## Traversable can't be derived for datatypes with unboxed arguments

Unlike `Functor`

and `Foldable`

, `Traversable`

cannot be derived for datatypes that contain arguments with unboxed types. A simple example:

```
{-# LANGUAGE DeriveTraversable, MagicHash #-}
import GHC.Prim (Int#)
data IntHash a = IntHash Int# deriving (Functor, Foldable, Traversable)
```

The generated `Traversable`

instance reveals the issue:

```
instance Traversable IntHash where
traverse f (IntHash a1) = fmap IntHash (pure a1)
```

```
Couldn't match kind `*' with `#'
When matching types
a0 :: *
Int# :: #
Expected type: a0 -> IntHash b
Actual type: Int# -> IntHash b
In the first argument of `fmap', namely `IntHash'
In the expression: fmap IntHash (pure a1)
When typechecking the code for `traverse'
in a derived instance for `Traversable IntHash':
To see the code I am typechecking, use -ddump-deriv
```

We have to avoid calling `pure`

on `a1`

, since `pure`

expects an argument with a `*`

-kinded type, not a `#`

-kinded one.

One way to fix this would be restructuring the derived `traverse`

implementation such that arguments which do not mention the last type parameter are moved to the function initially lifted with `pure`

, and doing nothing with them later. To better articulate what I mean, envision something like this:

`data IntHash2 a = IntHash2 Int# a (IntHash2 a) Int deriving (Functor, Foldable)`

Then a derived `Traversable`

instance that would type-check (and kind-check) would be:

```
instance Traversable IntHash2 where
traverse f (IntHash2 a1 a2 a3 a4) =
pure (\x2 x3 -> IntHash2 a1 x2 x3 a4) <*> f a2 <*> traverse f a3
```

Conceptually, this doesn't sound hard to implement. The tricky part is figuring out how much of the existing `Functor`

/`Foldable`

/`Traversable`

deriving machinery would need to be tweaked to make this work.