I don't know what to actually do with this after putting it in a *.lhs file.
You can :load *.lhs into ghci the same way you load .hs-files.
I'm not sure why you were getting the ambiguous type errors. I didn't:
test1 :: StateT Integer Identity (Integer, Integer)
test2 :: StateT [Char] Identity ([Char], [Char])
test3 :: StateT Integer (StateT [Char] Identity) (Integer, [Char])
test5 :: StateT Integer IO ()
test7 :: StateT Integer (StateT [Char] Identity) (Integer, [Char])
Hope that helps.
Arseniy.
On 2 January 2012 06:03, 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!!!
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