On Sun, Jun 28, 2009 at 12:41 AM, Henry Laxen wrote:

Dear Group,

If any of you are struggling with understanding monads, I've tried to put together a pretty through explanation of what is behind the Reader monad.  If you're interested, have a look at:

http://www.maztravel.com/haskell/readerMonad.html

Nice post. I didn't find how to add comments on your blog, so I post them here:

Areas of Confusion

1. What is the relationship between the Reader on the left hand side of the equals sign in the newtype definition and the Reader on the right hand side? 2. Why is there a Record field on the right hand side? 3. What is that r -> a doing there?

1) Reader on the left hand side be called a type constructor, Reader on the right hand side be called a data constructor, in Haskell 98 Report. You call them type definition and instance constructor, respectively, I'm not sure it's a good idea, or it is right. bug in the explanation: what you use to make something and instance of a Reader (left hand side) -> what you use to make something an instance of a Reader (left hand side) 2) runReader be called a selector function in Haskell 98 Report. 3) (->) is a type constructor, so r -> a is a function type. I used found 'instance Monad ((->) r)' hard to understand, but by follow the hit given by Brent Yorgey, i.e. the data constructor for type constructor (->) is called lambda abstraction, I found I can understand them by type inference. I have written a post about how I figure it out, maybe you want take a look: http://leeduhem.wordpress.com/2009/06/07/understanding-monad-instance-by-typ... bug in the explanation after (Reader f1) >>= f2 = Reader $ \e -> runReader (Reader b) e: Reader b is a function that takes and e and returns a c, -> Reader b is a function that takes an e and returns a c, lee