# Type arithmetic

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## Revision as of 06:26, 16 February 2006

**Type arithmetic** is calculations on types using fundeps as functions.

A simple example is Peano numbers:

data Zero

data Succ a

class Add a b ab | a b -> ab, a ab -> b instance Add Zero b b instance (Add a b ab) => Add (Succ a) b (Succ ab)

However, many other representations of numbers are possible, including binary and balanced base three. Type arithmetic may also include type representations of boolean values and so on.

## An Advanced Example : Type-Level Quicksort

An advanced example: quicksort on the type level.

Here is a complete example of advanced type level computation, kindly
provided by Roman Leshchinskiy. For further information consult Thomas
Hallgren's 2000 paper *Fun with Functional Dependencies*.

module Sort where -- natural numbers data Zero data Succ a -- booleans data True data False -- lists data Nil data Cons a b -- shortcuts type One = Succ Zero type Two = Succ One type Three = Succ Two type Four = Succ Three -- example list list1 :: Cons Three (Cons Two (Cons Four (Cons One Nil))) list1 = undefined -- utilities numPred :: Succ a -> a numPred = const undefined class Number a where numValue :: a -> Int instance Number Zero where numValue = const 0 instance Number x => Number (Succ x) where numValue x = numValue (numPred x) + 1 numlHead :: Cons a b -> a numlHead = const undefined numlTail :: Cons a b -> b numlTail = const undefined class NumList l where listValue :: l -> [Int] instance NumList Nil where listValue = const [] instance (Number x, NumList xs) => NumList (Cons x xs) where listValue l = numValue (numlHead l) : listValue (numlTail l) -- comparisons data Less data Equal data Greater class Cmp x y c | x y -> c instance Cmp Zero Zero Equal instance Cmp Zero (Succ x) Less instance Cmp (Succ x) Zero Greater instance Cmp x y c => Cmp (Succ x) (Succ y) c -- put a value into one of three lists according to a pivot element class Pick c x ls eqs gs ls' eqs' gs' | c x ls eqs gs -> ls' eqs' gs' instance Pick Less x ls eqs gs (Cons x ls) eqs gs instance Pick Equal x ls eqs gs ls (Cons x eqs) gs instance Pick Greater x ls eqs gs ls eqs (Cons x gs) -- split a list into three parts according to a pivot element class Split n xs ls eqs gs | n xs -> ls eqs gs instance Split n Nil Nil Nil Nil instance (Split n xs ls' eqs' gs', Cmp x n c, Pick c x ls' eqs' gs' ls eqs gs) => Split n (Cons x xs) ls eqs gs listSplit :: Split n xs ls eqs gs => (n, xs) -> (ls, eqs, gs) listSplit = const (undefined, undefined, undefined) -- zs = xs ++ ys class App xs ys zs | xs ys -> zs instance App Nil ys ys instance App xs ys zs => App (Cons x xs) ys (Cons x zs) -- zs = xs ++ [n] ++ ys -- this is needed because -- -- class CCons x xs xss | x xs -> xss -- instance CCons x xs (Cons x xs) -- -- doesn't work class App' xs n ys zs | xs n ys -> zs instance App' Nil n ys (Cons n ys) instance (App' xs n ys zs) => App' (Cons x xs) n ys (Cons x zs) -- quicksort class QSort xs ys | xs -> ys instance QSort Nil Nil instance (Split x xs ls eqs gs, QSort ls ls', QSort gs gs', App eqs gs' geqs, App' ls' x geqs ys) => QSort (Cons x xs) ys listQSort :: QSort xs ys => xs -> ys listQSort = const undefined

And we need to be able to run this somehow, in the typechecker. So fire up ghci:

___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.04, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Prelude> :l Sort Compiling Sort ( Sort.hs, interpreted ) Ok, modules loaded: Sort. *Sort> :t listQSort list1 Cons (Succ Zero) (Cons (Succ One) (Cons (Succ Two) (Cons (Succ Three) Nil)))