In Chapter 10 of Purely Functional Data Structures, Okasaki describes a technique called data structure bootstrapping. It’s a way to reuse existing implementation of data structures to construct (bootstrap) new ones.
In one of the examples he creates a new heap implementation with an efficient merge operation using another heap as basis, but it turns out that to implement this, we need to rely on mutually recursive modules, that is, two modules A and B, where A depends on B, and B depends on A.
In this post we’ll study the bootstrapped heap and learn how to implement mutually recursive modules in OCaml.
Heap with efficient merging
Assume we have a heap implementation with O(1)
insert
, and O(log n)
merge
, findMin
and deleteMin
operations. We’ve seen such an implementation with Skewed Binomial Heaps
We’ll see how to construct a new heap implementation which will improve the merge complexity to O(1)
.
Let’s call the base heap PrimaryHeap
and define our heap type as
type 'a heap = Empty | Heap of 'a * ('a heap) PrimaryHeap.heap |
this type can be either empty or a node with an element (root) and a primary heap whose element is the bootstrapped heap itself, that is, heap
and PrimaryHeap.heap
form a mutually recursive types. Note that the above is not a valid OCaml code. We’re using it to explain the theoretical concepts.
We can think of this as a k-ary tree where the element is the root and the children of that node are the subtrees, but these subtrees are stored in a heap instead of an array.
The root element at each node is the smallest among all of the subtrees. Hence, to obtain the minimum element for a heap, findMin
, is trivial: we can simply return that element:
let findMin (heap: heap): tv = | |
match heap with | |
| Empty -> raise Empty_heap | |
| Heap(elem, _) -> elem |
Merging two bootstrapped heaps is analogous to linking two trees. The only invariant we need to maintain is that the smallest root continues being the root.
let merge (heap1: heap) (heap2: heap) = match (heap1, heap2) with | |
| (Empty, heap2) -> heap2 | |
| (heap1, Empty) -> heap1 | |
| (Heap (element1, primaryHeap1), Heap (element2, primaryHeap2)) -> | |
if ((Element.compare element1 element2) < 0) | |
then Heap(element1, PrimaryHeap.insert heap2 primaryHeap1) | |
else Heap(element2, PrimaryHeap.insert heap1 primaryHeap2) | |
Since the primary heap has O(1)
insert
, the bootstrapped heap has O(1)
merge
, which was our goal. Note that we can implement insert
using merge
by creating a singleton node and merging it with an existing heap.
let singleton (elem: tv): heap = Heap(elem, PrimaryHeap.empty) | |
let insert (elem: tv) (heap: heap): heap = | |
merge heap (singleton elem) |
We need to handle the deletion of the minimum element, which is the more involved operation. It consists in discarding the root of the present node, and finding a new root from the primary heap.
Since each element in the primary heap is a bootstrapped heap, we first obtain the bootstrapped heap containing the smallest element:
(Heap (newMinElem, minPrimaryHeap)) = | |
PrimaryHeap.findMin primaryHeap |
then we remove this node from the primaryHeap
, and we merge the minPrimaryHeap
back into primaryHeap
.
let restPrimaryHeap = PrimaryHeap.deleteMin primaryHeap | |
in PrimaryHeap.merge minPrimaryHeap restPrimaryHeap |
finally we make newMinElem
the new root element of our top level bootstrapped heap. The complete code is
let deleteMin (heap: heap) = | |
match heap with | |
| Empty -> raise Empty_heap | |
| Heap(_, primaryHeap) -> | |
if PrimaryHeap.isEmpty primaryHeap then Empty | |
else | |
let (Heap (newMinElem, minPrimaryHeap)) = | |
PrimaryHeap.findMin primaryHeap | |
in let restPrimaryHeap = PrimaryHeap.deleteMin primaryHeap | |
in Heap (newMinElem, PrimaryHeap.merge minPrimaryHeap restPrimaryHeap) |
The only missing part is defining the correct type of the bootstrapped heap.
Mutually Recursive Modules

Drawing Hands by M. C. Escher
Okasaki mentions that recursive structures are not supported in Standard ML (at least at the time my copy of the book was printed), but they are supported in OCaml.
To make modules mutually depend on another, we need to mark it as recursive via the rec
keyword, and declaring both modules at the same time by using the and
connector. Let’s work with a toy example: two modules Even
and Odd
, where each depend on the other.
module rec Even = struct | |
type t = Zero | Succ of Odd.t | |
end | |
and Odd = struct | |
type t = Succ of Even.t | |
end |
This will lead to a compilation error:
Error: Recursive modules require an explicit module type.
We need to write the signatures explicitly:
module rec Even : sig | |
type t = Zero | Succ of Odd.t | |
end = struct | |
type t = Zero | Succ of Odd.t | |
end | |
and Odd : sig | |
type t = Succ of Even.t | |
end = struct | |
type t = Succ of Even.t | |
end |
This blog post from Jane Street describes a way to define mutually recursive modules by only listing its signatures:
module rec Even: sig | |
type t = Succ of Odd.t | |
end = Even | |
and Odd: sig | |
type t = Succ of Even.t | |
end = Odd |
The OCaml compiler can infer the implementation part from the type definitions, but unfortunately this won’t work if the module has function definitions, which is the case for our heap implementation.
Things get more complicated in our case because the primary heap implementation uses a functor to generate a heap taking the element’s module as parameter. In this case the element’s module is our bootstrapped heap. A valid module definition is given below:
module rec BootstrappedElement: sig | |
type t = Empty | Heap of Element.t * PrimaryHeap.heap | |
val compare: t -> t -> int | |
end = struct | |
type t = Empty | Heap of Element.t * PrimaryHeap.heap | |
let compare heap1 heap2 = match (heap1, heap2) with | |
| (Heap (x, _), Heap (y, _)) -> Element.compare x y | |
end | |
and PrimaryHeap: IHeapWithMerge | |
with type tv := BootstrappedElement.t = SkewBinomialHeap(BootstrappedElement);; |
Let’s understand what is going on.
type t = Empty | Heap of Element.t * PrimaryHeap.heap
is the definition we presented above. We also implement the methods from the Set.OrderedType
interface, namely compare
, since this is the interface the heap maker expects. The comparison is based solely on the root element.
Then we declare the PrimaryHeap type at the same time, with type IHeapWithMerge
, and because tv
is unbound in that interface, we need to bind it to BootstrappedElement.t
:
PrimaryHeap: IHeapWithMerge with type tv := BootstrappedElement.t
Finally we provide the implementation, using the result of the SkewBinomialHeap()
functor having the BootstrappedElement
module as element type:
PrimaryHeap (...) = SkewBinomialHeap(BootstrappedElement)
The syntax is pretty involved, but it accomplishes what we wanted. We can further refine this definition by adding
include Set.OrderedType with type t := t
to the BootstrappedElement
signature. This includes all the interface of Set.OrderedType
.
These newly defined modules are defined within a functor, the BootstrappedHeap
, together with the methods we defined above. Like other heap generators, the functor takes a module representing the element type as parameter. In this case we can also allow the primary heap type to be passed as parameter so we don’t have to use SkewBinomialHeap
as implementation. Any heap with merge
will do.
module BootstrappedHeap | |
(Element: Set.OrderedType) | |
(MakeHeap: functor (Element: Set.OrderedType) -> IHeapWithMerge with type tv = Element.t) | |
: IHeap with type tv = Element.t = | |
struct | |
module rec BootstrappedElement: sig | |
type t = Empty | Heap of Element.t * PrimaryHeap.heap | |
include Set.OrderedType with type t := t | |
end = struct | |
type t = Empty | Heap of Element.t * PrimaryHeap.heap | |
let compare heap1 heap2 = match (heap1, heap2) with | |
| (Heap (x, _), Heap (y, _)) -> Element.compare x y | |
end | |
and PrimaryHeap: IHeapWithMerge | |
with type tv := BootstrappedElement.t = MakeHeap(BootstrappedElement);; | |
(* Methods definition below… *) | |
end |
The constructors define within BootstrappedElement
are visible within BootstrappedHeap
but they need qualification, such as BootstrappedElement.Heap
. To avoid repeating this qualifier, we can use:
include BootstrappedElement
The complete implementation for BootstrappedHeap
can be found on github.
Conclusion
The idea of using implementations of a given data structure to yield improve implementations is amazing! The mutual recursion nature of the bootstrap heap got me at first, but making analogies with a k-ary tree made it easier to understand.
I was struggling a lot to get the syntax right for the recursive modules required for this implementation until I stumbled upon this github repository, from which I learned many new things about OCaml.
References
[1] Purely Function Data Structures, Chapter 10 – Chris Okasaki
[2] Jane Street Tech Blog – A trick: recursive modules from recursive signatures
[3] Github yuga/readpfds: bootStrappedHeap.ml