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190 lines
6.9 KiB
Markdown
190 lines
6.9 KiB
Markdown
---
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layout: developer-doc
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title: Polyglot Bindings
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category: polyglot
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tags: [polyglot, bindings, interop]
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order: 1
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---
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# Polyglot Bindings
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This document deals with the specification and design for the polyglot interop
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system provided in the Enso runtime. This system allows users to connect Enso to
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other supported programming languages, to both provide access to a wealth of
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libraries, and to integrate Enso into existing systems.
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The polyglot support in Enso is best-in class, and it supports this through two
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main mechanisms:
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1. **Polyglot FFI:** The low-level polyglot support provides a fairly low-level
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syntax sugar system for working with values from foreign languages.
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2. **Embedded Syntax:** This system allows users to write code from other
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languages directly in their `.enso` files, and to seamlessly share values
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between Enso and that foreign code.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Impedance Mismatch](#impedance-mismatch)
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- [The Polyglot FFI](#the-polyglot-ffi)
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- [Importing Polyglot Bindings](#importing-polyglot-bindings)
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- [Using Polyglot Bindings](#using-polyglot-bindings)
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- [Importing Polyglot Bindings \(Syntax\)](#importing-polyglot-bindings-syntax)
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- [Using Polyglot Bindings \(Syntax\)](#using-polyglot-bindings-syntax)
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- [Finding Polyglot Bindings](#finding-polyglot-bindings)
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- [Embedded Syntax](#embedded-syntax)
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- [Embedded Syntax Usage \(Syntax\)](#embedded-syntax-usage-syntax)
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<!-- /MarkdownTOC -->
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## Impedance Mismatch
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Polyglot interoperation in Enso has a significant impedance mismatch. In
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essence, this means that there is a mismatch between Enso's language semantics
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and the semantics of the foreign languages that are being worked with.
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While some of this mismatch can be worked around by manually wrapping the
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foreign constructs in Enso, there are still concepts that can't easily be
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represented by Enso.
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> The actionables for this section are:
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>
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> - Expand on the impedance mismatch and how it leads to the defined semantics.
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## The Polyglot FFI
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The low-level polyglot FFI mechanism refers to a way to use polyglot objects
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directly in Enso code. This can be used to underlie a library implementaion in
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Enso, or to interoperate with code running in other languages.
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The mechanism provides users with the facilities to import bindings from other
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languages and call them via a generic mechanism.
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### Importing Polyglot Bindings
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When importing a polyglot binding into scope in an Enso file, this introduces a
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_polyglot object_ into scope. This object will have appropriate fields and/or
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methods defined on it, as described by the foreign language implementation.
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> The actionables for this section are:
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>
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> - Expand greatly on the detail of this as the semantics of the imports become
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> clearer.
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### Using Polyglot Bindings
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With a polyglot object in scope, the user is free to call methods on it
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directly. These polyglot objects are inherently dynamically typed, meaning that
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any operation may _fail_ at runtime.
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Enso implements a generic variadic syntax for calling polyglot functions using
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vectors of arguments. In essence, this is necessary due to the significant
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impedance mismatch between Enso's runtime semantics (let alone the type system)
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and the runtime semantics of many of the polyglot languages.
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We went the way of the variadic call for multiple reasons:
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- It allows us to match up with a wide range of language semantics (such as
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subtyping and overloading).
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- It is flexible and easy to expand in the future.
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- We can easily build a more Enso-feeling interface on top of it.
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By way of illustrative example, Java supports method overloading and subtyping,
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two things which have no real equivalent in the Enso type system.
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> The actionables for this section are:
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>
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> - Expand greatly on the runtime semantics of working with polyglot bindings.
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> - Determine how to make the inherent 'failability' of polyglot objects safer.
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### Importing Polyglot Bindings (Syntax)
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Polyglot bindings can be imported using a polyglot import directive. This is
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constructed as follows:
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- The `polyglot` keyword
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- A language identifier (e.g. `java`).
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- The keyword `import`.
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- Optionally (where the language supports it), an identifier for the type of
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language entity being imported (e.g. `struct` for `c`).
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- A path that uniquely identifies the polyglot object to import.
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- Optionally, the keyword `as`, followed by a new name.
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For example:
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```ruby
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polyglot java import org.example.MyClass as MyClassJava
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polyglot c import struct NetworkPacket as NetworkPacketC
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```
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### Using Polyglot Bindings (Syntax)
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A polyglot binding is a polyglot object that has methods and/or fields defined
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on it. Due to an impedance mismatch between languages, Enso implements a
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variadic syntax for calling these polyglot bindings using vectors.
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In essence, we have a primitive function as follows:
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```ruby
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Polyglot.method : Polyglot.Object -> [Any] -> Any
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```
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It works as follows:
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- It is a method called `method` defined on the `Polyglot` type. The name
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`method` is, however, a stand-in for the name of the method in question.
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- It takes an object instance of the polyglot object.
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- It takes a vector of arguments (and is hence variadic).
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- And it returns some value.
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By way of example, the following is a valid usage of a polyglot binding:
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```ruby
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polyglot java import com.example.MyClass as MyClassJava
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main =
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x = MyClassJava.foo [1, 2, 3] # a static method
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inst = MyClassJava.new [a, b, c] # a constructor
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bar = inst.metod [x, y] # a static method
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```
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### Finding Polyglot Bindings
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Polyglot objects for various languages are found in the `polyglot` subdirectory
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of an Enso project. This folder is subdivided into directories based on the
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polyglot language. The name of each subdirectory must match the language
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identifier used in the source code.
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Inside each directory is an implementation-defined structure, with the polyglot
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implementation for that particular language needing to specify it. Please see
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the language-specific documentation for details.
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## Embedded Syntax
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The term "Embedded Syntax" is our terminology for the ability to use foreign
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language syntaxes from directly inside `.enso` files. This system builds upon
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the more generic mechanisms used by the [polyglot FFI](#the-polyglot-ffi) to
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provide a truly seamless user experience.
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### Embedded Syntax Usage (Syntax)
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A polyglot block is introduced as follows:
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- The `polyglot` keyword starts a block.
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- This must be followed by a language identifier (e.g. `java`).
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- After the language identifier, the remaining syntax behaves like it is an Enso
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function definition until the `=`.
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- After the `=`, the user may write their foreign code.
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```ruby
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polyglot python concat a b =
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def concat(a, b):
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str(a) + str(b)
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```
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In the above example, this defines a function `concat` that takes two arguments
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`a` and `b`, implemented in Python.
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> The actionables for this section are:
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>
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> - Greatly flesh out the syntax for the high-level polyglot functionality.
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