<div dir="ltr">Hi,<div><br></div><div style>just for the record, this split is already made in `MonadLib` (e.g., monadLib's `collect` method, which corresponds to what Petr called `contained`, is in its own class called `RunWriterM`; there are similar classes for readers and exceptions).</div>
<div style><br></div><div style>-Iavor</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Jan 28, 2013 at 9:49 AM, Edward Kmett <span dir="ltr"><<a href="mailto:ekmett@gmail.com" target="_blank">ekmett@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div>To be clear, I'm not yet making a concrete proposal for breaking up MonadReader and MonadWriter. Doing so would be fairly painful for at least some users in the short term. There are a lot of custom MonadReader and MonadWriter instances out there, and it also affects the signature of all combinators that consume these types that use pass, local or listen. That said the latter is probably only about 10% of the code that uses ask/tell. Moreso in the reader case than the writer case.</div>
<div><br></div><div>Given the number of users who have to support multiple platform releases, a concrete proposal would have to address the upgrade path, possibly including making MonadLocal and MonadListen stub subclasses a version or so early and then moving methods over later, etc. so users can upgrade appropriately, and library authors can follow suit, permitting both an upgrade path or deciding to rip the bandaid off in one go. The logistics of this are somewhat convoluted, and if we're going to go through that pain, it may be worth addressing other concerns at the same time.</div>
<span class="HOEnZb"><font color="#888888"><div><br></div><div>-Edward<br></div></font></span><div><div class="h5"><div><br>On Jan 28, 2013, at 12:10 PM, Petr P <<a href="mailto:petr.mvd@gmail.com" target="_blank">petr.mvd@gmail.com</a>> wrote:<br>
<br></div><blockquote type="cite"><div><div dir="ltr">Edward,<div class="gmail_extra"><br><div class="gmail_quote">2013/1/28 Edward A <span>Kmett</span> <span dir="ltr"><<a href="mailto:ekmett@gmail.com" target="_blank"><span>ekmett</span>@gmail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="auto"><div>I've often wanted to remove the Monoid constraint, so I'm a strong +1 on the first one.</div>
<div><br></div><div>The second one I'm -1 on, given that it can be expressed with the existing combinators and due in small part to a more ideological concern:</div><div><br></div><div>Over time I've come to view cramming pass/listen and local into the respective MonadWriter and MonadReader classes as a mistake. If we had the hierarchy to do over, I'd probably want them split into separate subclasses. This would permit more instances involving Cont, logging to disk, etc.</div>
</div></blockquote><div><br></div><div>I strongly support your idea. Currently, <span>MonadWriter</span> and <span>MonadReader</span> just reflect <span>ReaderT</span> and <span>WriterT</span> so that we can make <span>monad</span> stacks. But they don't reflect the ideas - a <span>monad</span> we write to or read from, without any additional constraints. I'd really love to have them split up: A more general class that has just 'tell' and 'writer', and another that also has 'listen' and 'pass' (perhaps also 'contained').</div>
<div>I tried to discuss it in <span>haskell</span>-cafe once <<a href="http://www.mail-archive.com/haskell-cafe@haskell.org/msg101756.html" target="_blank">http://www.mail-archive.com/<span>haskell</span>-cafe@<span>haskell</span>.org/msg101756.html</a>>, but without any feedback on the idea, so I thought everybody is happy with the current state.</div>
<div><br></div><div>In this case, it seems that nothing important would break by this change, because IMHO there are very very little custom instances of MonadWriter/Reader. I'm also a big fan of this proposal: <a href="http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances" target="_blank">http://<span>hackage</span>.<span>haskell</span>.org/<span>trac</span>/<span>ghc</span>/wiki/<span>DefaultSuperclassInstances</span></a></div>
<div>If it were implemented and standardized, we could refactor type classes without breaking existing code (or breaking it just very little).</div><div>[In particular, it'd be possible to make <span>Monad</span> a subclass of Applicative, something I've always regretted we don't have.]</div>
<div><br></div><div>Other than that, I'm not sure how adding "contained" goes against this idea. If we split MonadWriter into two type classes, "contained" would simply go into the one with "listen" and "pass".</div>
<div><br></div><div>Best regards,</div><div>Petr</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div dir="auto"><div><br></div><div>Also, the former can be done purely within the mtl, while the latter drags transformers into it.</div></div></blockquote><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<div dir="auto"><div><span><font color="#888888"><br><br>-Edward</font></span></div><div><div><div><br>On Jan 27, 2013, at 3:30 PM, Petr P <<a href="mailto:petr.mvd@gmail.com" target="_blank">petr.mvd@gmail.com</a>> wrote:<br>
<br></div><blockquote type="cite"><div><div dir="ltr"> Dear maintainers,<br><div class="gmail_quote"><div dir="ltr"><div><br></div><div>I have two suggestions for <span style="background-color:yellow">MonadWriter</span>:</div>
<div><br></div><div>(1) Remove the "<span style="background-color:yellow">Monoid</span> w" constraint from the definition.</div>
<div><br></div><div>The <span style="background-color:yellow">constraints</span> prevent creating new instances of the class that have only an implied <span style="background-color:yellow">monoid</span>. For example, I needed to create a simple writer which always stores the last written element. I had to wrap it into Last, which was a nuisance for users of my library. Without the constraint, my instance would be quite simpler and still satisfying all the laws. There are many other similar use cases, like counting the number of written values (and disregarding their actual content) etc.</div>
<div><br></div><div>The constraint is meant to ensure that instances of that class obey the <span style="background-color:yellow">monad</span> laws. But it's not the responsibility of a type class that its instances satisfy the laws. They could violate them even without this <span style="background-color:yellow">constraints</span>. Instead, this constraint should be specified (and it is) in the definition of their instances.</div>
<div><br></div><div>It has been discussed in <span style="background-color:yellow">haskell</span>-cafe <<a href="http://www" target="_blank">http://www</a>.<span style="background-color:yellow">haskell</span>.org/<span style="background-color:yellow">pipermail</span>/<span style="background-color:yellow">haskell</span>-cafe/2012-December/thread.html#105088> with <span style="background-color:yellow">arguments</span> for and against.</div>
<div><br></div><div><br></div><div>(2) Add</div><div><br></div><div><font face="courier new, monospace"> -- | @contained m@ executes the action @m@ in a contained environment and</font></div><div>
<font face="courier new, monospace"> -- returns its value and its output. The current output is not modified. </font></div><div><font face="courier new, monospace"> contained :: m a -> m (a, w)</font><br></div>
<div><br></div><div>to MonadWriter.</div><div><br></div><div>This generalizes "pass" and "listen" and has it's a sort of inverse to "writer" with these simple laws:</div>
<div><div><br></div><div><font face="courier new, monospace"> writer <=< contained = id</font></div><div><font face="courier new, monospace"> contained . writer = return</font><br></div></div><div>
<br></div><div>It seems as a understandable set of laws that its instances should obey.</div><div><br></div><div>It also expresses the same concept as "runWriterT" does, but inside the type class. In particular, for "WriterT" we have</div>
<div><font face="courier new, monospace"><br></font></div><div><div><font face="courier new, monospace"> contained :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w)</font></div><div><font face="courier new, monospace"> contained = lift . runWriterT</font></div>
</div><div><br></div><div>Current instances won't be affected as "contained" can be expressed using "pass" and "listen" (and vice versa).</div><div>Full details available at</div>
<div><a href="https://github.com/ppetr/mtl/compare/9cc075cfaa40028a54f1dedf62af67e912f9fd42...master" target="_blank">https://github.com/ppetr/mtl/compare/9cc075cfaa40028a54f1dedf62af67e912f9fd42...master</a><br></div><div>
[There "contained" is expressed without the "Monoid w" constraint as suggested in (1). If we keep the constraint, "contained" can be expressed more simply as</div><div><font face="courier new, monospace"> containde k = pass (listen k >>= \x -> return (x, const mempty</font><font face="courier new, monospace">))</font>.</div>
<div><br></div><div>Also, "contained" isn't probably a good name, I just couldn't think of anything better.]</div><div><br></div><div><br></div><div> Best regards,</div>
<div> Petr Pudlak</div></div>
</div><br></div>
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