Haskell.org is participating in Google Summer of Code again this year. If you're looking for a fun project to work on over the summer and want to learn something about Haskell compilation, a GHCJS-related project might just be the ticket for you. We've compiled a list of ideas to get you started:
GHCJS Google Summer of Code project ideas
GHCJS is a Haskell to JavaScript compiler that uses the GHC API. Our first official release is planned to coincide with the release of GHC 7.8.1.
GHCJS supports many modern Haskell features, including:
- All type system extensions supported by GHC
- Lightweight preemptive threading with blackholes, MVar, STM, asynchronous exceptions
- Weak references, CAF deallocation, StableName, StablePtr
- Unboxed arrays, emulated pointers
- Integer support through JSBN, 32 and 64 bit signed and unsigned arithmetic (
Word64
,Int32
etc.) - Cabal support, GHCJS has its own package database
And some JavaScript-specific features:
- new JavaScriptFFI extension, with convenient import pattens, asynchronous FFI and a JSRef FFI type,
- synchronous and asynchronous threads.
The GHCJS command line options are still in flux. Make sure that you have the
latest Cabal patch installed when you update GHCJS. You need both an updated
`cabal-install` executable and `Cabal` library. Rerun `ghcjs-boot --init` to get the
latest updates for the libraries. If you get `Not in scope: data constructor ‘GHCJS’`,
you need to update your `Cabal` library.
First install GHC 7.8 release candidate 2 or later and check with ghc --version
that it's the
compiler in your PATH
. Next, make sure that you have all the prerequisites for your platform:
- a recent version of
alex
andhappy
need to be in yourPATH
git
,make
,cpp
,autoreconf
,patch
need to be in yourPATH
- by default,
ghcjs-boot
will try to use the system GMP library, seeghcjs-boot --help
for more info.
- virus scanners often interfere with configure scripts (permission denied errors),
disable on-access scanning before running
ghcjs-boot
. - no extra programs need to be installed,
ghcjs-boot
will download an archive (around 100MB) with the required programs.
Run the following script to install an updated Cabal
and cabal-install
with GHCJS
support. Note that this will overwrite the cabal
executable in your cabal executable
installation path (typically ~/.cabal/bin
), you might want to backup your current version or
append a custom program suffix with a cabal
option such as --program-suffix=-js
.
#!/bin/sh
git clone https://github.com/ghcjs/cabal.git
cd cabal
git checkout ghcjs
cabal install ./Cabal ./cabal-install
Make sure that you're now running the new cabal-install
, GHCJS
support must be listed under the
compiler flags:
$ cabal install --help
...
build files (default dist)
-g --ghc compile with GHC
--ghcjs compile with GHCJS
--nhc98 compile with NHC
...
Now install GHCJS
itself:
$ git clone https://github.com/ghcjs/ghcjs.git
$ cabal install ./ghcjs
Check that you have the correct version of GHCJS
in your PATH:
$ ghcjs --version
The Glorious Glasgow Haskell Compilation System for JavaScript, version 0.1.0 (GHC 7.8.1)
$ ghcjs-boot --version
The Glorious Glasgow Haskell Compilation System for JavaScript, version 0.1.0 (GHC 7.8.1)
Build the base libraries for GHCJS
:
$ ghcjs-boot --init
or
$ ghcjs-boot --init --with-cabal cabal-js # if you used --program-suffix=-js earlier
ghcjs
can be invoked with the same command line arguments as ghc
. The generated programs can be run directly from
the shell with Node.js and SpiderMonkey jsshell.
for example:
$ ghcjs -o helloWorld helloWorld.hs
$ node helloWorld.jsexe/all.js
Hello world!
Use cabal install --ghcjs packageName
to install a package
Most packages from hackage should work out of the box. The main exception is packages with foreign (non-Haskell) dependencies. For these packages a JavaScript implementation of the dependencies must be provided. If a package you want to use does not work, please create a ticket.
Use ghcjs-pkg
to manipulate the GHCJS package database
The package database and runtime files from the shims repository are kept in the
GHCJS application data directory, typically ~/.ghcjs/
. Remove this directory to reset your GHCJS installation, you
need to run ghcjs-boot --init
again.
See GHCJS introduction for more examples.
If you want to hack on GHCJS, please join our friendly community on IRC at #ghcjs
on freenode (You're also
welcome if you only use the compiler or just want to chat about it!). The channel can be quiet from time
to time, but with a little patience you should be able to find people to get you started or point you in
the right direction.
The repositories you will be interested in are:
-
https://github.com/ghcjs/ghcjs
The compiler itself, code generator, linker, test suite
examples:
- Executables (
src-bin
) - Main code generator (
src/Gen2/Generator.hs
) - Optimizer (
src/Gen2/Optimizer.hs
) - Linker (
src/Gen2/Linker.hs
) - Generated runtime code (
h$ap_x_y
etc): (src/Gen2/RtsApply.hs
)
- Executables (
-
https://github.com/ghcjs/shims
Implementation of much of the runtime system and JavaScript dependencies. Code from this repository is linked into the
lib.js
andlib1.js
files when generating a.jsexe
. Installed in~/.ghcjs/version/shims
byghcjs-boot
.examples:
- thread scheduler, MVar implementation (
src/thread.js
) - STM implementation (
src/stm.js
) - IO system (
src/io.js
) (buffered IO forputStrLn
etc) - Heap scanner for weak references and finalizers (
src/gc.js
) - package-specific dependencies (
pkg
), dependencies listed inpackagename.yaml
(temporary measure until Cabal supports JavaScript dependencies directly) - third party libraries (
lib
)
- thread scheduler, MVar implementation (
-
https://github.com/ghcjs/ghcjs-base
GHCJS base library, basic marshalling between JavaScript and Haskell data
If you've changed something that affects the code generated by GHCJS, you'll need to rebuild the base packages (reboot the compiler). Often,
a full reboot is not required. You can run a quick boot to install only the essential packages (e.g. base
, integer-gmp
, array
, but not ghcjs-base
).
Adjust the -j4
flag (number of concurrent jobs) for your system:
$ ghcjs-boot --quick -j4
After a change, please run the test suite to
check that it does not break anything. Install jsshell
and node.js first to make sure that the tests run correctly (js
and node
executables should
be in your PATH
)).
When the compiler has been booted, run cabal test
, or run the test program directly for more options:
$ ./dist/build/test/test --help
For example to run a specific test or tests that match a pattern:
$ ./dist/build/test/test -t pattern
New test cases, in particular if you add new features, are always welcome. The testrunner automatically picks up new tests
(.hs
files that start with a lowercase letter) in the existing categories.
TODO add better instructions here
- link your program with
-debug
:ghcjs -debug -o test test.hs
, this adds debugging information to the generated code - get more informaton from the runtime system by enabling various tracing and logging options. See the code in the shims repository for more info. Examples:
-DGHCJS_TRACE_SCHEDULER
: messages from the thread scheduler-DGHCJS_TRACE_CALLS
: print all function calls from the main loop. warning: lots of output. Requires-debug
for name information-DGHCJS_TRACE_STACK
: print top of stack for every call in the main loop. warning: even more output-DGHCJS_TRACE_WEAK
: output related to weak references-DGHCJS_TRACE_STM
: output for software transactional memory-DGHCJS_TRACE_GC
: output garbage collector (heap scanner) related messages
- see the utility programs in
utils
in the ghcjs repository for tools for inspecting object files, quickly bisecting bugs in the optimizer etc.
You can use the JavaScript profiler to get a basic idea where the majority of time in your program is spent. Non-exported
names (like stack frame functions) start with h$$
, scroll up in the code to find an exported name to which they belong.
Unfortunately due to Haskell's lazy evaluation and GHCJS's tail-call optimization, the information here is far less
useful than GHC profiles for native code. Profiling support is planned, but implementation has not been started yet.
Profiling will support cost centres like in native GHC. In particular, we intend to add memory profiling support for
interactive (reactive) systems, keeping track of allocations and retention per event. If you're interested in helping
out or discussing features, please contact us in #ghcjs
on freenode.
Applications that use the following libraries should compile with GHCJS and run in a modern web browser and interface with DOM and JavaScript in the browser.
- webkit - Bindings for WebKitGTK+ that provide a low level DOM interface.
- webkit-javascriptcore - Low level bindings for JavaScriptCore
- jsc - Higher level inteface for JavaScriptCore
You can use these libraries without GHCJS to build a native version of your application (it will use WebKitGTK+ to run without a browser). If you want to find out more about making GHCJS compatible Haskell applications check out the GHCJS Examples