[Haskell-beginners] Typeclass question

[Stephen Tetley]

Hi Patrick

As you guessed, adding UndecidableInstances pragma would make it work.

Loosely speaking, the type of your instance 'contains' all the types
that the class can use anyway. Hopefully, someone more learned than me
will give a proper definition and add an explanation of the Paterson
Conditions.

http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#undecidable-instances


But why not just a function:

toInt :: Integral a => a -> Int
toInt = fromIntegral

Of course just a function won't be ideal if you were wanting to have a
special instance say for Integer and all other instances to use the
general version. There's more to say about this in the context of
Generics, but that's a somewhat advanced topic...

Best wishes

Stephen