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

[Carter Schonwald]

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 <dreixel at gmail.com>
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 <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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>
>
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