "true" | "false"
array ::= "[" (expr ",")* expr? "]"
object ::= "{" (field ",")* field? "}"

Name

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

Formatter

The formatter field configures moon fmt for this package. Currently it supports ignore, a list of file names that the formatter should skip.

This is useful for generated files or files that you intentionally keep in a different format. Files produced by pre-build are already skipped automatically, so formatter.ignore is mainly for additional files you want to exclude.

moon.pkg

options(
  formatter: {
    ignore: [ "generated.mbt", "snapshot.mbt" ],
  },
)

moon.pkg.json

{
  "formatter": {
    "ignore": ["generated.mbt", "snapshot.mbt"]
  }
}

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.

moon.pkg

options(
  "is-main": true,
)

moon.pkg.json

{
  "is-main": true
}

Importing dependencies

Import

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

For example, the following imports pkgA and pkgC, aliasing pkgC to c. User can write @c to access definitions from pkgC.

moon.pkg

import {
  "moonbit-community/language/packages/pkgA",
  "moonbit-community/language/packages/pkgC" @c,
  "moonbitlang/core/builtin",
}

moon.pkg.json

{
    "import": [
        "moonbit-community/language/packages/pkgA",
        {
            "path": "moonbit-community/language/packages/pkgC",
            "alias": "c"
        },
        "moonbitlang/core/builtin"
    ]
}

Most core packages are not special here: if you use @json, @test, or other ordinary core aliases, add the corresponding moonbitlang/core/... package to import to avoid core_package_not_imported warnings.

prelude is the exception. It is available by default, so the names it exposes do not need an explicit package import.

Test import

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

moon.pkg

import {
  "path/to/package1",
  "path/to/package2" @pkg2,
} for "test"

moon.pkg.json

{
  "test-import": {
    "path/to/package1",
    {
      "path": "path/to/package2",
      "alias": "pkg2"
    }
  }
}

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

White-box test import

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

moon.pkg

import {
  "path/to/package1",
  "path/to/package2" @pkg2,
} for "wbtest"

moon.pkg.json

{
  "wbtest-import": {
    "path/to/package1",
    {
      "path": "path/to/package2",
      "alias": "pkg2"
    }
  }
}

Maximum Concurrent Tests

The max-concurrent-tests field limits how many tests from this package may run at the same time when moon test executes the package.

This is useful when tests in the same package share ports, temporary files, or other external resources that should not all run in parallel.

moon.pkg

options(
  "max-concurrent-tests": 2,
)

moon.pkg.json

{
  "max-concurrent-tests": 2
}

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:

moon.pkg

options(
  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"]]
  }
)

moon.pkg.json

{
  "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"]]
  }
}

Supported Targets

The supported-targets option declares which backends a package is intended to support. It uses a target-set expression, not an array:

moon.pkg

options(
  "supported-targets": "js",
)

moon.pkg.json

{
  "supported-targets": "js"
}

Examples:

• js for a single backend

• +js+wasm-gc for an explicit set of backends

• +all-js for all backends except js

Legacy array syntax is still accepted for compatibility:

{
  "supported-targets": ["js", "native"]
}

This is package metadata, not a conditional compilation rule:

• use supported-targets to declare the package's supported backend set

• use targets to include or exclude individual files for different backends

• use preferred-target in moon.mod.json to choose the default backend for commands such as moon check, moon run, and moon build

When both the module and the package declare supported-targets, the effective backend set is their intersection.

Command behavior follows the selected backend:

• moon check, moon build, moon test, and moon bench keep only packages that support the selected backend

• moon run requires the selected package to support the selected backend

• moon info skips unsupported selected packages with a warning

• moon bundle skips package targets that do not support the selected backend

After root selection, Moon also checks reachable required dependencies. If a required dependency does not support the selected backend, the command fails with a normal user-facing error.

Notes:

• omitting supported-targets means all backends are supported

• --target all expands to wasm, wasm-gc, js, and native, but not llvm

• llvm is still a valid supported-targets value

• legacy array syntax is deprecated, but still accepted for compatibility

A common setup is:

• mark a native-only package with "supported-targets": "native"

• set "preferred-target": "native" in moon.mod.json

• use targets only when some files inside the package differ by backend

Native Stub Files

The native-stub field lists C stub source files that should be compiled with this package for native builds.

This is commonly used together with extern "C" declarations in the FFI documentation, where the stub file provides wrapper functions or adapter code that is easier to write in C than directly in MoonBit.

Paths are relative to the package directory.

moon.pkg

options(
  "native-stub": [ "stub.c", "helpers.c" ],
)

moon.pkg.json

{
  "native-stub": ["stub.c", "helpers.c"]
}

Link Options

By default, moon only links packages where is-main is set to true. If you need to link other packages, you can specify this with the link option.

The link option is used to specify link options, and its value can be either a boolean or an object.

Currently, link does not work for the native backend. The behavior described in this section applies to the wasm, wasm-gc, and js backends.

• When the link value is true, it indicates that the package should be linked. The output will vary depending on the backend specified during the build.

  moon.pkg

  options(
    link: true
  )
  

  moon.pkg.json

  {
    "link": true
  }
  

• When the link value is an object, it indicates that the package should be linked, and you can specify link options. For detailed configuration, please refer to the subpage for the corresponding backend.

Wasm Backend Link Options

Common Options

• The exports option is used to specify the function names exported by the Wasm backend.

  For example, in the following configuration, the hello function from the current package is exported as the hello function in the Wasm module, and the foo function is exported as the bar function in the Wasm module. In the Wasm host, the hello and bar functions can be called to invoke the hello and foo functions from the current package.

  moon.pkg

  options(
    link: {
      "wasm": {
        "exports": [ "hello", "foo:bar" ],
      },
      "wasm-gc": {
        "exports": [ "hello", "foo:bar" ],
      }
    }
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm": {
        "exports": [
          "hello",
          "foo:bar"
        ]
      },
      "wasm-gc": {
        "exports": [
          "hello",
          "foo:bar"
        ]
      }
    }
  }
  

• The import-memory option is used to specify the linear memory imported by the Wasm module.

  For example, the following configuration specifies that the linear memory imported by the Wasm module is the memory variable from the env module.

  moon.pkg

  options(
    link: {
      "wasm": {
        "import-memory": {
          "module": "env",
          "name": "memory",
        },
      },
      "wasm-gc": {
        "import-memory": {
          "module": "env",
          "name": "memory",
        },
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm": {
        "import-memory": {
          "module": "env",
          "name": "memory"
        }
      },
      "wasm-gc": {
        "import-memory": {
          "module": "env",
          "name": "memory"
        }
      }
    }
  }
  

• The memory-limits option is used to specify the minimum and maximum size of the linear memory used by the Wasm module.

• The shared-memory option is used to enable shared linear memory.

  For example, the following configuration sets memory limits and enables shared memory for both the wasm and wasm-gc backends.

  moon.pkg

  options(
    link: {
      "wasm": {
        "memory-limits": {
          "min": 1,
          "max": 65536,
        },
        "shared-memory": true,
      },
      "wasm-gc": {
        "memory-limits": {
          "min": 1,
          "max": 65535,
        },
        "shared-memory": true,
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm": {
        "memory-limits": {
          "min": 1,
          "max": 65536
        },
        "shared-memory": true
      },
      "wasm-gc": {
        "memory-limits": {
          "min": 1,
          "max": 65535
        },
        "shared-memory": true
      }
    }
  }
  

• The export-memory-name option is used to specify the name of the linear memory exported by the Wasm module.

  moon.pkg

  options(
    link: {
      "wasm": {
        "export-memory-name": "memory",
      },
      "wasm-gc": {
        "export-memory-name": "memory",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm": {
        "export-memory-name": "memory"
      },
      "wasm-gc": {
        "export-memory-name": "memory"
      }
    }
  }
  

Wasm Linear Backend Link Options

• The heap-start-address option is used to specify the starting address of the linear memory that can be used when compiling to the Wasm backend.

  For example, the following configuration sets the starting address of the linear memory to 1024.

  moon.pkg

  options(
    link: {
      "wasm": {
        "heap-start-address": 1024,
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm": {
        "heap-start-address": 1024
      }
    }
  }
  

Wasm GC Backend Link Options

• The use-js-builtin-string option is used to specify whether the JS String Builtin Proposal should be enabled when compiling to the Wasm GC backend. It will make the String in MoonBit equivalent to the String in JavaScript host runtime.

  For example, the following configuration enables the JS String Builtin.

  moon.pkg

  options(
    link: {
      "wasm-gc": {
        "use-js-builtin-string": true,
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm-gc": {
        "use-js-builtin-string": true
      }
    }
  }
  

• The imported-string-constants option is used to specify the imported string namespace used by the JS String Builtin Proposal, which is "_" by default. It should meet the configuration in the JS host runtime.

  For example, the following configuration and JS initialization configures the imported string namespace.

  moon.pkg

  options(
    link: {
      "wasm-gc": {
        "use-js-builtin-string": true,
        "imported-string-constants": "_",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "wasm-gc": {
        "use-js-builtin-string": true,
        "imported-string-constants": "_"
      }
    }
  }
  

  const { instance } = await WebAssembly.instantiate(bytes, {}, { importedStringConstants: "strings" });
  

JS Backend Link Options

• The exports option is used to specify the function names to export in the JavaScript module.

  For example, in the following configuration, the hello function from the current package is exported as the hello function in the JavaScript module. In the JavaScript host, the hello function can be called to invoke the hello function from the current package.

  moon.pkg

  options(
    link: {
      "js": {
        "exports": [ "hello" ],
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "js": {
        "exports": [
          "hello"
        ]
      }
    }
  }
  

• The format option is used to specify the output format of the JavaScript module.

  The currently supported formats are:

  • esm (default)

  • cjs

  • iife

  For example, the following configuration sets the output format of the current package to ES Module.

  moon.pkg

  options(
    link: {
      "js": {
        "format": "esm",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "js": {
        "format": "esm"
      }
    }
  }
  

Native Backend Link Options

• The cc option is used to specify the compiler for compiling the moonc-generated C source files. It can be either a full path to the compiler or a simple name that is accessible via the PATH environment variable.

  moon.pkg

  options(
    link: {
      "native": {
        "cc": "/usr/bin/gcc13",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "native": {
        "cc": "/usr/bin/gcc13"
      }
    }
  }
  

• The cc-flags option is used to override the default flags passed to the compiler. For example, you can use the following flag to define a macro called MOONBIT.

  moon.pkg

  options(
    link: {
      "native": {
        "cc-flags": "-DMOONBIT",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "native": {
        "cc-flags": "-DMOONBIT"
      }
    }
  }
  

• The cc-link-flags option is used to override the default flags passed to the linker. Since the linker is invoked through the compiler driver (e.g., cc instead of ld, cl instead of link), you should prefix specific options with -Wl, or /link when passing them.

  The following example strips symbol information from produced binary.

  moon.pkg

  options(
    link: {
      "native": {
        "cc-link-flags": "-s",
      },
    },
  )
  

  moon.pkg.json

  {
    "link": {
      "native": {
        "cc-link-flags": "-s"
      }
    }
  }
  

• The stub-cc option is similar to cc but controls which compiler to use for compiling stubs. Although it can be different from cc, it is not recommended and should only be used for debugging purposes. Therefore, we strongly recommend to specify cc and stub-cc at the same time and make them consistent to avoid potential conflicts.

• The stub-cc-flags is similar to cc-flags. It only have effects on stubs compilation.

• The stub-cc-link-flags are similar but have a subtle difference. Normally, stubs are compiled into object files and linked against object files generated from moonc-generated C source files. This linking is only controlled by cc-flags and cc-link-flags, as mentioned earlier. However, in specific modes, such as when the fast-debugging-test feature is enabled, there will be a separate linking procedure for stub objects files, where stub-cc-link-flags will take effect.

Default C compiler and compiler flags for the native backend

Here is a brief summarization to compiler_flags.rs

C Compiler

Search in PATH for the following items from top to bottom.

• cl

• gcc

• clang

• cc

• the internal tcc

For GCC-like compilers, the default compile & link command is as follows. [] is used to indicate the flags may not exist in some modes.

cc -o $target -I$MOON_HOME/include -L$MOON_HOME/lib [-g] [-shared -fPIC] \
   -fwrapv -fno-strict-aliasing (-O2|-Og) [$MOON_HOME/lib/libmoonbitrun.o] \
   $sources -lm $cc_flags $cc_link_flags

For MSVC, the default compile & link command is as follows.

cl (/Fo|/Fe)$target -I$MOON_HOME/include [/LD] /utf-8 /wd4819 /nologo (/O2|/Od) \
   /link /LIBPATH:$MOON_HOME/lib

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.

moon.pkg

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

moon.pkg.json

{
  "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
  #|

Warnings List

Used to disable warnings, enable warnings, or treat a warning as a fatal error. The warning list is a string composed of multiple warning name, each prefixed with a sign:

• - to disable the warning

• + to enable the warning

• @ to treat the enabled warning as a fatal error

For example, in the following configuration, -unused_value disables the unused functions and variables warning.

moon.pkg

warnings = "-unused_value"

moon.pkg.json

{
  "warn-list": "-unused_value"
}

If multiple warnings need to be disabled, they can be directly connected and combined.

moon.pkg

warnings = "-unused_value-unreachable_code"

moon.pkg.json

{
  "warn-list": "-unused_value-unreachable_code"
}

If it is necessary to activate certain warnings that were originally prohibited, use the plus sign.

moon.pkg

warnings = "+unused_optional_argument"

moon.pkg.json

{
  "warn-list": "+unused_optional_argument"
}

To treat a warning as a fatal error, use the @.

moon.pkg

warnings = "@deprecated"

moon.pkg.json

{
  "warn-list": "@deprecated"
}

You can also use warning numbers in warn-list. In the output below, mnemonic is the symbolic warning name used in warning lists, while id is the numeric form of the same warning.

The current list from moonc check -warn-help is:

Available warnings:
mnemonic                   description                                                     id state
unused_value               Unused variable or function.                                     1 warn
unused_value               Unused variable.                                                 2 warn
unused_type_declaration    Unused type declaration.                                         3 warn
missing_priv               Unused abstract type.                                            4 warn
unused_type_variable       Unused type variable.                                            5 warn
unused_constructor         Unused constructor.                                              6 warn
unused_field               Unused field or constructor argument.                            7 warn
redundant_modifier         Redundant modifier.                                              8 warn
struct_never_constructed   Struct never constructed.                                        9 warn
unused_pattern             Unused pattern.                                                 10 warn
partial_match              Partial pattern matching.                                       11 error
unreachable_code           Unreachable code.                                               12 warn
unresolved_type_variable   Unresolved type variable.                                       13 warn
alert or alert_<category>  All alerts or alerts with specific category.                    14 warn
unused_mut                 Unused mutability.                                              15 error
parser_inconsistency       Parser inconsistency check.                                     16 warn
ambiguous_loop_argument    Ambiguous usage of loop argument.                               17 warn
useless_loop               Useless loop expression.                                        18 warn
deprecated                 Deprecated API usage.                                           20 warn
missing_pattern_arguments  Some arguments of constructor are omitted in pattern.           21 warn
ambiguous_block            Ambiguous block.                                                22 warn
unused_try                 Useless try expression.                                         23 warn
unused_error_type          Useless error type.                                             24 warn
test_unqualified_package   Using implicitly imported API in test.                          25 off
unused_catch_all           Useless catch all.                                              26 warn
deprecated_syntax          Deprecated syntax.                                              27 warn
todo                       Todo                                                            28 warn
unused_package             Unused package.                                                 29 warn
missing_package_alias      Empty package alias.                                            30 warn
unused_optional_argument   Optional argument never supplied.                               31 off
unused_default_value       Default value of optional argument never used.                  32 off
text_segment_excceed       Text segment exceed the line or column limits.                  33 warn
implicit_use_builtin       Implicit use of definitions from `moonbitlang/core/builtin`.    34 warn
reserved_keyword           Reserved keyword.                                               35 warn
loop_label_shadowing       Loop label shadows another label.                               36 warn
unused_loop_label          Unused loop label.                                              37 warn
missing_invariant          For-loop is missing an invariant.                               38 off
missing_reasoning          For-loop is missing a proof_reasoning.                          39 off
multiline_string_escape    Deprecated escape sequence in multiline string.                 40 error
missing_rest_mark          Missing `..` in map pattern.                                    41 warn
invalid_attribute          Invalid attribute.                                              42 warn
unused_attribute           Unused attribute.                                               43 warn
invalid_inline_wasm        Invalid inline-wasm.                                            44 error
unused_rest_mark           Useless `..` in pattern                                         46 warn
invalid_mbti               Invalid mbti file                                               47 warn
missing_definition         Unused pub definition because it does not exist in mbti file.   49 warn
method_shadowing           Local method shadows upstream method                            50 warn
ambiguous_precedence       Ambiguous operator precedence                                   51 warn
unused_loop_variable       Loop variable not updated in loop                               52 warn
unused_trait_bound         Unused trait bound                                              53 warn
ambiguous_range_direction  Ambiguous looping direction for range e1..=e2                   54 off
unannotated_ffi            Unannotated FFI param type                                      55 error
missing_pattern_field      Missing field in struct pattern                                 56 warn
missing_pattern_payload    Constructor pattern expect payload                              57 warn
unaligned_byte_access      Unaligned byte access in bits pattern                           59 warn
unused_struct_update       Unused struct update                                            60 warn
duplicate_test             Duplicate test name                                             61 warn
invalid_cascade            Calling method with non-unit return type via `..`               62 warn
syntax_lint                Syntax lint warning                                             63 warn
unannotated_toplevel_array Unannotated toplevel array                                      64 warn
prefer_readonly_array      Suggest ReadOnlyArray for read-only array literal               65 off
prefer_fixed_array         Suggest FixedArray for mutated array literal                    66 off
unused_async               Useless `async` annotation                                      67 warn
declaration_unimplemented  Declaration is unimplemented                                    68 warn
declaration_implemented    Declaration is already implemented                              69 off
deprecated_for_in_method   using `iterator()` method for `for .. in` loop.                 70 off
core_package_not_imported  Packages in `moonbitlang/core` need to be explicitly imported.  71 warn
unqualified_local_using    unqualified local using                                         72 off
unnecessary_annotation     unnecessary type annotation                                     73 off
missing_doc                Missing documentation for public definition                     74 off
A                          all warnings
state: warn = enabled, error = promoted to error, off = disabled
note: default alert exceptions: alert_unsafe=off

Note

Use moonc check -warn-help to see the list of preset compiler warnings.

Alert Warning

Alerts are special warnings that indicate the usage of API marked with #internal attribute.

All alerts has a category associated with it, which is customized by the author of the API. You can enable or disable specific alert categories using the alert_<category> warning name, or use alert to control all alert warnings at once.

For example, in the following configuration, all warnings for alerts are treated as fatal errors, except for the unsafe category, which is disabled.

moon.pkg

warnings = "@alert-alert_unsafe" 

moon.pkg.json

{
  "warn-list": "@alert-alert_unsafe" 
}

Virtual Package

A virtual package serves as an interface of a package that can be replaced by actual implementations.

Declarations

The virtual field is used to declare the current package as a virtual package.

For example, the following declares a virtual package with default implementation:

moon.pkg

options(
  virtual: {
    "has-default": true,
  },
)

moon.pkg.json

{
  "virtual": {
    "has-default": true
  }
}

Implementations

The implement field is used to declare the virtual package to be implemented by the current package.

For example, the following implements a virtual package:

moon.pkg

options(
  implement: "moonbitlang/core/abort",
)

moon.pkg.json

{
  "implement": "moonbitlang/core/abort"
}

Overriding implementations

The overrides field is used to provide the implementations that fulfills an imported virtual package.

For example, the following overrides the default implementation of the builtin abort package with another package:

moon.pkg

options(
  overrides: [ "moonbitlang/dummy_abort/abort_show_msg" ],
)

moon.pkg.json

{
  "overrides": ["moonbitlang/dummy_abort/abort_show_msg"]
}