This blog post is an HTML translation of a note I wrote for myself some time ago. I am publishing it here with the hope that somebody may find it useful.

King and Wadler claimed in [3] that the list monad can be composed with any monad. As observed in [2] and more recently in [4], the definitions given in [3] are not correct: e.g., they do not work if one tries to compose the list monad with itself. We show how these definitions can be falsified using QuickCheck.

The notion of distributive law was introduced by Beck [1] in order to classify monad compositions. Roughly speaking, if $K$ and $H$ are monad on a category $\mathcal{C}$, then a distributivity law of $K$ over $H$ is a natural transformation $\lambda : HK\to KH$ subject to some axioms. Given such a distributivity law, the composition (of functors) $KH$ can be equipped with the structure of a monad. Composing (or, more generally, combining) monads plays an important role in programming languages theory, because it allows us to combine various types of computations. For example, the list monad $L$ can be used to model a form of nondeterminism, where instead of returning one value, a list of values is returned, and the empty list indicates failure. Therefore, composing the list monad with another monad $M$ amounts in practice to adding nondeterminism to the class of computations modeled by $M$. In [3], King and Wadler claimed that the list monad can be composed with any monad. Unfortunately, the definitions given in [3] are not correct, as observed, e.g., by Jones and Duponcheel [2] and more recently by Manes and Mulry [4]. We show how this error could have been spotted by translating these definitions into Haskell and QuickCheck’ing them. This blog post is a literate Haskell program.

First, we need a few imports:

> import Control.Monad
> import Test.QuickCheck


Given any monad m and type a, King and Wadler observe that m [a] becomes a monoid with respect to the following operation:

> (<>) :: Monad m => m [a] -> m [a] -> m [a]
> a <> b = do  x <- a
>              y <- b
>              return (x ++ y)


Equivalently, (<>) = liftM2 (++). The identity element for (<>) is

> e :: Monad m => m [a]
> e = return []


We can also multiply any list of elements of type m [a]:

> prod :: Monad m => [m [a]] -> m [a]
> prod = foldl (<>) e


King and Wadler then define a function

> joinML :: Monad m => m [m [a]] -> m [a]
> joinML = join . liftM prod


and claim that together with

> unitML :: Monad m => a -> m [a]
> unitML = return . return


and

> mapML :: Monad m => (a -> b) -> m [a] -> m [b]
> mapML = liftM . map


it makes the composition of m and [] into a monad. However, we can easily check that the associativity does not hold for joinML if m is specialized to []. Namely, if we define a QuickCheck property

> associativity :: [[[[[[Int]]]]]] -> Bool
> associativity t
>     = (joinML . joinML $t) == (joinML . mapML joinML$ t)


then testing it with QuickCheck yields

*Main> quickCheck associativity
*** Failed! Falsifiable (after 5 tests and 14 shrinks):
[[[[[[],[]]]],[[[[0]]],[]]]]


In fact, King and Wadler make even a stronger claim, namely that any monad m admits a distributivity law over the list monad and it is given by

> cp :: Monad m => [m a] -> m [a]
> cp = prod . map (liftM return)


However, it is easy to check that one of the distributive law axioms fails, namely the one called (cp-4) in [3] and (DL B) in [4]. Again, it is easily QuickCheck’ed:

> cp4 :: [[[Int]]] -> Bool
> cp4 xsss
>     = (cp . map concat $xsss) == (concat . map cp . cp$ xsss)

*Main> quickCheck cp4
*** Failed! Falsifiable (after 5 tests and 10 shrinks):
[[[0,0]],[[1],[2]]]


It is shown in [4] that a distributivity law of m over the list monad exists provided that m is a commutative monad (e.g., m is a reader monad, or the writer monad associated with a commutative monoid). Let us describe this distributivity law. The list monad is a linear quotient of the monad of binary trees:

> data Tree a  =  Empty
>              |  Leaf a
>              |  Branch (Tree a) (Tree a)

> tau                 ::  Tree a -> [a]
> tau Empty           =   []
> tau (Leaf x)        =   [x]
> tau (Branch t1 t2)  =   (tau t1) ++ (tau t2)


The distributivity law lambda :: Monad m => [m a] -> m [a] of m over the list monad is then uniquely determined by the equation psi == lambda . tau (see [4], p. 204), where psi is defined as follows:

> psi                :: Monad m => Tree (m a) -> m [a]
> psi Empty          =  e
> psi (Leaf x)       =  liftM return x
> psi (Branch t1 t2) =  (psi t1) <> (psi t2)


It is easy to check that the function cp satisfies the equation psi == cp . tau. Indeed, we need to show that psi t == cp . tau $t for any tree t :: Tree (m a). The proof is by induction on the structure of t. If t == Empty, we obtain:  cp . flatten$ Empty
== {- definition of tau -}
cp []
== {- definition of map -}
prod []
== {- definition of foldl -}
e
== {- definition of psi -}
psi Empty


If t == Leaf x, then

   cp . flatten $Leaf x == {- definition of tau -} cp [x] == {- property of map -} prod [liftM return x] == {- property of foldl -} liftM return x == {- definition of psi -} psi (Leaf x)  Finally, if t == Branch t1 t2, then  cp . flatten$ Branch t1 t2
== {- definition of tau -}
cp $(tau t1) ++ (tau t2) == {- definition of cp -} prod . map (liftM return)$ (tau t1) ++ (tau t2)
== {- map is a homomorphism of monoids-}
prod $(map (liftM return) (tau t1)) ++ (map (liftM return) (tau t2)) == {- (<>) is associative and foldl is a homomorphism of monoids -} (prod . map (liftM return)$ tau t1)
<> (prod . map (liftM return) $tau t2) == {- induction hypothesis -} (psi t1) <> (psi t2) == {- definition of psi -} psi (Branch t1 t2)  Therefore, the distributivity law whose existence is proven in [4] coincides with cp. Where does the commutativity of the monad m show up? The proof of the distributivity law axioms for cp relies on the following property of prod (stated as (prod-4) in [3]): prod . map (join . liftM prod) == join . lift prod . prod  which does not hold in general (and can be falsified using QuickCheck likewise cp). The left hand side of the equation applied to a list [a1, a2, ..., an] :: [m [m [a]]] can be seen to be equal to prod . map (join . liftM prod)$ [a1, a2, ..., an]
== do  x1 <- a1
z1 <- prod x1
x2 <- a2
z2 <- prod x2
...
xn <- an
zn <- prod xn
return (z1 ++ z2 ++ ... ++ zn)


whereas the right hand side applied to [a1, a2, ..., an] can be identified with

join . liftM prod . prod \$ [a1, a2, ..., an]
== do  x1 <- a1
x2 <- a2
...
xn <- an
z1 <- prod x1
z2 <- prod x2
...
zn <- prod xn
return (z1 ++ z2 ++ ... ++ zn)


If m is commutative, then the order of the statements in the do block does not matter, and the obtained expressions are equal.

The existence of a distributivity law for a general m is a subtle question. In fact, it is an open problem whether the list monad admits a distributivity law over itself. It is worth pointing out that the monad of nonempty lists does admit a distributive law over itself (see [4], Example 5.1.10).

### References

[1] Jon Beck, Distributive laws, In Sem. on Triples and Categorical Homology Theory (ETH, Zürich, 1966/67), pp. 119-140. Springer, Berlin, 1969.

[2] Mark P. Jones, Luc Duponcheel, Composing monads, 1993; available here.

[3] David King and Philip Wadler, Combining Monads, Mathematical Structures in Computer Science, 1992, pp. 61-78.

[4] Ernie Manes and Philip Mulry, Monad Composition I: General Constructions and Recursive Distributive Laws, Theory and Applications of Categories, Vol. 18, No. 7, 2007, pp. 172-208.