BinaryIndexedListCoerce.hs

Prelude> :set -XGADTs
Prelude> :set -XEmptyDataDecls 
Prelude> :{
Prelude| data List l a where
Prelude|   Nil :: List Empty a
Prelude|   (:-) :: a -> List l a -> List NE a
Prelude| :}
Prelude> 
Prelude> :set -XStandaloneDeriving 
Prelude> deriving instance (Show a) => Show (List l a)
Prelude> (12::Int) :- ((13::Int) :- Nil)
(:-) 12 ((:-) 13 Nil)

Prelude> import Unsafe.Coerce 
Prelude Unsafe.Coerce> let indexedAs = (12::Int) :- ((13::Int) :- Nil)
Prelude Unsafe.Coerce> unsafeCoerce indexedAs :: [Int]
[12,13]
Prelude Unsafe.Coerce> let indexedAs = (Nil :: List Empty Int)
Prelude Unsafe.Coerce> unsafeCoerce indexedAs :: [Int]
[]
Prelude Unsafe.Coerce> let indexedErrs = (12::Int) :- ((undefined::Int) :- Nil)
Prelude Unsafe.Coerce> unsafeCoerce indexedAs :: [Int]
Prelude Unsafe.Coerce> head $ (unsafeCoerce indexedErrs :: [Int])
12
Prelude Unsafe.Coerce>