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A Terra-backed ClojureScript compiler
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ClojureScript-Terra

ClojureScript Terra is a ClojureScript compiler that targets Terra.

This is the extracted design information from the developer log.

The original project was called Clojure/Cyclone Inspired C, or CLIC, based on a design question. The project in here is referred to as ClojureScript-Terra, given the path forward (enhancing Lua and Terra, writing a CLJS compiler)

Timothy Baldridge has also been pursuing projects with similar goals. There could be a point where I migrate the efforts found here into one of his projects.

Please see the Implementation Notes below for various modifications this project has made to ClojureScript and Terra

There will be a refresh of this project (and the final missing pieces will be completed) when the cljs-bootstrap work settles a bit more.

Rationale

Clojure's opinions make for an attractive, well-balanced, powerful, and simple language, but there are no comparable options for highly-constrained applications:

  • Operating systems and kernels
  • Real-time computation
  • resource-sensitive and resource-constrained systems

While Clojure and the JVM (and to some degree CLJS and V8) are advantageous in larger systems-of-systems engineering, they are not ideal or even acceptable for the fields listed above. Typically, such domains require or strive for:

  • Binary compatiblity with C (for library and ABI support)
  • LLVM compilation targets
  • Runtimes smaller than 500K
  • Deterministic runtime characteristics

In some cases, Clojure (and the JVM) could be adapted to achieve some of the desired goals. Interfacing and adapting the use of something like Javolution would allow Clojure to be more deterministic and fulfill real-time requirements.

Ideally, producing LLVM intermediate code would allow for a compilation chain to take advantage of HSA tooling and other advancements from LLVM targets. This is ideal for High-Performance computing as well as embedded (but highly specialized) Systems-on-a-Chip (SoC).

The main design question is:

 What would C look like if it were designed today?

C inspired by:

To a lesser degree

I would like something C-like/native that supports:

  • Namespaces
  • Protocols & ideally predicate dispatch
  • Persistent data structures with transients (or pods) and metadata
  • Region-based memory management with region inference
  • First class functions and proper closures
  • Destructuring/binding via data structures or pattern matching
  • The Sequence abstraction
  • The separation of state and identity (as found in Clojure)
  • Lazy and eager evaluation - lazy by default with functions for eagerness
  • Improved pointer safety and region checking
  • Immutability as default, while maintaining convenient array manipulation
  • Structs as Records with property access
  • Proper promises
  • Common FP functions/operations provided (all sequence oriented functions)
  • Some notion of memory safety
  • No requirement of an external runtime/virtual or operating system
    • Although using one should always remain an option

The language should explore:

  • Separating the syntax from the language
  • Pluggable, Optional type systems, with a separate hook/phase in compilation
    • This includes the type system, checker (if any), and inference
    • The only requirement is that all type systems need to resolve to C types/OS memory
  • Pluggable, Optional exception handling (exception handling as a library)
  • Multi-stage programming
  • What the separation of state and identity looks like at low-level interactions
  • Can this all be implemented cleanly on top of C (ala SAClib)?
  • Optional GC - either in place of the region system, or on top of the region system
    • Maybe this implies regions are opt-in?

Additional (but orthogonal) explorations include:

  • Multiple Language feature expressions with LLVM cross-compilation and linking
    • This could be built on something like Julia's llvmcall
  • Concurrency that supports Futures and CSP (state machines in parallel) at a minimum
    • This will most likely be built on Ray (libuv) or Richard Hundt's predecessor Luv (potentially his ray branch)
    • You could add the OS-Thread worker queue, but not allow any global access (new or cloned Terra/Lua states per work item), in a share-nothing style.
    • Terra interops with pthreads just fine; You could always fall back to low-level threading if appropriate.

The language is never concerned with:

  • Binary compatibility with C++ (but won't be explicitly avoided)
  • Image-based runtimes (ala Smalltalk)

Approach

The Developer Log contains log entries as the ideas are explored one-by-one, and the design is evolved and shaped. Below is the most recent, collective view of the design. Please cross-reference that doc for additional information/links.

The current approach is:

Write a ClojureScript compiler on top of Terra (which in turn uses LuaJit). This allows a developer to choose from a language spectrum that covers: C -> Terra -> Lua(JIT) -> ClojureScript. In this spectrum C exists primarily for legacy reasons - where you'd want to embed Terra into an existing system, or where you'd want Terra's FFI to use a C library (legacy or hand-written). Terra's LLVM backing allows open, extensible, low-level compilation to any LLVM Target, while providing the tooling for such low-level code to be generated from higher level languages (Lua and ClojureScript). The generated/compiled code requires no runtime whatsoever. LuaJIT's resident memory space is about 300K, but executes code considerably faster than Python, PyPy, Julia, and V8. Terra statically links the LLVM libs, requiring no third-party dependencies for running the packaged runtime.

In summary:

  • Binary compatibility with C
    • Extremely fast and efficient FFI, even when used dynamically
    • FFI Terra code can compile down (to an executable or shared-lib)
  • Embed in C applications
  • Small footprint (between 300K - 4mb when statically linked)
  • Compilation with no runtime dependency
  • Optional Runtime for dynamic scripting (or live code generation/optimization), with better performance characteristics than alternative scripting languages
  • ClojureScript for the highest-level of abstraction; AOT compilation (for the time being)
    • CLJS can be used purely for Lua (scripting on LuaJIT), Terra (low-level code), or both

This approach allows for:

  • Talking directly to hardware if needed via C libs or something like Snabb Switch
    • See testimony here and follow links as needed.
  • Embedding into kernels, even real-time ones like L4
  • Building applications, scripts, runtime libraries, or AOT-compiled shared-libs
  • Building hard real-time applications on top of stock Linux with some modifications

But currently lacks:

  • Library support to the degree found in other languages (Python, JavaScript, Java)
    • limited to C, ClojureScript, and lua libs; Virtually no Terra libs (language is new)
  • Complexity associated with the spectrum of multi-stage programming
    • For example, different tiers have different dispatch mechanisms that don't interop; Different exception handling mechanisms
  • Data structures differ between multi-stage tiers and must conform to interop rules

The tiers

Host, typed

This is just C.

Low-level, typed

At this tier, code is targeting LLVM compilation and everything is typed. Mutability is default, and there is no memory safety. There is no formal notion of metadata or separation of state and identity.

  • Dict (to be written, based on HMap here)
  • Array
  • Vector (this is special-purposed for SIMD; it's namespaced to avoid clashing)
  • Struct

Dispatch is:

  • Function call
  • Interfaces (Go-like/protocol-like dispatch)

Runtime, dynamic

At this tier, code is dynamic and part of an active runtime (LuaJIT). This code is used for low-level generation, or the foundation for a dynamic application. Mutability is the default, there is memory safety. Exception handling is protective calls, signaling, and recovery. There is no formal notion of metadata or separation of state and identity.

  • Table
  • TableList
  • Tree
  • Container (an object with no properties, only functions)
  • Object (a prototypical object; uses Lua's OO metatables pattern)
  • Peg (an LPeg pattern Object)

Dispatch is:

  • Function call
  • Multimethods without hierarchies

Generation, dynamic

At this tier, code is compiled AOT to target the lower two tiers (runtime and low-level). This tier is very high-level and extensible. It champions simplistic systems, functional programming, immutability, and memory safety by default. There is full and extensible exception handling. There is full support for metadata and the separation of state and identity.

  • Map
  • List
  • Vector
  • ArrayMap
  • Set
  • Record

Dispatch is:

  • Function call
  • Multimethods with hierarchies
  • Protocols
  • core.match-based

TODO

The current TODO is (See Jan 31,2014 and after in the dev log):

  • Build a better stdlib with LPeg included (and wrapped as an Object)
  • Integrate and refactor Terra with the new stdlib (details in dev log)
  • Add the Interface (Protocol) piece into Terra with reflection added
  • Write CLJS-Lua upon the new stdlib (supporting LuaJIT FFI and FFI calls) and LDoc
  • Make CLJS array operations protocol-based, add a cljs.terra namespace for the Terra specific features; Array opts should work on Arrays, Vectors, and TableLists
  • Replace std.functional with a Mori-like precompiled CLJS core
    • Data structures are first class in std (not in functional)
  • Rope in pieces from Terra Utils
  • Add llvm_call to Terra and expose in cljs.terra
  • Package up the MPS Memory Pool for Terra and expose in cljs.terra.mempool

For details of these items (and their rationale), please read the Developer Log.

Implementation notes

ClojureScript

For now, the compiler infrastructure all comes from ClojureScript, which requires the JVM. It will output a single Terra file.

One notable difference is the absence of (js* ...) forms, replaced with (host* ...) forms. This is intentionally done to make the compiler less js-specific. Note that Terra sits on top of Lua, so you can put valid Lua or Terra code in the host* form. It also means that this compiler uses an adapted CLJS analyzer.

Lua optimizes tail calls. You should still write idiomatic Clojure (with explicit recur calls), but under the hood TCO is available and utilized at different points in the compiler.

RegEx objects/literals are PEG literals in CLJS-Terra. Regular expressions can be captured as PEG Patterns. To make regex more convenient, any place that accepts a Match/Pattern/PEG literal, can also take a string. If a string is passed, it's treated as a Regular Expression.

The LuaJIT compiler takes the place of the Google Closure compiler in CLJS-Terra. Controls are not currently in place, but you can see the levels of optimizations and options.

Low-level code (as defined by defnf, etc) can optionally be compiled to executables or shared libraries during runtime or compilation time (where ever the terra/saveobj call appears).

Terra

CLJS-Terra uses a custom terralib. Shims are in place to allow for full interoperability with the one found within Terra itself.

Usage

TODO


License

Copyright © 2014 Paul deGrandis

The use and distribution terms for this software are covered by the Eclipse Public License 1.0 which can be found in the file epl-v10.html at the root of this distribution.

By using this software in any fashion, you are agreeing to be bound by the terms of this license.

You must not remove this notice, or any other, from this software.


The ClojureScript compiler is distributed under the Eclipse Public License version 1.0.

Terra, Terra-utils, Lua, LPeg, stdlib, and LuaJIT are all under the MIT License.

MPS Memory Pool is distributed under the BDB / Sleepycat License

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