RE: composed contexts
class (Monad m, Monad (t m)) = MonadT t m where lift :: m a - t m a instance (Monad m, Monad (StateT s m)) = MonadT (StateT s) m where lift m = \s - m = \x - return (s,x) If the definitions from the paper can be turned into valid Haskell 98 w.l.o.g. now, then I'm happy. No, neither can, and that's not going to change for Haskell 98. There is a raft of generalisations to the class system (implemented in Hugs and GHC) but which would require a much bigger upheaval to Haskell 98. They are all going to be in Haskell 2 (IMHO). But for H98, sorry. S
RE: composed contexts
class (Monad m, Monad (t m)) = MonadT t m where lift :: m a - t m a instance (Monad m, Monad (StateT s m)) = MonadT (StateT s) m where lift m = \s - m = \x - return (s,x) If the definitions from the paper can be turned into valid Haskell 98 w.l.o.g. now, then I'm happy. No, neither can, and that's not going to change for Haskell 98. There is a raft of generalisations to the class system (implemented in Hugs and GHC) but which would require a much bigger upheaval to Haskell 98. They are all going to be in Haskell 2 (IMHO). But for H98, sorry. Oops. Sorry, I forgot that the MPC stuff, etc. wasn't going into Haskell 98. I guess with all the crossfire I'm getting confused about H98 and H2. --FC
RE: composed contexts
Simon Peyton-Jones [EMAIL PROTECTED] writes - The simple-context restriction. ... My default position is not to change. Question: who, apart from Ralf, has actually tripped over the lack of contexts of the form (C (a t1 .. tn)) in Haskell 1.4? Is their lack a real problem in practice? Are you talking about contexts in general, or only contexts in function signatures? For me, the most powerful argument in favor of generalizing contexts is the possibility of defining monad transformers, as described in "Monad Transformers and Modular Interpreters" by Liang, Hudak and Jones. (Because I love this paper so much. :) I think I convinced you of this once before, when MPC support was being added to GHC. One has applications in class declarations, and non-variable arguments in instance declarations: class (Monad m, Monad (t m)) = MonadT t m where lift :: m a - t m a instance (Monad m, Monad (StateT s m)) = MonadT (StateT s) m where lift m = \s - m = \x - return (s,x) If the definitions from the paper can be turned into valid Haskell 98 w.l.o.g. now, then I'm happy. --FC
