[Haskell-cafe] Grok Monad Transformers - some help needed

[Steve Horne]

I'm having another go at figuring out Monad Transformers, starting at 
the same point I've started and stopped the last couple of times. That's 
this tutorial...

http://blog.sigfpe.com/2006/05/grok-haskell-monad-transformers.html

Onion layers, lift etc - I get that. But I've never actually got 
anything to work - and therefore never built the confidence to move on.

Problem 1 - this is literate Haskell. I've tried feeding it to haddock - 
all I get is an error about it lacking a main. I don't know what to 
actually do with this after putting it in a *.lhs file.

No big deal, but...

Problem 2 - even cutting the code out, shoving it in a *.hs file, and 
:loading it into GHCi, I still get a lot of errors.

For the functions test1 and test2, the fixes were explicit type 
signatures. Easy enough to figure out...

   test1 :: State Int (Int, Int)
   test2 :: State String (String, String)

I guess the basic issue is the same with test3 (and though I haven't 
tried the others today, probably test5 and test7 too). The trouble there 
is that I don't know what those type signatures should be because I 
don't know that much about monad transformers and how they can work. I 
can see what's going on in the body of test3, but that is a very small 
amount of understanding - the type signatures are important. And I can't 
use :type because GHCi is rejecting the code.


In short... HELP!!!