[Haskell-cafe] Can't Haskell catch upwith Clean's uniqueness typing?

Robert Dockins robdockins at fastmail.fm
Tue Dec 6 18:19:22 EST 2005

On Tuesday 06 December 2005 04:00 pm, haskell-cafe.mail.zooloo at xoxy.net wrote:
> From: "Shae Matijs Erisson - shae at ScannedInAvian.com"
> Sent: Tuesday, December 06, 2005 6:16 PM
> > haskell-cafe.mail.zooloo at xoxy.net writes:
> > > being occupied with learning both languages, I'm getting curious if
> > > Haskell couldn't achieve most of the performance gains resulting from
> > > uniqueness typing in Clean by *automatically* determining the reference
> > > count of arguments wherever possible and subsequently allowing them to
> > > be physically replaced immediately by (the corresponding part of) the
> > > function's result. Are there any principal obstacles, or *could* this
> > > be done, or *is* this even done already, e. g. in ghc?
> >
> > Maybe you're describing speculative evaluation?
> >
> > Optimistic Evaluation: An Adaptive Evaluation Strategy for Non-Strict
> > Programs http://citeseer.ist.psu.edu/ennals03optimistic.html
> > --
> Thanks for the pointer - I have heard a little about optimistic evaluation
> already, but don't know much of the details (yet). Anyway, from what I
> know, I think it's a different thing.
> In Clean, you can (and often are required to) assign uniqueness attributes
> to some parts of a function's type signature. The extended type checker
> ensures that none of those parts is referred to more than once during a
> single run of the program. Based on this guarantee, a function does not
> have to allocate new memory at all to store a unique result but can
> overwrite the unique arguments in place.
> Apparently, the uniqueness assignments have to comply with very tight laws
> - getting a program through the Clean type checker can be tough, once it
> reports an uniqueness coercion error. I suppose, no explicit uniqueness
> attributing is going to be implemented in Haskell, anyway.
> My question is - and this might better suit to Haskell -, can't uniqueness
> be inferred (and exploited) automatically in many cases?

Yes, probably.  There is a technique called sharing analysis that attempts to 
determine when a datastructure is only referenced once (ie, NOT shared).  If 
you can prove a datastructure node is not shared then you can reuse it 

Here is a paper on the technique.  It's written for lisp cons cells, but one 
might be able to generalize the technique to ADT.  I don't know where to find 
a free copy.


There has also been some similar work done along these lines for Mercury (a 
logic programming language).


Search for papers with the word "reuse" in the title.  I'm not very familiar 
with this work, so I don't know how applicable this might be.

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