Re: [Haskell-cafe] Need help with scrap-your-boilerplate

I think the missing bit is simply you need to add a deriving Typeable to your data Foo declaration On Thu, Oct 2, 2014 at 2:52 PM, José Pedro Magalhães wrote:

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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