Why not to define it for any monad, for example STM (TVars) and whatever?<br><br><div class="gmail_quote">2010/9/7 Maciej Piechotka <span dir="ltr"><<a href="mailto:uzytkownik2@gmail.com">uzytkownik2@gmail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im">On Tue, 2010-09-07 at 02:15 +0200, Bas van Dijk wrote:<br>
> On Mon, Sep 6, 2010 at 11:55 PM, Maciej Piechotka <<a href="mailto:uzytkownik2@gmail.com">uzytkownik2@gmail.com</a>> wrote:<br>
> > Reference is class which generalizes references and monads they exists<br>
> > in. It means that IORef, STRef and others can be accessed by common<br>
> > interface.<br>
> ><br>
> > Currently it is of form:<br>
> ><br>
> > class Reference r m where<br>
> ><br>
> > 1. There was a proposal to rename the class to MonadRef or<br>
> > MonadReference. IMHO it would imply m -> r functional dependency and<br>
> > therefore disallow the instances for both MVar IO and IORef IO<br>
> ><br>
> > 2. Should the functional dependencies or type famillies be introduced?<br>
> > Personally I don't think so as I would like to allow all of the<br>
> > following:<br>
> ><br>
> > - IORef IO<br>
> > - MVar IO<br>
> > - IORef (ContT IO)<br>
> > - MVar (ContT IO)<br>
> ><br>
> > Any feedback mostly welcome.<br>
> ><br>
> > Regards<br>
> > PS. Darcs repository will be available soon<br>
> ><br>
> > _______________________________________________<br>
> > Haskell mailing list<br>
> > <a href="mailto:Haskell@haskell.org">Haskell@haskell.org</a><br>
> > <a href="http://www.haskell.org/mailman/listinfo/haskell" target="_blank">http://www.haskell.org/mailman/listinfo/haskell</a><br>
> ><br>
> ><br>
><br>
<br>
<br>
</div>First I'd like to say sorry for error in cafe mailing list address. The<br>
whole e-mail is included above.<br>
<div class="im"><br>
> I've played with a somewhat similar idea:<br>
><br>
> darcs get <a href="http://code.haskell.org/~basvandijk/code/only-read-or-write-vars" target="_blank">http://code.haskell.org/~basvandijk/code/only-read-or-write-vars</a><br>
><br>
> API Docs + Hyperlinked source:<br>
> <a href="http://code.haskell.org/~basvandijk/code/only-read-or-write-vars/doc/html/only-read-or-write-vars/" target="_blank">http://code.haskell.org/~basvandijk/code/only-read-or-write-vars/doc/html/only-read-or-write-vars/</a><br>
><br>
> This is not released (yet) because I'm unsure about the design.<br>
><br>
> Especially, I'm unsure whether parameterizing Readable with α is a good idea:<br>
><br>
> class Readable v m α | v → m where read v → m α<br>
><br>
> and whether parameterizing Writable with α and β is a good idea:<br>
><br>
> class Writable v α m β | v → m where write v → α → m β<br>
><br>
</div>> They do allow some some nice instances like:<br>
<div class="im">><br>
> instance Readable (TMVar α) STM (Maybe α) where<br>
> read = TMVar.tryTakeTMVar<br>
><br>
> instance Writable (TMVar α) α STM Bool where<br>
> write = TMVar.tryPutTMVar<br>
><br>
> instance Writable (Chan α) (End α) IO () where<br>
> write t = Chan.unGetChan t unEnd<br>
><br>
> -- | Writing @End x@ to a 'Chan' or 'TChan' writes @x@ to the end of<br>
the channel<br>
> -- instead of to the front. Also see 'unGetChan' and 'unGetTChan'.<br>
> newtype End α = End {unEnd α}<br>
><br>
> Regards,<br>
><br>
> Bas<br>
><br>
<br>
</div>I guess they are to some extend like Foldable to FoldableLL and possibly<br>
describing different aspects. Yours seems to be a bit like StateVar[1]<br>
generalized for different monads. Support for ptr & others is additional<br>
bonus.<br>
<br>
I wanted clear 1-1 mapping with references - i.e. basic 2 operations<br>
with added third for 'atomic' support.<br>
<br>
For example one of unmentioned law is that:<br>
<br>
write r x >> read r === write r x >> return r -- If it's only thread<br>
write r x >> write r y === write r y -- If it's only thread<br>
read r >> read r === read r<br>
read r >>= write r === id -- If it's only thread<br>
read r >>= const f === f -- If it's only thread?<br>
<br>
In fact I guess each on them could be used as RULES as IMHO each case in<br>
which it is not necessary true it is undeterministic anyway.<br>
<br>
While Chan is clearly writable and readable it cannot be considered a<br>
reference - it do have it purposes but they are usually different then<br>
references.<br>
<br>
To sum up - I would not be willing to use Readable/Writable in place of<br>
Reference but I don't say they don't have their purposes.<br>
<br>
PS. I took the liberty of continuing sentence.<br>
<br>
[1] <a href="http://hackage.haskell.org/package/StateVar-1.0.0.0" target="_blank">http://hackage.haskell.org/package/StateVar-1.0.0.0</a><br>
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<br></blockquote></div><br>