Simon Marlow wrote:
Dan Licata wrote:
Simon PJ and I are implementing view patterns, a way of pattern matching against abstract datatypes, in GHC. Our design is described here:
http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns
If you have any comments or suggestions about this design, we'd love to hear them. You can respond to this list (and we can take it to haskell-cafe if the thread gets long) or, if you prefer, directly to me.
At the risk of being a spoil-sport, I have a somewhat negative take on view patterns. Not because I think they're particularly bad, but because I don't think they're significantly useful enough to warrant adding to the language, at least if we also have pattern guards.
All of the examples on the wiki page can be written in the same number of lines, sometimes with fewer characters, using pattern guards or some other existing Haskell idiom (e.g. the bit parsing example is much more nicely expressed using a bit-parsing monad). I believe adding yet another idiom will be detrimental: too much choice is bad.
I agree that only few examples from the wiki page are that compelling. Nevertheless, I like view patterns since they can be put to really good use http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns#UsesofViews Views are especially useful for Data.Graph. Also, I favor views instead of pattern guards. However, I don't like the current proposal, mainly because it doesn't have the "Transparent ordinary Patterns"-feature. Also, I consider definitions like foldr f z [] = z foldr f z (x : foldr f z -> xs) = x `f` xs abuse.
Data.Set, Data.IntSet, Data.Map, Data.IntMap Data.Graph, Data.Sequence, Data.Bytestring, Data.Array, Data.HashTable, Data.Ratio(Rational)
So I don't think there's an overwhelming amount of stuff that would change if we had view patterns. In GHC itself most of our data structures are already abstract too.
While the implementation of the abstract data structures themselves is unlikely to change, views make it much easier to use them. I think it would be a big win to have ByteStrings or Data.Sequence pattern matched like ordinary lists and I think that Data.Graph will blossom with proper view patterns. Regards, apfelmus