Code from a meetup presentation: http://www.meetup.com/N-Languages-in-N-Months-NYC/events/202521322/

Main.hs

-- Code from a meetup presentation: http://www.meetup.com/N-Languages-in-N-Months-NYC/events/202521322/
-- Implements half adder and full adder circuits

-- Installation: `brew install ghc cabal-install`
-- Check compilation: `ghc Main.hs`
-- Load into repl:
-- $ cabal repl`
-- Prelude> :l Main.hs
module Main where

--resolve name clash by doing:
import Prelude hiding (and, or)

main :: IO ()
main = undefined
--returns nothing, always starts off with main

--define a bit
--'data' is a keyword, defining two constructors, the symbol High and Low
--Bit is now a type, we tried to use it below in a type signature
data Bit = High | Low
	-- a way to show our output
	deriving (Show)
	--expression problem - sometimes there is no silver bullet

--make a function
--take a bit and return two bits, one sum and one carry
halfAdder :: Bit -> Bit -> (Bit, Bit)
--descriptive naming, whether inside or outside
--we want to describe the output from the input
--input is a pair, so is output
halfAdder inputOne inputTwo = (inputOne `xor` inputTwo, inputOne `and` inputTwo)

xor :: Bit -> Bit -> Bit
xor High Low = High
xor Low High = High
xor _	_	 = Low

and :: Bit -> Bit -> Bit
and High High = High
and _	 _    = Low

fullAdder :: Bit -> Bit -> Bit -> (Bit, Bit)
--these are lazy, if we wanted to evaluate immediately with bangs `!`
fullAdder inputOne inputTwo carryIn =
	(sumOutputTwo, carryOutput `or` carryOutputTwo)
	where
		--need input from one half adder, output of first half adder
		--'destructure' is used here, with some pattern matching as in the rest of the file
	    (sumOutput, carryOutput)       = halfAdder inputOne inputTwo
	    (sumOutputTwo, carryOutputTwo) = halfAdder sumOutput carryIn

or :: Bit -> Bit -> Bit
or Low Low = Low
or _   _   = High