Hello Mike, Typeable is essentially monomorphic, so I don't think you can use mkT with a polymorphic function... Cheers, Pedro On Thu, Oct 2, 2014 at 2:17 PM, Michael Sperber wrote:

I'm a casual Haskell user, and am trying to use scrap-your-boilerplate to write a transformation - and failing. The rub is that the base function is polymorphic, boiling down to this:

data Foo a = Foo a

bar :: Foo a -> Foo a bar x = x

Now, I'm trying to use SYB like so:

foo :: Typeable a => a -> a foo = mkT bar

... but I get:

Could not deduce (Typeable a0) arising from a use of ‘mkT’ from the context (Typeable a) bound by the type signature for foo :: Typeable a => a -> a at foo.hs:... The type variable ‘a0’ is ambiguous Note: there are several potential instances:

Is there any way I could make this work?

Any help would be much appreciated!

-- Regards, Mike

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On 02/10/14 16:17, Michael Sperber wrote:

I'm a casual Haskell user, and am trying to use scrap-your-boilerplate to write a transformation - and failing. The rub is that the base function is polymorphic, boiling down to this:

data Foo a = Foo a

bar :: Foo a -> Foo a bar x = x

Now, I'm trying to use SYB like so:

foo :: Typeable a => a -> a foo = mkT bar

... but I get:

Could not deduce (Typeable a0) arising from a use of ‘mkT’ from the context (Typeable a) bound by the type signature for foo :: Typeable a => a -> a at foo.hs:... The type variable ‘a0’ is ambiguous Note: there are several potential instances:

Is there any way I could make this work?

Any help would be much appreciated!

I assume you want to apply bar to all monomorphic instances of Foo a, and not just a specific one (which would be much easier). I think the following should work (with GHC 7.8). But you definitely should test it — with this kind of code, the fact that it compiles doesn't really mean much. {-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable import GHC.Prim data Foo a = Foo a deriving Typeable bar :: Foo a -> Foo a bar x = x getTyCon :: TypeRep -> TyCon getTyCon = fst . splitTyConApp fooTyCon :: TyCon fooTyCon = getTyCon $ typeRep (Proxy :: Proxy Foo) foo :: Typeable a => a -> a foo x | thisTyCon == fooTyCon = unsafeCoerce# (bar (unsafeCoerce# x :: Foo Any)) | otherwise = x where thisTyCon = getTyCon $ typeOf x Roman