Yhc/Erlang/Proof of concept

Introduction

This Wiki article describes an experiment targeting execution of Haskell programs on top of the Erlang Virtual Machine (BEAM). Haskell source code is compiled to Yhc Core with York Haskell Compiler (Yhc), then the program further discussed converts Yhc Core to Core Erlang; finally Erlang Compiler (erlc) compiles Core Erlang to the BEAM file format which can be loaded and executed by the Erlang VM.

There have been numerous discussions about Haskell (mainly GHC) runtime lacking some properties that are available in Erlang environment, as well as about possible improvements in Erlang language syntax and type system to bring some elements available in Haskell.

This experiment is an attempt to answer the critics from both sides. Once it becomes possible to execute Haskell programs in Erlang environment, Haskell users get access to the robust concurrency-oriented runtime, still being able to use Haskell native syntax. Erlang users get possibility to develop some algorithms with regard to the Haskell strong type system, while still being able to code directly in Erlang, where it seems more appropriate implementation-wise.

Implementation details

This section discusses in deep the approach taken in this experiment. It is good to remember that nothing yet is final; some of the techniques described may possibly make it into the mainstream code, while others may not. This is only the beginning.

Core Erlang overview

The Core Erlang initiative project is a "collaboration between the High-Performance Erlang (HiPE) project at the Department of Information Technology of Uppsala University, and Ericsson's OTP/Erlang developers".

Core Erlang is an intermediate form of Erlang source compilation. It provides a desugared (compared to Erlang) syntax of a strict functional language. Erlang source may be compiled to Core Erlang, and Core Erlang may be compiled to BEAM bytecode.

Core Erlang plays the same role in the Erlang compilation process as Yhc Core in the Haskell (Yhc) compilation process. So, it turns out to be the most convenient to do the conversion between these formats rather than between e. g. Haskell source and Erlang source.

There was an attempt made earlier to do similar things, only converting from Haskell source (indeed, a subset of Haskell syntax) to Core Erlang. This project is called Haskerl, developed by Torbjörn Törnkvist, (not to be confused with Will Partain's Haskerl. Some source code from Haskerl was used in this experiment, in particular, the algebraic data type to represent Core Erlang internally, and the pretty printer module for Core Erlang, both with some necessary extensions.

The whole compilation chain looks like this:

1. Haskell source modules are compiled into Yhc Core and linked;
2. Some overall program optimizations (functionality provided with the Yhc Core library) are performed on the linked Yhc Core;
3. Yhc Core is converted to Core Erlang;
4. The Erlang compiler erlc produces a BEAM file.

Haskell on BEAMs ;)

From the Erlang VM standpoint, a Haskell program is just a large(ish) Erlang module. To run the program, certain function exported from that module (usually, main) needs to be called with arguments as necessary. Often the special force function has to be applied to values returned from Haskell-originated module: otherwise some unfinished computation may be returned instead of the expected result.

From Haskell program standpoint, Erlang VM is just an execution environment providing system calls that are strict on all their arguments, and may have variable number of arguments. Haskell program may spawn concurrent processes able to receive messages of certain types (the idea of typed processes was borrowed from this Livejournal article (in Russian). The message distribution/transport mechanism is entirely provided by the Erlang VM runtime.

Examples

Sample code to demonstrate results of the experiment was checked into the Yhc Darcs repo. Haskell and Erlang sources as well as compiled BEAM files are located at http://darcs.haskell.org/yhc/src/translator/erlang/00proof/ .