This post is about implementing a function for inverting Map of Maps, that is,

(typescript)

```
Map<A, Map<B, T>>
// to
Map<B, Map<A, T>>
```

I’ll be using Haskell and Typescript throughout the post and show my best attempt in the end.

## Table of Contents

## Motivation

Recently I got to deal with lots of instances of a type `T`

. Since there were so many of them, I had to group them by some criterion for performance reasons. For example, `T`

uses resource `A`

so I group `T`

s by `A`

thereby I can process all the `T`

s that share the same `A`

at once when it is allocated.

In my case there were two criteria, `A`

and `B`

, that I had to take into account simultaneously so naturaly I got to build a data structure of

(typescript)

```
Map<A, Map<B, T[]>>
```

(haskell)

```
Map A (Map B [T])
```

The problem was that in some cases it needed to be expressed in the form of `Map<B, Map<A, T[]>>`

which is alteration of switching the outter Map’s Key and inner Map’s Key.

Of course I could just implement it imperatively something like this …

(pseudo imperative code)

```
// boring imperative code …
func invertMap(ABTs: Map<A, Map<B, T[]>>):
BATs = new Map<B, Map<A, T[]>>
for (a, BTs) of ABTs:
for (b, Ts) of BTs:
if BATs has no key b then
BATs[b] = new Map<A, T[]>
BATs[b][a] = Ts
else
if BATs[b] has no key a then
BATs[b][a] = Ts
else
BATs[b][a].add Ts
return BATs
```

But I thought I could do better. They were the algebraically same data structure therefore there must have been a natural way of transforming into each other. I only needed to find them.

## Applicative Map?

At first I thought about conforming `Map<B, _>`

to *Applicative* then just `sequence`

ing it. It was possible because Map is *Traversable*. But soon I found Map couldn’t conform to *Applicative* because there is simply no way to implement `pure`

(or `of`

, if you will) nor `ap`

(or `<*>`

, `liftA`

, if you will). I mean how are you going to construct a Map of functions that satisfies Identity law?!

So I gave up on Applicative Map.

## Combining Monoidal Operations

I got thinking that it was evident in the imperative version of implementation above that the whole process is just combination of Monoidal operations: creating empty Map, concatenating list of Ts and ultimately combining all to define monoidal operation on `Map<B, Map<A, T>>`

.

This observation led me to this functional implementation.

## Implementation

(Haskell)

```
invertMap :: (Monoid t, Ord a, Ord b) => Map a (Map b t) -> Map b (Map a t)
invertMap = foldr (unionWith mappend) empty . mapWithKey (fmap . singleton)
```

(Typescript, fp-ts)

```
export function invertMap<KA, KB, T>(
kaOrd: Eq<KA>,
kbOrd: Eq<KB>,
monT: Monoid<T>
) {
const monAT = map.getMonoid(kaOrd, monT);
const monBAT = map.getMonoid(kbOrd, monAT);
return (mm: Map<KA, Map<KB, T>>) =>
getFoldableWithIndex(kaOrd).foldMapWithIndex(monBAT)(mm, (a, bt) =>
pipe(
bt,
map.map((t) => new Map<KA, T>().set(a, t))
)
);
}
```

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