Comonads in Haskell

Comonads: what are they and what can you do with them?
Melbourne Haskell Users Group
David Overton
29 May 2014

Table of Contents
1 Motivation
2 Theory
3 Examples
4 Applications
5 Other Considerations
6 Further Reading

Motivation
• Monads are an abstract concept from category theory, have turned out to be
surprisingly useful in functional programming.
• Category theory also says that there exists a dual concept called comonads. Can
they be useful too?
• Intuition:
• Monads abstract the notion of eﬀectful computation of a value.
• Comonads abstract the notion of a value in a context.
• “Whenever you see large datastructures pieced together from lots of small but
similar computations there’s a good chance that we’re dealing with a comonad.”
—Dan Piponi

What is a Comonad?
• A comonad is just a comonoid in the category of endofunctors. . .
• A comonad is the category theoretic dual of a monad.
• A comonad is a monad with the “arrows” reversed.

What is a Comonad?
Both monads and comonads are functors. (Functor is its own dual.)
class Functor f where
fmap :: (a → b) → (f a → f b)
class Functor m ⇒ Monad m where class Functor w ⇒ Comonad w where
return :: a → m a extract :: w a → a
bind :: (a → m b) → (m a → m b) extend :: (w b → a) → (w b → w a)
join :: m (m a) → m a duplicate :: w a → w (w a)
join = bind id duplicate = extend id
bind f = fmap f ◦ join extend f = fmap f ◦ duplicate
(>>=) :: m a → (a → m b) → m b (=>>) :: w b → (w b → a) → w a
(>>=) = flip bind (=>>) = flip extend

Intuition
• Monadic values are typically produced in eﬀectful computations:
a → m b
• Comonadic values are typically consumed in context-sensitive computations:
w a → b

Monad/comonad laws
Monad laws
Left identity return ◦ bind f = f
Right identify bind return = id
Associativity bind f ◦ bind g = bind (f ◦ bind g)
Comonad laws
Left identity extract ◦ extend f = f
Right identity extend extract = id
Associativity extend f ◦ extend g = extend (f ◦ extend g)

Example: reader/writer duality
-- Reader monad
instance Monad ((→) e) where
return = const
bind f r = λc → f (r c) c
-- CoReader (a.k.a. Env) comonad
instance Comonad ((,) e) where
extract = snd
extend f w = (fst w, f w)
-- Writer monad
instance Monoid e ⇒ Monad ((,) e)
where
return = ((,) mempty)
bind f (c, a) = (c ♦ c’, a’)
where (c’, a’) = f a
-- CoWriter (a.k.a. Traced) comonad
instance Monoid e ⇒ Comonad ((→) e)
where
extract m = m mempty
extend f m = λc →
f (λc’ → m (c ♦ c’))

Example: state
newtype State s a = State { runState :: s → (a, s) }
instance Monad (State s) where
return a = State $ λs → (a, s)
bind f (State g) = State $ λs →
let (a, s’) = g s
in runState (f a) s’
data Store s a = Store (s → a) s -- a.k.a. ‘‘Costate’’
instance Comonad (Store s) where
extract (Store f s) = f s
extend f (Store g s) = Store (f ◦ Store g) s
One deﬁnition of Lens:
type Lens s a = a → Store s a
Hence the statement that lenses are “the coalgebras of the costate comonad”.

Example: stream comonad
data Stream a = Cons a (Stream a)
instance Functor Stream where
fmap f (Cons x xs) = Cons (f x) (fmap f xs)
instance Comonad Stream where
extract (Cons x ) = x
duplicate xs@(Cons xs’) = Cons xs (duplicate xs’)
extend f xs@(Cons xs’) = Cons (f xs) (extend f xs’)
• extract = head, duplicate = tails.
• extend extends the function f :: Stream a → b by applying it to all tails of
stream to get a new Stream b.
• extend is kind of like fmap, but instead of each call to f having access only to a
single element, it has access to that element and the whole tail of the list from
that element onwards, i.e. it has access to the element and a context.

Example: list zipper
data Z a = Z [a] a [a]
left, right :: Z a → Z a
left (Z (l:ls) a rs) = Z ls l (a:rs)
right (Z ls a (r:rs)) = Z (a:ls) r rs
instance Functor Z where
fmap f (Z l a r) = Z (fmap f l) (f a) (fmap f r)
iterate1 :: (a → a) → a → [a]
iterate1 f = tail ◦ iterate f
instance Comonad Z where
extract (Z a ) = a
duplicate z = Z (iterate1 left z) z (iterate1 right z)
extend f z = Z (fmap f $ iterate1 left z) (f z)
(fmap f $ iterate1 right z)

Example: list zipper (cont.)
• A zipper for a data structure is a transformed structure which gives you a focus
element and a means of stepping around the structure.
• extract returns the focused element.
• duplicate returns a zipper where each element is itself a zipper focused on the
corresponding element in the original zipper.
• extend is kind of like fmap, but instead of having access to just one element, each
call to f has access to the entire zipper focused at that element. I.e. it has the
whole zipper for context.
• Compare this to the Stream comonad where the context was not the whole
stream, but only the tail from the focused element onwards.
• It turns out that every zipper is a comonad.

Example: array with context
data CArray i a = CA (Array i a) i
instance Ix i ⇒ Functor (CArray i) where
fmap f (CA a i) = CA (fmap f a) i
instance Ix i ⇒ Comonad (CArray i) where
extract (CA a i) = a ! i
extend f (CA a i) =
let es’ = map (λj → (j, f (CA a j))) (indices a)
in CA (array (bounds a) es’) i
• CArray is basically a zipper for Arrays.
• extract returns the focused element.
• extend provides the entire array as a context.

Application: 1-D cellular automata – Wolfram’s rules
rule :: Word8 → Z Bool → Bool
rule w (Z (a: ) b (c: )) = testBit w (sb 2 a .|. sb 1 b .|. sb 0 c) where
sb n b = if b then bit n else 0
move :: Int → Z a → Z a
move i u = iterate (if i < 0 then left else right) u !! abs i
toList :: Int → Int → Z a → [a]
toList i j u = take (j - i) $ half $ move i u where
half (Z b c) = b : c
testRule :: Word8 → IO ()
testRule w = let u = Z (repeat False) True (repeat False)
in putStr $ unlines $ take 20 $
map (map (λx → if x then ’#’ else ’ ’) ◦ toList (-20) 20) $
iterate (=>> rule w) u

Application: 2-D cellular automata – Conway’s Game of Life
data Z2 a = Z2 (Z (Z a))
instance Functor Z2 where
fmap f (Z2 z) = Z2 (fmap (fmap f) z)
instance Comonad Z2 where
extract (Z2 z) = extract (extract z)
duplicate (Z2 z) = fmap Z2 $ Z2 $ roll $ roll z where
roll a = Z (iterate1 (fmap left) a) a (iterate1 (fmap right) a)

Application: 2-D cellular automata – Conway’s Game of Life
countNeighbours :: Z2 Bool → Int
countNeighbours (Z2 (Z
(Z (n0: ) n1 (n2: ): )
(Z (n3: ) (n4: ))
(Z (n5: ) n6 (n7: ): ))) =
length $ filter id [n0, n1, n2, n3, n4, n5, n6, n7]
life :: Z2 Bool → Bool
life z = (a && (n == 2 | | n == 3))
| | (not a && n == 3) where
a = extract z
n = countNeighbours z

Application: image processing
laplace2D :: CArray (Int, Int) Float → Float
laplace2D a = a ? (-1, 0)
+ a ? (0, 1)
+ a ? (0, -1)
+ a ? (1, 0)
- 4 ∗ a ? (0, 0)
(?) :: (Ix i, Num a, Num i) ⇒ CArray i a → i → a
CA a i ? d = if inRange (bounds a) (i + d) then a ! (i + d) else 0
• laplace2D computes the Laplacian at a single context, using the focused element
and its four nearest neighbours.
• extend laplace2D computes the Laplacian for the entire array.
• Output of extend laplace2D can be passed to another operator for further
processing.

Application: Env (CoReader) for saving and reverting to an initial value
type Env e a = (e, a)
ask :: Env e a → e
ask = fst
local :: (e → e’) → Env e a → Env e’ a
local f (e, a) = (f e, a)
initial = (n, n) where n = 0
experiment = fmap (+ 10) initial
result = extract experiment
initialValue = extract (experiment =>> ask)

Other applications of comonads
• Signal processing: using a stream comonad.
• Functional reactive programming: it has been postulated (e.g. by Dan Piponi,
Conal Elliott) that some sort of “causal stream” comonad should work well for
FRP, but there don’t yet seem to be any actual implementations of this.
• Gabriel Gonzalez’s three examples of “OO” design patterns:
• The Builder pattern: using CoWriter / Traced to build an “object” step-by-step.
• The Iterator pattern: using Stream to keep a history of events in reverse
chronological order.
• The Command pattern: using Store to represent an “object” with internal state.

Syntactic sugar
At least two diﬀerent proposals for a comonadic equivalent of do notation for
comonads:
• Gonzalez’s method notation – “OOP-like” with this keyword representing the
argument of the function passed to extend.
• Orchard & Mycroft’s codo notation – resembles Paterson’s arrow notation.
Unsugared Gonzalez Orchard & Mycroft
λwa →
let wb = extend (λthis → expr1) wa
wc = extend (λthis → expr2) wb
in (λthis → expr3) wc
method
wa> expr1
wb> expr2
wc> expr3
codo wa ⇒
wb ← λthis → expr1
wc ← λthis → expr2
λthis → expr3
λwa →
let wb = extend func1 wa
wc = extend func2 wb
in func3 wc
method
wa> func1 this
wb> func2 this
wc> func3 this
codo wa ⇒
wb ← func1
wc ← func2
func3

Comonad transformers
-- Monad transformers
class MonadTrans t where
lift :: Monad m ⇒ m a → t m a
-- Comonad transformers
class ComonadTrans t where
lower :: Comonad w ⇒ t w a → w a
The comonad package provides a few standard transformers:
• EnvT – analogous to ReaderT
• StoreT – analogous to StateT
• TracedT – analogous to WriterT

Cofree comonads
-- Free monad
data Free f a = Pure a | Free (f (Free f a))
instance Functor f ⇒ Monad (Free f) where
return = Pure
bind f (Pure a) = f a
bind f (Free r) = Free (fmap (bind f) r)
-- Cofree comonad
data Cofree f a = Cofree a (f (Cofree f a))
instance Functor f ⇒ Comonad (Cofree f) where
extract (Cofree a ) = a
extend f w@(Cofree r) = Cofree (f w) (fmap (extend f) r)
• Cofree Identity is an inﬁnite stream.
• Cofree Maybe is a non-empty list.
• Cofree [] is a rose tree.

A bit of category theory
-- Kleisli category identity and composition (monads)
return :: Monad m ⇒ a → m a
(>=>) :: Monad m ⇒ (a → m b) → (b → m c) → (a → m c)
f >=> g = λa → f a >>= g
-- Co-Kleisli category identity and composition (comonads)
extract :: Comonad w ⇒ w a → a
(=>=) :: Comonad w ⇒ (w a → b) → (w b → c) → (w a → c)
f =>= g = λw → f w =>> g
• Each monad has a corresponding Kleisli category with morphisms a → m b,
identity return and composition operator (>=>).
• Each comonad has a corresponding Co-Kleisli category with morphisms w a → b,
identity extract and composition operator (=>=).

Category laws
Monad laws
Left identity return >=> f = f
Right identify f >=> return = f
Associativity (f >=> g) >=> h = f >=> (g >=> h)
Comonad laws
Left identity extract =>= f = f
Right identify f =>= extract = f
Associativity (f =>= g) =>= h = f =>= (g =>= h)
Category laws
Left identity id ◦ f = f
Right identify f ◦ id = f
Associativity (f ◦ g) ◦ h = f ◦ (g ◦ h)

Further reading
• http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html
• http://blog.sigfpe.com/2008/03/comonadic-arrays.html
• http://blog.sigfpe.com/2008/03/transforming-comonad-with-monad.html
• http://blog.sigfpe.com/2014/05/cofree-meets-free.html
• https://www.fpcomplete.com/user/edwardk/cellular-automata
• http://www.jucs.org/jucs_11_7/signals_and_comonads/jucs_11_7_1311_1327_
vene.pdf
• http://www.haskellforall.com/2013/02/you-could-have-invented-
comonads.html
• http://www.cl.cam.ac.uk/~dao29/publ/codo-notation-orchard-ifl12.pdf
• http://conal.net/blog/tag/comonad

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