Nice. I've written similar stuff a couple of times before, but the formulation using Maybe and modify definitely solves some problems I started to notice as I used it on bigger structures. However, it might be better to separate a class of "never failing" references, where the reader is guaranteed to succeed, from "potentially failing" references which give a Maybe value. I'd also not use the names "get" and "modify" because they are already used by MonadState. I always used "frGet", "frSet", and "frModify", and then tended to not use them directly as I was generally working in a state monad. Here's a few more primitives that I found quite useful:
fetch :: MonadState s m => FRef s a -> m (Maybe a) fetch ref = liftM (frGet ref) get
(=:) :: MonadState s m => FRef s a -> a -> m () r =: v = modify (frSet r v)
($=) :: MonadState s m => FRef s a -> (a -> a) -> m () r $= f = modify (frModify r f)
You should package this up and put it on hackage.
-- ryan
On Thu, Sep 4, 2008 at 4:51 PM, Tim Newsham
I'm playing with functional references and looking for some feedback on a small FRec library I put together:
http://www.thenewsh.com/~newsham/FRef.hs
Novel (I think) is that the library is applied to some data accesses that are not normally covered by functional references -- ie. extracting words or replacing words in a string or values in an association list. I'm hoping it will provides a useful framework for editing complex values such as data embedded in Base64 cookies in an HTTP header.
Tim Newsham http://www.thenewsh.com/~newsham/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe