[Haskell-cafe] Re: [Haskell] ANNOUNCE: reference 0.1

[Alberto G. Corona]

Why not to define it for any monad, for example STM (TVars) and whatever?

2010/9/7 Maciej Piechotka <uzytkownik2 at 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 <uzytkownik2 at 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
> > > Haskell at 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
>
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>
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