[Haskell-cafe] Haskell Weekly News: Issue 92 - November 8, 2008

Austin Seipp mad.one at gmail.com
Mon Nov 10 18:38:26 EST 2008

> Anyway, I don't see it anywhere in the release notes, but I get the vibe 
> that type families are supposed to be "fully working" now. Is that 
> correct? If so, why no mention anywhere?

Type families have been completely reimplemented and should be stable
now, but there are some bugs - notably equality constraints in
superclasses are not supported in GHC 6.10.1, i.e.

> class (F a ~ b) => C a b where
>   type F a

As indicated by this bug report: http://hackage.haskell.org/trac/ghc/ticket/2715
And here: http://haskell.org/haskellwiki/GHC/Indexed_types#Equality_constraints

> Also, the release notes tantelisingly hint that the long-awaited 
> parallel-array stuff is finally working in this release, but I can't 
> find any actual description of how to use it. All the DPH stuff seems on 
> the wiki was last updated many months ago. You would have thought that 
> such a big deal would be well-documented. It must have taken enough 
> effort to get it to work! You'd think somebody would want to shout about 
> it...

I put up a DPH version of the binarytrees benchmark in the shootout:


There are some notes there; the only documentation I really used was
the documentation built by the GHC build process on the 'dph-*'
libraries (you can see them in 6.10 by just doing 'ghc-pkg list' and
looking through it.)

I was thinking of porting more of the parallel shootout entries to use
DPH, but I'm busy right now - results could be interesting.


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