finitary-derive-2.0.0.0: Flexible and easy deriving of type classes for finitary types.
Copyright (C) Koz Ross 2019 GPL version 3.0 or later koz.ross@retro-freedom.nz Experimental GHC only None Haskell2010

Data.Finitary.PackWords

Description

If a type a is Finitary, each inhabitant of a has an index, which can be represented as an unsigned integer, spread across one or more machine words. This unsigned integer will have fixed length (as the number of inhabitants of a is finite). We can use this to derive a Unbox instance, by representing Vector as a large array of machine words. We can also derive a Storable instance similarly.

This is the most efficient encoding of an arbitrary finitary type, both due to the asymptotics of encoding and decoding (logarithmic in Cardinality a with base Cardinality Word) and the fact that word accesses are faster than byte and bit accesses on almost all architectures. Unless you have concerns regarding space, this encoding is a good choice.

Unless your type's cardinality is extremely large (a non-trivial multiple of Cardinality Word), this encoding is wasteful. If your type's cardinality is smaller than that of Word, you should consider Data.Finitary.PackInto instead, as you will have much larger control over space usage at almost no performance penalty.

Synopsis

# Documentation

data PackWords (a :: Type) Source #

An opaque wrapper around a, representing each value as a fixed-length array of machine words.

#### Instances

Instances details

pattern Packed :: forall (a :: Type). (Finitary a, 1 <= Cardinality a) => PackWords a -> a Source #

To provide (something that resembles a) data constructor for PackWords, we provide the following pattern. It can be used like any other data constructor:

import Data.Finitary.PackWords

anInt :: PackWords Int
anInt = Packed 10

isPackedEven :: PackWords Int -> Bool
isPackedEven (Packed x) = even x

Every pattern match, and data constructor call, performs a $$\Theta(\log_{\texttt{Cardinality Word}}(\texttt{Cardinality a}))$$ encoding or decoding of a. Use with this in mind.