GHC targetting Java?
simonpj at microsoft.com
Wed Nov 23 04:32:07 EST 2005
| On a similar note, how about the .NET support? Can it work on a
| platform where the only .NET support is Mono?
This comes up regularly. Here's my message from Jan:
It'd make a lot of sense to give GHC a .NET back end, and it's a
question that comes up regularly. The reason that we haven't done it
here, at GHC HQ, is because it's a more substantial undertaking than
might at first appear (see below). Furthermore, it'd permanently add a
complete new back-end platform for us to maintain. Given our rather
limited development effort (= Simon and me), we have so far not bitten
the bullet, and we have no immediate plans to do so.
It'd be a good, well-defined project for someone else to tackle, and
there is some good groundwork already done:
* Sigbjorn Finne did a simple interop implementation that allows a
Haskell program to be compiled to native code (as now) but to call
.NET programs via a variant of the FFI. I don't think this work is
in active use, and I'd be surprised if it worked out of the box, but
it could probably be revived with modest effort
* Andre Santos and his colleagues at UFPE in Brazil are working on a
.NET back end, that generates CLR IL, though I don't know where they
are up to.
* [This para wasn't in the original message]
Nigel Perry and Oliver Hunt have a Haskell.NET prototype that works
using GHC to compile to Core, and then compiling Core to NET. I'm
not sure what stage it is at.
* GHC.Net would be extra attractive if there was a Visual Studio
integration for GHC. Substantial progress on this has been made in
2004 by Simon Marlow, Krasimir Angelov, and Andre Santos and
There may be others that I don't know of. If anyone wants to join in
this effort, do contact the above folk. And please keep us informed!
Here's a summary of why it's a non-trivial thing to do:
* The first thing is to generate native CLR Intermediate Language
(IL). That's not really hard. Requires thinking about
representations for thunks and functions, and it may not be
particularly efficient, but it can surely be done. An open question
is about whether to generate verifiable IL or not. The trouble here
is that Haskell's type system is more expressive than the CLR's in
some ways, notably the use of higher-kinded type variables. So, to
generate verifiable IL one is bound to need some run-time casts, and
it's not clear how to minimise these.
At first blush this is *all* you need do. But it isn't!
* Next, you need to think about how to inter-operate with .NET
libraries. You don't really want to write "foreign import..." for
each and every import. You'd like GHC to read the CLR meta-data
directly. But there are lots of tricky issues here; see the paper
that Mark Shields and I wrote about "Object-oriented style
overloading for Haskell".
* Now you need to figure out how to implement GHC's primitive
the I/O monad
arbitrary precision arithmetic
software transactional memory
Not all of these are necessary, of course, but many are used in the
libraries. The CLR supports many of them (e.g. concurrency) but
with a very different cost model.
* Last, you have to figure out what to do for the libraries. GHC has
a pretty large library, and you either have to implement the primops
on which the library is based (see previous point), or re-implement
it. For example, GHC's implementation of I/O uses mutable state,
concurrency, and more besides. For each module, you need to decide
either to re-implement it using .NET primitives, or to implement the
stuff the module is based on.
These challenges are mostly broad rather than deep. But to get a
production quality implementation that runs a substantial majority of
Haskell programs "out of the box" requires a decent stab at all of them.
More information about the Glasgow-haskell-users