Good questions. You can't have a polymorphic typecase like "`extQ` (show :: Show a => a -> String )" because that's not really a *type*case. It is too polymorphic. You can have a polymorphic typecase like "`extQ` ( lshow :: [ a ] -> String )" because that's covered by the SYB2 paper; you need ext1Q (extQ1, depending on version). With SYB3, the second case is easier; it's just an instance. The first case ... still doesn't work ... this time for reasons of the class system. The type system doesn't allow to write one "default" instance for the case that a certain constraint is satisfiable (here: Show a) and to have yet another "default" instance to kick in *otherwise* to the SYB version. It would be great to have typeclass case to deal with this issue. Questions asking for typeclass case pop every now and then. The idea of the example is that you would need to define your own generic show function from scratch. The limitation of SYB1/SYB2 is that you would hit a closed-world-assumption. That's why SYB3! It allows you to add instances to the new generic show, as you go. Because of the lack of typeclass case, you still wouldn't be able to use the normal Prelude show function as default. You can use it instance-per-instance, by adopting each (attractive) show instances to be become instance of the new generic show function. However, you need to clone code because you probably want to re-tie the recursive knot in the new generic function. SYB3 is shipped separately, please use the distribution you pointed to. Simon PJ and I are still undecided regarding the modalities for adding SYB3 to the GHC libraries. Basically, we don't like the idea of having two libraries; SYB3 is more powerful but the combinator types are somewhat more complicated. However, using the separate SYB3 distribution is safe because it comes with TH support and a standalone mini SYB library. Regards, Ralf
-----Original Message----- From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org]
On Behalf Of Johannes Waldmann Sent: Monday, November 28, 2005 7:33 AM To: haskell@haskell.org Subject: [Haskell] Data.Generics question
Dear all, in Data.Generics.Text http://cvs.haskell.org/cgi- bin/cvsweb.cgi/fptools/libraries/base/Data/Generics/Text.hs?rev=1.10 I find this nice example
gshow = ( \t -> "(" ++ showConstr (toConstr t) ++ concat (gmapQ ((++) " " . gshow) t) ++ ")" ) `extQ` (show :: String -> String)
but I couldn't figure out how to use this to declare a useful show instance. I would need something like
`extQ` (show :: Show a => a -> String )
i. e. use Show instance if available, or
`extQ` ( lshow :: [ a ] -> String )
i. e. use another function if argument type is a list type
BTW: I guess the above is related to http://homepages.cwi.nl/~ralf/syb3/ Do the current (6.4) ghc compiler/libraries correspond to what's in that paper? -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- ---- http://www.imn.htwk-leipzig.de/~waldmann/ -------
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