On 12/12/06, Brandon S. Allbery KF8NH wrote:

Read the thread more completely; ((->) a) is indeed a monad, but you need to include an additional module to get the definition.

Ahh, okay I did see that. I thought that importing Control.Monad would be sufficient. What I didn't realize is that even though the documentation for Control.Monad says that (->(r)) is an instance, it must not be defined there. Importing Control.Monad.Reader indeed does make it work, thanks for pointing that out again. So I guess my next question would be, looking here http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html How do I figure out that the instance declaration is actually in the Reader module? Is it just because the (->) constructor is special -- or just not hyperlinked? -- Lou.

On 12/12/06, Cale Gibbard wrote:

It's pretty subtle, but the Haddock there does list an instance of MonadReader r ((->) r).

Yup, I see that one. What I meant was that in the Mondad package it says that ((->) r) is an instance of Monad, but I can't find way to get from there to Reader.

In newer GHC's the instance is also found in Control.Monad.Instances. In my opinion, it belongs in the Prelude. :)

Hmm. Yeah, it seems like there aren't many monad instances imported into prelude. Just [], Maybe and IO. I'll have to keep that in mind. Thanks for your help everybody. -- Lou.