Portability	non-portable
Stability	experimental
Maintainer	vector-th-unbox@liyang.hu
Safe Haskell	None

Data.Vector.Unboxed.Deriving

Description

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). (See http://hackage.haskell.org/packages/archive/vector/latest/doc/html/Data-Vector-Unboxed.html.) Using derivingUnbox, we can define the same instances much more succinctly:

derivingUnbox "Complex"
    [d| instance (Unbox a) => Unbox' (Complex a) (a, a) |]
    [| \ (r :+ i) -> (r, i) |]
    [| \ (r, i) -> r :+ i |]

Requires the MultiParamTypeClasses, TemplateHaskell and TypeFamilies LANGUAGE extensions.

The dummy Unbox' class provides a convenient way to pass in the source and representation types, along with any requisite constraints.

Synopsis

Documentation

class Unbox' src rep Source

A dummy class for passing arguments to derivingUnbox.

derivingUnbox Source

Arguments

:: String	Unique constructor suffix for the MVector and Vector data families
-> DecsQ	Quotation of the form `[d\| instance ctxt => Unbox' 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 can 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 (see the data-default package) constraint. Thus:

derivingUnbox "Maybe"
    [d| instance (Default a, Unbox a) => Unbox' (Maybe a) (Bool, a) |]
    [| maybe (False, def) (\ x -> (True, x)) |]
    [| \ (b, x) -> if b then Just x else Nothing |]