Words

These definitions below can be found in Basic.hs.

A word in the context of Bernstein's Salsa20 specification is a bit vector of size 32. Representing bit vectors in ReWire is done via the built-in type constructor for words, W. It is size-indexed so that, for example, 32-bit words are represented by W 32. There are a host of operators defined for words parameterized by size. For example, the type of addition on words has type (checking its type in the GHCi interpreter):

λ> :type (+)
(+) :: KnownNat n => W n -> W n -> W n

This means that, for any natural number n, (+) takes two (W n) and returns a (W n).

Most bit-vector operations in Verilog or VHDL have a corresponding operation in ReWire.

The Basic Usage TL;DR

The first lines of code for the Salsa20 quarterround function are below and illustrate how to load the built-in bit-vector types and operations in ReWire.

{-# LANGUAGE DataKinds #-}                    -- 1
module QuarterRound(quarterround) where

import Prelude hiding ((+) , (^))             -- 2
import ReWire                                 -- 3
import ReWire.Bits (lit , rotL , (^) , (+))   -- 4

To use the built-in bit vector type, one must simply include import ReWire (line 3) and also the language directive (1). To use specific operations, one can include them from ReWire.Bits (line 4) and some of these operations have the same name as Haskell Prelude operations, so one can exclude these as in line 2.

Some additional type constructors for Salsa20

Some additional type constructors used in this specification are included below. We could use Haskell built-in tuple types for these, but it is useful for various technical reasons to define our own larger vector types instead (e.g., it makes pretty-printing values as we wish easier). It's not important to understand the precise details, however.

data X16 a  = X16 a a a a a a a a a a a a a a a a 

type Quad a = (a , a , a , a)
type Oct a  = (a , a , a , a , a , a , a , a)
type Hex a  = X16 a
    
data X64 a  = X64 a a a a a a a a a a a a a a a a
                  a a a a a a a a a a a a a a a a
                  a a a a a a a a a a a a a a a a
                  a a a a a a a a a a a a a a a a 

These definitions can be found in Basic.hs.