How to get started with a new backend?

[Justin Bailey]

Someone wrote a Haskell library that could interoperate with .NET -
you might get ideas for how to express .NET's type system in Haskell
from it:

http://www.haskell.org/haskellwiki/Salsa

On Mon, Jan 28, 2013 at 8:53 AM, Jason Dagit <dagitj at gmail.com> wrote:
>
>
> On Jan 28, 2013, at 3:21 AM, Simon Peyton-Jones <simonpj at microsoft.com>
> wrote:
>
> I would like to explore making a backend for .NET. I've done a lot of
> background reading about previous .NET and JVM attempts for Haskell. It
> seems like several folks have made significant progress in the past and,
> with the exception of UHC, I can't find any code around the internet from
> the previous efforts. I realize that in total it's a huge undertaking and
> codegen is only one of several significant hurdles to success.
>
>
>
> Someone should start a wiki page about this!  It comes up regularly.  (That
> doesn’t mean it’s a bad idea; just that we should share wisdom on it.)
> Maybe there is one, in which case perhaps it could be updated?
>
>
>
> Some thoughts (which, if someone would like to transfer these to the wiki
> page, perhaps in more coherent form):
>
> ·        Check out Frege.  Maybe others.  I think several folk are working
> on .NET or JVM projects.
>
> ·        Do you want to inter-operate with .NET, or compile to .NET
> bytecode.  These are very different?  I’ll assume the latter.
>
>
> Yes, the latter.
>
> ·        Do you want to compile to verifiable .NET bytecode?  This is hard.
> GHC’s type system is more expressive than .NET’s in many ways (higher kinded
> type variables, kind polymorphism...).  And less expressive in others
> (sub-typing). Compiling to verifiable bytecode would require lots of
> run-time type-tests (which we know will succeed) to reassure .NET that it’s
> all kosher.  Figuring out how to minimise these tests would be a challenge
> in itself.
>
>
> I'm okay with slow code. That's something I can improve later. :)
>
> ·        And of course this affects whether you can go via C#, since that
> requires the program to be well typed in the .NET sense.
>
>
> I'm not sure yet about going via C# or directly to CIL.
>
> ·        Do you want simply to run Haskell programs on .NET or do you want
> Haskell programs to call .NET libraries?  I assume the latter.  But that
> means that .NET types must be manifested somehow as Haskell types.   A big
> issue is how much of .NET’s type system you want to suck into Haskell.  A
> stressful example: can you define in Haskell a type that sub-classes an
> imported .NET type.
>
> Daan and Mark and Sigbjorn and I wrote papers about this stuff. Look at the
> papers under "Foreign language integration" on
> http://research.microsoft.com/en-us/um/people/simonpj/papers/papers.html.
> Oh, and this:
> http://research.microsoft.com/en-us/um/people/simonpj/papers/oo-haskell/index.htm
>
>
> Thanks. I'll make sure to read over those.
>
> ·        I’d be inclined to start from Core rather than STG, perhaps the
> output of CorePrep.  STG is a bit encrusted (though we’re about to simplify
> it a bit).  But honestly they are pretty similar.
>
> Cmm would be a bad choice. It’s far too low level.
>
>
> Good to know.
>
> ·        Another major issues is that of primops (prelude/primops.txt.pp).
> Things like add Int# or Float# are fine, but what about forking a thread, or
> throwing an exception?
>
> o   One possibility is to try to emulate them all.  I have no idea how hard
> that would be, but GHC’s I/O libraries rely heavily on lightweight
> concurrency.
>
> o   Another is to trim the list of PrimOps, and only support some of them.
> That’s easier, but you’d have to dump a lot of the ‘base’ package
> (particularly I/O) and re-implement it using the .NET libraries instead.
>
>
> I think I will do whatever gets Fibonacci working first, and then
> reevaluate.
>
> Thanks for the pointers.
>
> Jason
>
>
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