The dreaded M-R

Cale Gibbard cgibbard at
Sat Jan 28 22:37:02 EST 2006

Aha, okay. Yeah, I can reproduce that now, and it makes good sense
what's going on. It is in fact quite sensible that x be evaluated
twice with that sort of polymorphism.

Hmm... however, could we not assign to each instance a unique
identifier which could be compared? Say a cryptographic hash of the
source code for the instance? (Which of course would never be exposed
to the user.) That should be enough to tell them apart. The
translation would then partition the work according to the incoming
instances, and share all computation possible.

 - Cale

On 28/01/06, Lennart Augustsson <lennart at> wrote:
> Oh, I guess I did one more change.  I put b in a separate module.
> Your type signature isn't the most general, the most general is
> b :: (Num a, Num b) => (a, b)
> And that is the source of the problem.  You need to pass two
> dictionaries.  To keep sharing you'd need some very clever
> runtime machinery to find that the dictionaries are the same.
>         -- Lennart
> Cale Gibbard wrote:
> > On 28/01/06, Lennart Augustsson <lennart at> wrote:
> >
> >>Remove the type signature for b and you will see the
> >>loss of sharing.
> >
> >
> > Nope, still not seeing it with either profiling or Debug.Trace. Also
> > -- the type signature I gave was polymorphic, so what's the deal? If
> > adding a polymorphic type signature fixes the problem, and a
> > polymorphic type signature can be inferred, why not simply treat the
> > source as if one had been written there?
> >
> >
> >>It mostly hurts people like John Hughes that don't
> >>have the energy to put in type signatures. ;)
> >
> >
> > Well, sure. I don't think that we should see exponential blowup in
> > complexity of some programs by leaving out type signatures (though if
> > it was only in sufficiently rare cases, I could put up with it).
> >
> >
> >>On the subject of type signatures, I don't want to
> >>make them mandatory, but I think they should be strongly
> >>encouraged.  I don't buy the argument that they make
> >>refactoring programs that much harder.  It's still
> >>very easy to do, the type checker will tell you exactly
> >>where. :)
> >
> >
> > Me too. It's nice to be able to write quick programs where you leave
> > out the type signatures, but including them is always good for real
> > code. I also think that type signatures (and the type system in
> > general), makes it much easier to refactor code and work on code with
> > which you're unfamiliar.
> >
> >  - Cale
> >

More information about the Haskell-prime mailing list