Hi Neil, thanks for the response. On Mon, May 30, 2011 at 8:48 PM, Neil Mitchell <ndmitch...@gmail.com> wrote:
> Hi John, > > >> > While I'm on the topic, I recently wrote a tool that wanted to > >> > traverse deep data structures as produced by haskell-src-exts. ?I > >> > wound up with about 50 lines of case expressions and around the time > >> > my hands were literally beginning to hurt decided that enough was > >> > enough and I should try a generic approach. ?I heard uniplate was > >> > pretty easy to use, and was pretty pleased to turn the entire thing > >> > into a single line. ?It took me a little longer to figure out I needed > >> > to use universeBi since all the examples were monotyped, but once I > >> > did it Just Worked. ?Amazing. ?So thanks again! ?And maybe you could > >> > mention universeBi in the instant introduction? > >> > >> Yes, I probably should - I'll try and get to that. Of course, I'd also > >> happily accept a patch against > >> http://community.haskell.org/~ndm/darcs/uniplate > >> > >> I use Uniplate inside HLint, and it's invaluable - there are a lot of > >> times when List Comp + universeBi really hits the spot. > > > > Does Uniplate include an instance for: > >> instance Uniplate a => Biplate [a] a > > No, it only includes: > > instance Biplate [Char] Char where > biplate (x:xs) = plate (:) |* x ||* xs > biplate x = plate x > > I am slightly curious why I didn't include the more general a instead > of Char version, but perhaps it doesn't quite work - polymorphic > versions of the Direct instances can have problems if you pick weird > types. I'll have a think, and if it does always work, I'll include it. > > Note that if you use the Typeable or Data versions this instance is > automatically available. In practice I almost always end up using the > Data versions of Uniplate, they require no instance definitions are > are good to get started with - you can switch to Direct only if you > need the additional performance. > I started with Data, but writing the Direct instance was so simple that I didn't see a reason not to do it. My type doesn't have many constructors yet though, and several of them aren't recursive, so maybe it was easier than normal. John
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