Generalize op2 type parameter.

[dmjio]

Generalizes op2 return type so lookup can unify.

src/ArrayFire/Index.hs:52:22: error: [GHC-25897]
    • Couldn't match type ‘a’ with ‘Int’
      Expected: Array a
        Actual: Array Int
      ‘a’ is a rigid type variable bound by
        the type signature for:
          ArrayFire.Index.lookup :: forall a.
                                    Array a -> Array Int -> Int ->
Array a
        at src/ArrayFire/Index.hs:(44,1)-(51,12)
    • In the second argument of ‘op2’, namely ‘b’
      In the first argument of ‘($)’, namely ‘op2 a b’
      In the expression:
        op2 a b $ \ p x y -> af_lookup p x y (fromIntegral n)
    • Relevant bindings include
        a :: Array a (bound at src/ArrayFire/Index.hs:52:8)
        lookup :: Array a -> Array Int -> Int -> Array a
          (bound at src/ArrayFire/Index.hs:52:1)
   |
52 | lookup a b n = op2 a b $ \p x y -> af_lookup p x y (fromIntegral
n)
   |                      ^
Error: cabal: Failed to build arrayfire-0.7.0.0.