Or, if people have easy-enough extensible records that /will/ work with funky types, I'd be happy to use those! -Ron On Thu, Jul 10, 2008 at 10:29 AM, Ron Alford wrote:

On Wed, Jul 9, 2008 at 11:01 PM, Antoine Latter wrote:

...

It isn't immediately obvious to me that the "Typeable" family of classes deal at all with higher-kinded type constructors, but I didn't look that hard.

Yes, that's what I'm worried about. For people's fun and amusement, I've attached the file. The trailing comments show what I'm trying to accomplish (getName, setName, and so forth).

-Ron

On Thu, Jul 10, 2008 at 2:15 PM, Ron Alford wrote:

I'm making progress, but how would I make the following a Typeable instance: data (f :+: g) e = Inl (f e) | Inr (g e) deriving Eq

Here is what I'm using for Expr: data Expr f = In (f (Expr f)) instance Typeable1 f => Typeable (Expr f) where typeOf (In x) = mkTyConApp (mkTyCon "Data.Trie.General.ListGT") [typeOf1 x]

I don't think I can use this for ':+:', because the typeOf instance only has access to a member of one type at a time. This may be similar to a definition of Typeable2 for Either, but I can't find an example to follow for that.

Maybe something like: instance (Typeable1 f, Typeable1 g) => Typeable (f :+: g) where typeOf in@(InL f) = (some function of 'f' and 'g') where InR g = undefined `asTypeOf` in typeOf in@(InR g) = (some function of 'f' and 'g') where InL f = undefined `asTypeOf` in would work? -Antoine

This is a bit similar to Either. Is there a way to see the generated instance code for deriving instance Data Either ? On Thu, Jul 10, 2008 at 6:38 PM, Ron Alford wrote:

Close - it compiles now! I made a minor change, going to Typeable1 instead of Typeable:

instance (Typeable1 f, Typeable1 g) => Typeable1 (f :+: g) where typeOf1 l@(Inl x) = mkTyConApp (mkTyCon "Planning.Wouter.:+:") [typeOf1 x, typeOf1 y] where (Inr y) = undefined `asTypeOf` l typeOf1 r@(Inr y) = mkTyConApp (mkTyCon "Planning.Wouter.:+:") [typeOf1 x, typeOf1 y] where (Inl x) = undefined `asTypeOf` r

Except this gives me a runtime error: *WouterTest> getName testNamed "*** Exception: Prelude.undefined

The only thing I can think of is to have a class that gives default values to type - ick!

-Ron Alford

On Thu, Jul 10, 2008 at 4:16 PM, Antoine Latter wrote:

...

On Thu, Jul 10, 2008 at 2:15 PM, Ron Alford wrote:

...

I'm making progress, but how would I make the following a Typeable instance: data (f :+: g) e = Inl (f e) | Inr (g e) deriving Eq

Here is what I'm using for Expr: data Expr f = In (f (Expr f)) instance Typeable1 f => Typeable (Expr f) where typeOf (In x) = mkTyConApp (mkTyCon "Data.Trie.General.ListGT") [typeOf1 x]

I don't think I can use this for ':+:', because the typeOf instance only has access to a member of one type at a time. This may be similar to a definition of Typeable2 for Either, but I can't find an example to follow for that.

Maybe something like:

instance (Typeable1 f, Typeable1 g) => Typeable (f :+: g) where typeOf in@(InL f) = (some function of 'f' and 'g') where InR g = undefined `asTypeOf` in

typeOf in@(InR g) = (some function of 'f' and 'g') where InL f = undefined `asTypeOf` in

would work?

-Antoine

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

On Fri, Jul 11, 2008 at 7:07 AM, Ron Alford wrote:

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)

The problem is that the List instance is playing the same dirty tricks with it's 'typeOf' implementation as we are: it's asking us the type of one of the list elements by passing in "undefined" to our "typeOf1" implementation. And then your "typeOf1" implementation tries to do pattern matching on undefined. Here is what will work: instance (Typeable1 f, Typeable1 g) => Typeable1 (f :+: g) where typeOf1 x = mkTyConApp (mkTyCon "Planning.Wouter.:+:") [typeOf1 left, typeOf1 right] where (Inr right) = Inr undefined `asTypeOf` x (Inl left) = Inl undefined `asTypeOf` x Now we never do pattern matching on our input. This has been pretty educational. -Antoine