Why not to define it for any monad, for example STM (TVars) and whatever? 2010/9/7 Maciej Piechotka <uzytkown...@gmail.com>
> On Tue, 2010-09-07 at 02:15 +0200, Bas van Dijk wrote: > > On Mon, Sep 6, 2010 at 11:55 PM, Maciej Piechotka <uzytkown...@gmail.com> > wrote: > > > Reference is class which generalizes references and monads they exists > > > in. It means that IORef, STRef and others can be accessed by common > > > interface. > > > > > > Currently it is of form: > > > > > > class Reference r m where > > > > > > 1. There was a proposal to rename the class to MonadRef or > > > MonadReference. IMHO it would imply m -> r functional dependency and > > > therefore disallow the instances for both MVar IO and IORef IO > > > > > > 2. Should the functional dependencies or type famillies be introduced? > > > Personally I don't think so as I would like to allow all of the > > > following: > > > > > > - IORef IO > > > - MVar IO > > > - IORef (ContT IO) > > > - MVar (ContT IO) > > > > > > Any feedback mostly welcome. > > > > > > Regards > > > PS. Darcs repository will be available soon > > > > > > _______________________________________________ > > > Haskell mailing list > > > hask...@haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell > > > > > > > > > > > First I'd like to say sorry for error in cafe mailing list address. The > whole e-mail is included above. > > > I've played with a somewhat similar idea: > > > > darcs get > http://code.haskell.org/~basvandijk/code/only-read-or-write-vars > > > > API Docs + Hyperlinked source: > > > http://code.haskell.org/~basvandijk/code/only-read-or-write-vars/doc/html/only-read-or-write-vars/ > > > > This is not released (yet) because I'm unsure about the design. > > > > Especially, I'm unsure whether parameterizing Readable with α is a good > idea: > > > > class Readable v m α | v → m where read v → m α > > > > and whether parameterizing Writable with α and β is a good idea: > > > > class Writable v α m β | v → m where write v → α → m β > > > > They do allow some some nice instances like: > > > > instance Readable (TMVar α) STM (Maybe α) where > > read = TMVar.tryTakeTMVar > > > > instance Writable (TMVar α) α STM Bool where > > write = TMVar.tryPutTMVar > > > > instance Writable (Chan α) (End α) IO () where > > write t = Chan.unGetChan t unEnd > > > > -- | Writing @End x@ to a 'Chan' or 'TChan' writes @x@ to the end of > the channel > > -- instead of to the front. Also see 'unGetChan' and 'unGetTChan'. > > newtype End α = End {unEnd α} > > > > Regards, > > > > Bas > > > > I guess they are to some extend like Foldable to FoldableLL and possibly > describing different aspects. Yours seems to be a bit like StateVar[1] > generalized for different monads. Support for ptr & others is additional > bonus. > > I wanted clear 1-1 mapping with references - i.e. basic 2 operations > with added third for 'atomic' support. > > For example one of unmentioned law is that: > > write r x >> read r === write r x >> return r -- If it's only thread > write r x >> write r y === write r y -- If it's only thread > read r >> read r === read r > read r >>= write r === id -- If it's only thread > read r >>= const f === f -- If it's only thread? > > In fact I guess each on them could be used as RULES as IMHO each case in > which it is not necessary true it is undeterministic anyway. > > While Chan is clearly writable and readable it cannot be considered a > reference - it do have it purposes but they are usually different then > references. > > To sum up - I would not be willing to use Readable/Writable in place of > Reference but I don't say they don't have their purposes. > > PS. I took the liberty of continuing sentence. > > [1] http://hackage.haskell.org/package/StateVar-1.0.0.0 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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