Can I create a representation polymorphic datatype?

[Clinton Mead]

Hi All

I can make a list of unboxed ints like below:

{-# LANGUAGE MagicHash #-}

import GHC.Exts (Int#, Float#)

data IntList = IntTail | IntNode Int# IntList

intListLength :: IntList -> Int
intListLength IntTail = 0
intListLength (IntNode _ rest) = 1 + intListLength rest

I can then make a list of unboxed floats similarly:

data FloatList = FloatTail | FloatNode Int# FloatList

floatListLength :: FloatList -> Int
floatListLength FloatTail = 0
floatListLength (FloatNode _ rest) = 1 + floatListLength rest

But as you can see, this is getting a bit copy-pasta, which is not good. So
instead, lets try this:

newtype GeneralList (a :: l) = Tail | Node a (GeneralList a)

This is not allowed here, I believe because `GeneralList` is expected to
have one representation for all `a`, instead of a representation which
depends on `a`. This is so that if one writes a function:

generalListLength :: GeneralList a -> Int
generalListLength Tail = 0
generalListLength (Node _ rest) = 1 + generalListLength rest

You can't actually compile this into one function, because the relative
location of the "next" pointer can change based on the size of `a`
(assuming `a` is stored first).

However, I can achieve what I want with copy pasting or Template Haskell
hackery.

Is there a way to get GHC to do the copy pasting for me? Or do I have to
make a choice between extra runtime indirection and avoiding ugly code or
having ugly code but avoiding the runtime indirection? A representation
polymorphic list here is something that languages like C++, Rust, and even
C# will handle happily, so Haskell seems behind here unless I'm missing
something,
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