[Haskell-cafe] Lazy language on JVM/CLR

[Tom Lokhorst]

About a year ago, Jeroen Leeuwenstein and I worked on CLR backend for
the Utrecht Haskell Compiler (UHC) [1].
That was a one-month project for a seminar at Utrecht University, and
the backend is far from being complete. But we did make some
interesting observations.

A particular caveat of the UHC is that it does whole program analysis,
so we had access to the entire program and all libraries at compile
time.

A benefit of using the CLR was that it does support tail calls. So a
mutual recursive function definition can loop a million times without
creating a stack overflow.

Our main problem (in efficiency) was lazy evaluation, not knowing the
difference between an evaluated `int` and a possible thunk
`Lazy<int>`. That meant we had to wrap _everything_ in a layer of
indirection, e.g.:

> add :: Int -> Int -> Int
> add x y = x + y

> add 2 4

Becomes something equivalent to:

> public int add(Lazy<int> x, Lazy<int> y)
> {
>   return x.Force() + y.Force();
> }

> add(new Lazy(() => 2), new Lazy(() => 4));

Having a strictness analyser would have helped tremendously.

Also, I wonder if there is some efficient way of implementing the Lazy
class, perhaps by having the Force method using runtime code
generation to override itself. I don't know if this is possible, but I
vaguely remember the Dynamic Language Runtime on .NET doing something
like that.

I find this an interesting topic, so when you do have something more,
please let us know on this list.

- Tom Lokhorst

[1]: http://tom.lokhorst.eu/ehc/clr/ehc-clr-handout.pdf


On Tue, Feb 9, 2010 at 1:42 AM, Tony Morris <tonymorris at gmail.com> wrote:
> I have hypothesised a pure, lazy language on the JVM and perhaps the
> .NET CLR with FFI to .NET/Java libraries. I foresee various problems but
> none that are catastrophic; just often requiring a compromises,
> sometimes very unattractive compromises. I have authored several
> libraries in the same vain as pure, lazy programming to run on the JVM
> in Java and Scala programming languages.
>
> I expect others have forethought and perhaps even experimented with such
> a language. Are there any dangers to be wary of that undo the entire
> endeavour?
>
> Thanks for any insights.
>
> --
> Tony Morris
> http://tmorris.net/
>
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