On Thu, Feb 28, 2008 at 7:28 AM, Aaron Altman
newtype CircularFuncList funcList arg = CircularFuncList (funcList -> arg -> (arg, funcList))
instance Monad (CircularFuncList funcList arg) where return a = CircularFuncList (\funcList a -> (a, funcList)) CircularFuncList currentFuncList currentArg >>= argTransform = let result = argTransform $ (head currentFuncList) currentArg newFuncList = map argTransform $ shiftActionList currentFuncList in CircularFuncList newFuncList result
So some standard monads are: instance Monad [] where ... instance Monad Maybe where ... Note how they are all "missing" a type argument, i.e. not: instance Monad (Maybe Int) where ... So your monad needs to be, say: instance Monad (CircularFuncList funcList) where ... But see below...
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I get an error that CircularFuncList funcList arg has kind * while Monad is looking for * -> *. This is a first attempt so it may be I'm a ways off from a monad that implements a circular list of functions. That is the goal though. What advice can you offer?
I'm not sure how CircularFuncList is a monad. In fact it doesn't even look like a Functor (because arg appears both as an argument and a return in your data type definition). How do you intend your monad to be used? That is, when a user writes: foo :: CircularFuncList func a foo = do x <- ... y <- ... x ... What are the primitive operations (the ...s here) and what does it mean to sequence them like this? Describing this in words might help you implement this, or more likely, help you realize that a monad isn't what you thought it is :-) Luke