Re: [Haskell-cafe] Re: Combining Wouter's expressions with extensible records

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 wrote:

2008/7/10 Ron Alford :

...

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