I'm pleased to announce that I've uploaded the <i>tardis</i> package to hackage.<br><br><a href="http://hackage.haskell.org/package/tardis-0.2.0.0">http://hackage.haskell.org/package/tardis-0.2.0.0</a><br><br><i>TardisT</i> is a monad transformer that combines the (regular, "forwards") State monad<br>
with the "backwards State monad", allowing you to communicate on two channels of state:<br>one is "backwards travelling" state, while the other is "forwards travelling" state.<br>Its primitives are comparable to the State monad's "get" and "put", but the names<br>
disambiguate which channel they operate on:<br><pre><div class="line" id="LC102"><span class="nf">getPast</span> <span class="ow"> ::</span> <span class="kt">Monad</span> <span class="n">m</span> <span class="ow">=></span> <span class="kt"> TardisT</span> <span class="n">bw</span> <span class="n">fw</span> <span class="n">m</span> <span class="n">fw</span></div>
<span class="nf">getFuture</span> <span class="ow"> ::</span> <span class="kt">Monad</span> <span class="n">m</span> <span class="ow">=></span> <span class="kt"> TardisT</span> <span class="n">bw</span> <span class="n">fw</span> <span class="n">m</span> <span class="n">bw</span><span class="nf"><br>
sendPast</span> <span class="ow"> ::</span> <span class="kt">Monad</span> <span class="n">m</span> <span class="ow">=></span> <span class="n">bw</span> <span class="ow">-></span> <span class="kt">TardisT</span> <span class="n">bw</span> <span class="n">fw</span> <span class="n">m</span> <span class="nb">()</span><span class="nf"><br>
sendFuture</span> <span class="ow">::</span> <span class="kt">Monad</span> <span class="n">m</span> <span class="ow">=></span> <span class="n">fw</span> <span class="ow">-></span> <span class="kt">TardisT</span> <span class="n">bw</span> <span class="n">fw</span> <span class="n">m</span> <span class="nb">()</span><br>
</pre>These primitives allow you to write some very amusing timey-wimey code,<br>but it only works in the presence of sufficient laziness.<br>If you run into an infinite loop, try adding laziness annotations everywhere,<br>
and make sure that what you are trying to do is not a time paradox.<br><br>I've provided both <i>transformers</i> and <i>mtl</i> style modules:<br>Control.Monad.Trans.Tardis contains the transformer,<br>while Control.Monad.Tardis.Class contains the MonadTardis class,<br>
and Control.Monad.Tardis re-exports both as well as the TardisT instance<br>for MonadTardis.<br><br>See the readme on github for more details:<br><br><a href="https://github.com/DanBurton/tardis#readme">https://github.com/DanBurton/tardis#readme</a><br>
<br>As a side note, since the code base is relatively small, it can also serve<br>as a simple demonstration of how to use a cabal flag<br>in conjunction with CPP to selectively include swaths of code<br>(see Control/Monad/Tardis.hs and tardis.cabal).<br>
<br>I'm not entirely sold on the naming conventions for the package (including the package name),<br>and am open to suggestions.<br><br clear="all">-- Dan Burton<br>