Copyright	© 2012−2015 Liyang HU
License	BSD3
Maintainer	vector-th-unbox@liyang.hu
Stability	experimental
Portability	non-portable
Safe Haskell	Trustworthy
Language	Haskell2010

Data.Vector.Unboxed.Deriving

Description

Synopsis

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.

derivingUnbox #

Arguments

:: String	Unique constructor suffix for the MVector and Vector data families
-> TypeQ	Quotation of the form `[t\| ctxt ⇒ src → rep \|]`
-> ExpQ	Quotation of an expression of type `src → rep`
-> ExpQ	Quotation of an expression of type `rep → src`
-> DecsQ	Declarations to be spliced for the derived Unbox instance

Let's consider a more complex example: suppose we want an Unbox instance for Maybe a. We could encode this using the pair (Bool, a), with the boolean indicating whether we have Nothing or Just something. This encoding requires a dummy value in the Nothing case, necessitating an additional Default constraint. Thus:

derivingUnbox "Maybe"
    [t| ∀ a. (Default a, Unbox a) ⇒ Maybe a → (Bool, a) |]
    [| maybe (False, def) (\ x → (True, x)) |]
    [| \ (b, x) → if b then Just x else Nothing |]