[Haskell-cafe] jhc speed

[John Meacham]

On Sun, Feb 22, 2009 at 03:36:34PM +0100, Peter Verswyvelen wrote:
> Would it be possible to separate the frontend (Haskell to Core) and backend
> (Core to machine code) from the Haskell compilers (requiring a standard Core
> language?)
> I'm not sure how many extensions required a change to the Core language.

Well, it depends on what you mean by 'core'. If you mean a desugared
version of haskell, I think such a front end could be quite useful. in
particular, I'd like to see a standalone implementation of template
haskell. If you mean something lower level, as in the ghc core
intermediate language the compiler uses internally, or jhc's core or
grin representation, things get a bit more tricky.

Although many core languages are somewhat similar, based on a typed
lambda calculus of some sort, the details will differ, and translating
between them can be lossy. 

For instance, looking at jhc core:
http://repetae.net/computer/jhc/manual.html#jhc-core-type-system
you can see it has a very rich language for dealing with strictness and
boxedness. For instances, a boxed value known to be in WHNF actually has a
different _type_ than one that is possibly unevaluated. Such
distinctions are quite useful for jhc's back end but not so much for
ghc's, hence ghc core doesn't make that distinction and any translation
between the two would 'lose' that useful information. 

In other cases things are even worse, for instance ghc has a powerful
type equality concept in its core language which jhc has no counterpart
for, so that information will be lost on translation. But losing that
information will actually cause the core to not type check, since ghc
core can type some things jhc core cannot (and vice versa) so coercions
or other mechanisms to bypass the type system will have to be
introduced.

So, it is certainly possible to translate between the two, in fact, I
made a jhc core -> ghc core translator, but the code it produced was
necessarily riddled with unsafeCoerce#'s for everywhere the type systems
didn't quite match up.

        John


-- 
John Meacham - ⑆repetae.net⑆john⑈