What's odd is that it works directly (typeOf ... (Expr (f :+: g)) returns a type), but if you enclose the expression in a list, it fails with Prelude.undefined. Do I also need a custom instance for Typeable [Expr ...] ? (See previous message for code)
-Ron On Fri, Jul 11, 2008 at 1:56 AM, Antoine Latter <[EMAIL PROTECTED]> wrote: > 2008/7/10 Ron Alford <[EMAIL PROTECTED]>: >> Ok, I'm closer, but I'm running into a problem with typeOf and lists, >> of all things: >> *WouterTest> typeOf (eVar "v" :: TermExpr) >> Planning.Wouter.Expr (Planning.Wouter.:+: WouterTest.Const WouterTest.Var) >> *WouterTest> typeOf ([eVar "v"] :: [TermExpr]) >> *** Exception: Prelude.undefined >> >> I'm pretty sure this is the culprit for getName: >> *WouterTest> getName testNamed >> "*** Exception: Prelude.undefined >> >> Any hints? >> >> Also, anyone have hints for how to get automatic derivation of Data (Expr f) >> ? >> I don't want to proliferate the last lines: >> deriving instance Data (Expr (And :+: Atomic (Expr (Const :+: Var)))) >> deriving instance Data (Expr (Const :+: Var)) >> >> > > > I screwed up the example code - it typechecks, but it'll fail at runtime. > > If you say: > >> Inr x = undefined > > and then try to pass 'x' off to another function, you're trying to > evaluate the "undeifned", which is a runtime error. > > You'll want something more like: > > typeOf1 in@(InR f) = [...] > where InL f = (InL undefined) `asTypeOf` in > > This is approaching silliness, but I've tested the code this time > around - so it should even work. > > -Antoine > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
