pulumi/docs/mupack.md

Mu Package Metadata (MuPack)

This document describes the overall concepts, capabilities, and serialization format for MuPack. It also describes the intermediate language (IL) and type system for MuPack, something we refer to as MuIL.

Overview

Each MuPack file is called a MuPackage and contains four things:

• Package metadata.
• Symbol names and tokens.
• Module, type, function, and variable definitions.
• Data and computations encoded in an intermediate language (IL).

The metadata section describes attributes about the overall MuPackage, like its name and version.

All data and computation AST nodes are fully bound, and ready for interpretation/execution. Higher level MetaMu language compilers are responsible for performing this binding, and encoding the results. Those results are symbol names and tokens that are registered and available for lookup within any given MuPackage. These symbols provide a quick, and binding logic-free, way of resolving any bound node to its target abstraction (module, type, or function). From there, any data or computations associated with those abstractions may be retrieved thanks to the definitions.

MuPack is serialized in JSON/YAML form, although in the future we may explore more efficient file formats. Examples in this document will use a YAML syntax for brevity's sake.

Metadata

Each package may contain self-describing metadata, such as a name, and optional attributes that are common in package managers, like a description, author, website, license, and so on. For example:

name: acmecorp/elk
description: A fully functioning ELK stack (Elasticsearch, Logstash, Kibana).
author: Joe Smith <joesmith@elk.com>
website: https://github.com/joesmith/elk

TODO(joe): describe the full informational attributes available.

Most of this information is taken from the source MetaMu program and carried forward during compilation.

Note that a version number is not part of the metadata prelude. The version number is managed by a version management system outside of the purview of the package contents. For example, packages checked into Git are managed by SHA1 hashes and tags, while packages registered with a traditional package management system might be versioned manually.

Symbols

Each symbol refepresents one of four kinds of abstractions: module, variable, function, and type:

• A module represents a collection of said abstractions. No type, function, or const can exist outside of one. Every MuPackage consists of at least one top-level module, with optional nested modules inside of it.

• A variable represents a named, typed storage location.

• A function represents a named computation with typed parameters and an optional typed return value.

• A type is either a record or class. A record type is pure data and is restricted to a "JSON-like" subset of interoperable data types. A class type, on the other hand, can contain "behavior" by way of functions, although Mu severely restricts this to ensure cross-language interoperability with languages that don't support OOP.

Tokens

Each symbol is keyed by a token, which is a unique identifier used to reference the symbol from elsewhere inside and/or outside of the defining module. Each token is just a string name that encodes the entire context necessary to resolve it to a concrete module and, within that module, a symbol definition:

• A protocol (e.g., https://).
• A base URL (e.g., hub.mu.com/, github.com/, etc).
• A fully qualified name (e.g., acmecorp, aws/s3/Bucket, etc).

For example, https://hub.mu.com/acmecorp#latest refers to the latest version of the acmecorp module, while https://github.com/aws/s3/Bucket refers to the Bucket class exported from the aws/s3 module (itself exported from the aws package). The URLs are present so that package managers can download dependencies appropriately.

Each MuPackage contains a concrete list of module dependencies. For example:

dependencies:
    - https://hub.mu.com/aws#^1.0.6
    - https://hub.mu.com/github#~1.5.2

Now, throughout the rest of the MuPackage, any symbol tokens prefixed with https://hub.mu.com/aws and https://hub.mu.com/github will be resolved to the artifacts exported by the repsective packages. Note that dependencies are required to be acyclic.

Notice that dependency names are like ordinary token names, but must also carry a version number:

• An # followed by version number (e.g., #^1.0.6, #6f99088, #latest, etc).

This version number ensures that the same dependency used for compilation is used during evaluation. Mu supports multiple versioning formats (semantic versioning, Git SHA1 hash versioning, and "tip" (latest)). Please refer to Mu Dependencies for more information about token and dependency names and the resolution process.

MuPackages may export other symbols in the form of modules, types, variables, and functions as members.

Naming Conventions

TODO: talk about casing.

Definitions

Each package contains a definitions map containing all modules, types, variables, and functions:

declarations:

This map is laid out in a hierarchical manner, so that any types belonging to a module are nested underneath it, etc., making the name resolution process straightforward. It contains both internal and exported members.

Modules

Because each package has an implicit top-level module, the declarations: element itself is actually a module specification. Every module specification may contain up to the four kinds of members underneath it listed earlier:

declarations:
    functions:
        # functions, keyed by name
    modules:
        # submodules, keyed by name
    types:
        # types, keyed by name
    variables:
        # variables, keyed by name

Each of these elements may be made accessible outside of the package by attaching the export: true attribute:

export: true

A module may contain definitions that aren't exported simply by leaving off export: true or explicitly marking a definition as export: false. These are for use within the package only. No additional accessibility level is available at the MuPack level of abstraction, although of course MetaMu languages may project things however they wish.

Modules cannot contain statements or expressions outside of functions and types. This is unlike some programming languages that permit "global" code that runs at module load time. The only code that is permitted at this level is variable initializers, which is run in a deterministic order at module load time. Such initializers might depend on other variable initializers, in which case, this dependency tree must provably form a DAG. This ensures determinism.

Types

This section describes MuIL's type system, plus the type definition metadata formats.

MuPack's type system was designed to be supported by a broad cross-section of modern programming languages. That said, it's entirely possible that MuPack exposes a construct that a certain language doesn't support. Because MuIL is designed for interpretation, determinism, and predictability -- and not runtime speed -- all type coercions are checked and fail-fast if an illegal coercion is attempted. It is obviously a better choice to verify such conversions where possible in the MetaMu compilers themselves, however this approach naturally accomodates dynamic languages.

There is a single top-type that may refer to any record or class value: the any type.

All instances of records and classes in MuIL are called objects. They are allocated on the heap, in map-like data structures that have strong type identity, facilitating dynamic and structural conversions, in addition to classical RTTI and OOP patterns. In a sense, this is a lot like how ECMAScript works. Furthermore, there is no notion of a pointer in MuIL and so the exact storage location is kept hidden from MetaMu languages and their semantics.

Because all instances are objects, we must talk about null. By default, types do not include the special value null in their domain. To include it in a type's domain, suffix athat type T with a question mark, as in T?.

Primitives

At the core, all types are built out of the primitives:

• The basic primitive types: bool, number, and string.

• Any record S can be modified by appending [] to make an array type S[]: e.g., number[] and string[].

• Similarly, two types can be paired up to make a map type using map[K]V, where K is the type of keys used to index into the map and V is the type of value inside: e.g., map[string]number and map[string]record, and so on. Note that only the primtive types bool, number, and string can be used as keys for a map type. A map type with a value type V that belongs to the record subset of types is also a record; otherwise, it is a class.

As with JSON, all numbers are IEEE 754 64-bit floating point numbers.

TODO(joe): we likely want ints/longs. Perhaps not in the JSON-like subset, however. Maybe even bignum.

Records

Records are pure data, and instances are representable in JSON, ensuring interoperability with languages and Internet protocols. This is in contrast to objects which may represent types with invariants that make them unappealing to serialize and deserialize. There may be additional constraints placed on records, to enforce contracts, but this does not alter their runtime representation.

The special type record may refer to any record type. This represents the JSON-like subset of types.

It is of course possible to define new record types, comprised solely out of primitives and other record types.

Each custom record type has the following attributes:

• A name (its key).
• An optional base type.
• An optional informative description.
• Either of these:
  • An optional set of properties; or,
  • An optional set of value constraints.

For instance, here is an example of a custom Person record type:

Person:
    description: A record describing a person.
    properties:
        firstName:
            type: string
            description: The person's given name.
        lastName:
            type: string
            description: The person's family name.
        age:
            type: number
            description: The person's current age.

Properties

In the case of properties, each property has the following attributes:

• A name (its key).
• An required type, indicating its primitive or record type.
• An optional default value.
• An optional optional indicator.
• An optional readonly indicator.
• An optional informative description.

By default, each property is mutable. The readonly attribute on a property indicates that it isn't:

Person:
    properties:
        firstName:
            type: string
            readonly: true
        # ...

As with most uses of readonly in other programming languages, it is shallow (that is, the property value cannot be changed by if the target is a mutable record, properties on that record can be).

By default, each property is also required. The optional attribute on a property indicates that it isn't:

Person:
    properties:
        # ...
        age:
            type: number
            optional: true

Any property can be given a default value, in which case it is implicitly optional, for example as follows:

Person:
    properties:
        # ...
        age:
            type: number
            default: 42

TODO(joe): secret keyword for Amazon NoEcho-like cases.

Subtyping

Record types may subtype other record type using the base: element.

For example, imagine we want an Employee which is a special kind of Person:

Employee:
    description: A record describing an employee.
    base: Person
    properties:
        company:
            type: string
            description: The employee's current employer.
        title:
            type: string
            description: The employee's current title.

This facilitates easy conversion from an employee value to a person. Because MuIL leverages a nominal type system, the parent/child relationship between these types is preserved at runtime for purposes of RTTI. This caters to MetaMu languages that use nominal type systems as well as MetaMu languages that use structural ones.

At the moment, there is support for covariance (i.e., strengthening properties). All base-type properties are simply inherited "as-is".

Conversions

Although schemas are nominal, they also enjoy convenient structural conversions in the language without undue ceremony.

IDENTITY.

Classes

Advanced Types

MuIL supports some additional "advanced" type system features.

Constraints

To support rich validation, even in the presence of representations that faciliate data interoperability, MuIL supports additional constraints on number, string, and array types, inspired by JSON Schema:

• For numbers:
  • Minimum: number<M:>, where M is a constant number of the minimum (inclusive) value.
  • Maximum: number<:N>, where N is a constant number of the maximum (inclusive) value.
  • Both minimum and maximum: number<N:M>.
• For strings:
  • Exact length in characters: string<L>, where L is a constant number of the exact length.
  • Minimum length in characters: string<M:>, where M is a constant number of the minimum (inclusive) length.
  • Minimum length in characters: string<:N>, where N is a constant number of the maximum (inclusive) length.
  • Both minimum and maximum: string<N:M>.
  • A regex pattern for legal values: string<"R">, where "R" is a regex representing valid patterns.
• For arrays:
  • Exact length: T[L], where L is a constant number of the exact length.
  • Minimum length: T[M:], where M is a constant number of the minimum (inclusive) length.
  • Maximum length: T[:N], where N is a constant number of the maximum (inclusive) length.
  • Both minimum and maximum: T[N:M].

As examples of these, consider:

number<512:>            // min 512 (incl)
number<:1024>           // max 1024 (incl)
number<512:1024>        // min 512, max 1024 (both incl)

string<8>               // exactly 8 chars
string<2:>              // min 2 chars (incl)
string<:8>              // max 8 chars (incl)
string<2:8>             // min 2, max 8 chars (incl)

string<"a-zA-Z0-9">     // regex only permits alphanumerics

number[16]              // exactly 16 numbers
number[8:]              // min 8 numbers (incl)
number[:16]             // max 16 numbers (incl)
number[8:16]            // min 8, max 16 numbers (incl)

These constructs are frequently useful for validating properties of schemas without needing custom code.

Union and Literal Types

A union type is simply an array containing all possible types that a value might resolve to. For example, the type [ string, number ] resolves to either a string or number value at runtime.

A literal type is a type with an arbitrary string value. A literal type is silly to use on its own, however, when combined with union types, this provides everything we need for strongly typed enums.

For example, imaging we wish our state property to be confined to the 50 states:

properties:
    state:
        type: [ "AL", "AK", ..., "WI", "WY" ]

A compiler should check that any value for the state property has one of the legal string values. If it doesn't, MuIL runtime validation will ensure that it is the case.

Type Aliases

Any type A can be used as an alias for another type B, simply by listing B as A's base type:

Employees:
    base: Employee[]

This can be particularly useful for union/literal enum types, such as our state example above:

State:
    base: [ "AL", "AK", ..., "WI", "WY" ]

Now, given this new State type, we can simplify our state property example from the previous section:

properties:
    state:
        type: State

Data and Computations

Possibly-Controversial Decisions

It's worth describing for a moment some possibly-controversial decisions about MuPack and MuIL.

These might come as a surprise to higher level programmers, however, it is worth remembering that MuIL is attempting to strike a balance between high- and low-level multi-language representations. In doing so, some opinions had to be discard, while others were strengthened. And some of them aren't set in stone and may be something we revisit later.

Generics

MuIL does not support generics. MetaMu languages are free to, however they must be erased at compile-time.

This admittedly sacrifices some amount of generality. But it does so at the benefit of simplicity. Some MetaMu languages simply do not support generics, and so admitting them to the core would be problematic. Furthermore, languages like Go demonstrate that modern cloud programs of considerable complexity can be written without them.

Perhaps the most unfortunate and apparent aspect of MuIL's lack of generics is the consequently missing composable collection types. To soften the blow of this, MuIL has built-in array, map, and enumerable object types.

Operators

MuIL does come with a number of built-in operators.

MuIL does not care about operator precedence, however. All expressions are evaluated in the exact order in which they appear in the tree. Parenthesis nodes may be used to group expressions so that they are evaluated in a specific order.

MuIL does not support operator overloading. The set of operators is fixed and cannot be overridden, although a higher-level MetaMu compiler may decide to emit calls to intrinsic functions rather than depending on MuIL operators.

Smaller Items

MuIL doesn't currently support "attributes" (a.k.a., decorators). This isn't for any principled reason other than the lack of a need for them and, as such, attributes may be something we consider adding at a lter date.

Open Questions

AST shapes

Exporting classes: how much do you get? E.g., Go is a good litmus test.

Exceptions: fail-fast

Abstract

Virtuals

Inheritance

RTTI/Casting/Conversion

Lambda types Numeric types (long, int, etc)

Main entrypoint (vs. open-ended code)

Boxing/unboxing?