I'm not exactly a Haskell beginner but haven't grasped much of it yet. I have a feeling the answer to your question has something to do with the fact that the monad transformer library provides instances that allow the use of, for example, "get" and "put" in any transformed monad that has a StateT somewhere in it. Or throwError in anything that has an ErrorT in it. In other words, you don't need to lift everything into the right level. <br>
<br>Okay, that's a crude way of putting it, but someone will probably come along and clarify.<br>Dennis<br><br><div class="gmail_quote">On Sat, Aug 25, 2012 at 1:15 AM, Song Zhang <span dir="ltr"><<a href="mailto:vxanica@gmail.com" target="_blank">vxanica@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br clear="all"><div>When I use a State Monad transformer to combine with a Writer Monad</div><div>StateT s (Writer w) a. is it different from composition of State Monad and Writer Monad. It is State s (Writer w a) ?</div>
<div>StateT is defined as (s -> m (a, s)), so StateT s (Writer w) a can be regarded as (s -> Writer w a) , which is (s -> ((a,w),s)</div><div>and on the other hand State s (Writer w a) is (s -> ((a,w),s). I suppose the are similar and if so, what is the point we still get Monad transformers? Thanks</div>
<div><br></div><div><br></div><br>
<br>_______________________________________________<br>
Beginners mailing list<br>
<a href="mailto:Beginners@haskell.org">Beginners@haskell.org</a><br>
<a href="http://www.haskell.org/mailman/listinfo/beginners" target="_blank">http://www.haskell.org/mailman/listinfo/beginners</a><br>
<br></blockquote></div><br>