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

[José Pedro Magalhães]

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 <sperber at deinprogramm.de>
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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