Nicolas Frisby wrote:
Your intended behavior for Reader indicates stateful computational features. The "read later" roughly expands to "be read by some monadic action on the rhs of a >>=" as in
(read >>= \x -> read {-this is later-} >>= ...)
Recognizing the stateful nature gives you two options:
1) Implement yours as a restricted version of Control.Monad.State
type ReaderAC = State readAC = get >>= \x -> put (tail x) >> return (head x)
I'll at least have a look at that...
2) or (as Isaac demonstrated) look to the definition of Control.Monad.State.State for guidance own how to structure your monad.
Ah yes. I just sat down to do this, and... oh look. There in the first few lines, a non-standard language extension that I don't understand. ...which neatly brings us back to the reason I'm trying to reimplement this by hand in the first place. ;-)