[Haskell-cafe] More on the random idea

[Stefan O'Rear]

On Sat, May 26, 2007 at 07:41:19PM +0100, Andrew Coppin wrote:
> Donald Bruce Stewart wrote:
> >The #haskell people have been working on this for about 3 years now.
> >The result is the 'runplugs' program, which I've talked about in
> >previous mails.
> >
> >    http://www.cse.unsw.edu.au/~dons/code/lambdabot/scripts/RunPlugs.hs
> >
> >It uses hs-plugins for the evaluation, along with the points about IO
> >prevention via type checking, resource limits controlled by the OS, 
> >language extension preventions, and a trusted (audited) module base.
> >
> >The security mechanisms were briefly described in the 2004 hs-plugins
> >paper, if I recall, but otherwise, I don't think we've documented the 
> >techniques. Maybe we should, as many issues have been encountered over
> >the years, further and further constraining the kinds of things that are
> >allowed.
> >  
>
> For me, the unfortunate thing is that there seems to be no interpreter 
> for Haskell written in Haskell. (Except for GHC, which is *huge*.) We

Actually, there exists no interpreter for Haskell.  Period.

> have a Haskell parser [which is complex beyond usability], but no 
> interpreter. It would be nice if one existed, and I'm trying to 
> implement one... but on the other hand, one doesn't want to say to 
> people "hey, come learn Haskell, try this web interpreter" and then have 
> them find it doesn't implement Haskell precisely. So it looks like I'm 

Haskell has never been implemented.  Every implementation has a large
collection of corner cases that aren't correctly parsed; these can be
recognized as the wontfix bugs. 

> stuck with present technology - and that essentially means GHC. (Other 
> compilers? What other compilers??)
> 
> As far as I know, hs-plugins works by taking an expression, writing it 
> to a file, calling GHC to parse it, transform it to Core, optimise it,
> transform it to STG, optimise it, transform it to C--, optimise it, 
> transform it to ANSI C, optimise it, pass it to GCC, compile it, link 
> it, and *then* using the GHC runtime linker to load the generated object 
> code into memory, type-check it, and, finally, execute it.

No, the link step is omitted - the runtime loader works directly on .o
files.  This cuts several seconds off the sequence, making it neglible
compared to IRC lag. 

> OTOH, GHCi just takes an expression, parses it and interprets it. This 

Haha, no.

GHCi takes an expression, parses it, wraps it in module Main { main =
print (expr) }, type checks it, transforms it to Core, optimizes,
transforms to STG, optimizes, transforms to object code for a bytecode
machine, links it, and emulates the result.  Three times, since it
needs to try each of expr, expr >>= print, and print expr.

Hugs is also a full compiler, but for some dumb reason doesn't support
saving the object code to disk, so you have to recompile everything
each time. 

> appears to be a *much* more lightweight approach. I have had some 

Not at all.  GHCi seems much faster because GHC's nontrivial startup
overhead is amortized over the entire run. 

> success in using the GHC API to write a program that is essentially GHCi 
> - you type stuff in, it executes it. Now, if I could write a complete 
> full-featured HTTP server and feed that into it instead of stdin and 
> stdout, I could make a web server for executing Haskell... But, hmm, 
> that's not going to happen any time soon.
> 
> ...or, I could use GHC -e as I described earlier. But I have to look at 
> figuring out how to prevent it doing Bad Things. (Lambdabot gives me an 
> idea. Write a module that imports all the stuff you might want 
> interactively, and process the user-supplied script to make sure it 
> doesn't issue any imports, and then you can precisely control what 
> functions the code can call.)
> 
> Whatever... I'd just like to see an online way to run Haskell, and since 
> the Lambdabot webpage still shows no sign of working...

That's my fault - I designed a much simpler configuration interface
for lambdabot, but nobody has been motivated to un-bitrot GOA yet. 

Stefan