Documentation

Writing Unbox instances for new data types is tedious and formulaic. More often than not, there is a straightforward mapping of the new type onto some existing one already imbued with an Unbox instance. The example from the vector package represents Complex a as pairs (a, a). Using derivingUnbox, we can define the same instances much more succinctly:

derivingUnbox "Complex"
    [t| ∀ a. (Unbox a) ⇒ Complex a → (a, a) |]
    [| \ (r :+ i) → (r, i) |]
    [| \ (r, i) → r :+ i |]

Requires the MultiParamTypeClasses, TemplateHaskell, TypeFamilies and probably the FlexibleInstances LANGUAGE extensions. Note that GHC 7.4 (but not earlier nor later) needs the Vector and MVector class method names to be in scope in order to define the appropriate instances:

#if __GLASGOW_HASKELL__ == 704
import qualified Data.Vector.Generic
import qualified Data.Vector.Generic.Mutable
#endif

Consult the sanity test for a working example.