Veit's Blog

Let’s Write A Hashmap in Carp


An exciting new feature landed in Carp a while ago, and it is perfect material for a tutorial-style blog post!

The feature I’m talking about is generic types for structs, and in this blog post we’ll be building a simple hash map. Hash maps are fun to write, and easy to implement inefficiently, and today we’re going to do just that!

As I always do with these types of blog posts, I’ll start by defining an API, and then we’ll try and implement the whole thing!


A hashmap is usually used for storing and retrieving values, so we need to primarily be concerned with that. There are also a few convenience functions that we would like to have, but for the purposes of this blog post they’ll be left as an exercise to the reader. If you want you can also check out the repository I linked to above where I implemented a more complete interface.

(let [x (Map.create)]
    (Map.put x "my-key" "my-value")
    (Map.get x "my-key")))
Fig. 1: A usage example for maps.

This is fairly sparse but already provides a lot of the functionality we need.

Before going any further, let’s think about how we deal with getting keys that don’t exist. We’re in a statically typed language without any standard error mechanism—sorry about that—, and as such, we need to opt for an implementation using defaults or opt for the interface zero that returns a zero value for all of the types it is defined in. In this implementation we’ll use zero and hope it is defined for all of the value types we will want to put into our map. This differs from the version I linked to above, but makes for a simpler interface, since we don’t always have to specify a default value.

As this is a hashmap, we’ll also have to think about hashing. I’ll provide you with an interface for a hash function, but not its implementation for all data types. You can either also see this as an exercise or just copy-paste it from my ready-made solution.

(definterface hash (Fn [a] Int))
Fig. 2: An interface for hashing.

Let’s get to coding!

An implementation

First, let’s define some types using our beautiful generic syntax. To be able to do that, we have to answer some questions as to how we want our implementation to work, so brace yourselves for some tradeoffs!

The types

What is a hashmap?

(deftype (Map a b) [
  n-buckets Int
  buckets (Array (Bucket a b))
Fig. 3: A hashmap type.

Okay, so a map is a struct that contains a number of buckets and a list of buckets from type a to type b, where those letters are type variables. Type variables are always written lower-case, and we have to specify them as arguments to the type definition.

So, what are buckets?

(deftype (Bucket a b) [
  entries (Array (Entry a b))
Fig. 4: A bucket type.

Turns out a bucket is just an array of entries. It, too, just hands down its type arguments to the entry type.

Right around now you’re probably asking yourself what the heck we’re doing. Turns out that when writing a hashmap we have to deal with collisions. The easiest way to do so is to make each list a bucket of values that we then search linearly once we encounter a collision.

So, what is an entry?

(deftype (Entry a b) [key a value b])
Fig. 5: An entry type.

An entry is a key value pair. Here we finally use our type variables to make those key-value pairs generic. This enables us to write generic hashmaps. One restriction this poses on our hashmaps, though, is that the type variables a and b always resolve to the same types in the same map. This means that you can have a map from integers to strings or strings to floats, for instance, but not from anything to anything.

These are all the types we need, so now it’s time to actually implement some functionality and see how that goes.

The functions

We will implement these functions in a top-down manner, starting with the ones that are public and working our way down the stack to the nitty-gritty details. Let’s start with something simple, creating a map.


(defmodule Map
  (def default-nbuckets 256)

  (defn create []
    (init default-nbuckets
          (repeat default-nbuckets Bucket.empty)))
Fig. 6: Creating an empty map.

When creating a map, we have to initialize the buckets. We decide to have 256 buckets in our hashmap, which is a number that I chose completely arbitrarily. Usually you’d want to experiment with the number a bit and see what’s best for you: if it is lower, the initial memory penalty will be low; if it is higher, we have less collisions.

We also use Bucket.empty to initialize the buckets themselves, so we have to define that. As described on the packaging, it will create and return an empty bucket.

(defmodule Bucket
  (defn empty []
    (Bucket.init []))
Fig. 7: Creating an empty bucket.

And that’s all we need to implement to be able to bootstrap maps. Now let’s try and figure out how to put stuff into it using Map.put.


(defmodule Map
  (defn put [m k v]
    (let [idx (Int.mod (hash k) @(n-buckets &m))
          bs (buckets &m)
          b (nth bs idx)
          e (Entry.init @k @v)]
      (set-buckets m (aset @bs
                           (Bucket.grow b e)))))
Fig. 8: Putting things into a map.

Putting a new entry into a map is a three step process: first we need to find the index of the bucket we need to put the element in. We do this by hashing the key and using a modulus operation to make it fit into the number of buckets1.

We then grow this bucket—an operation we haven’t defined yet—, which will put the entry element into the bucket. And, finally, we write the result back into our buckets array.

The meat of this function lies within Bucket.grow, which is surprisingly simple as well. Have a look at it:

(defmodule Bucket
  (defn grow [b e]
    (set-entries b (push-back @(entries b) e)))
Fig. 9: Putting things into a bucket.

All we have to do is take the bucket and push our new entry to the back of the array. Simple housekeeping.

We’re only missing a final puzzle-piece, retrieval, to make our hashmap useful.


(defmodule Map
  (defn get [m k]
    (let [idx (Int.mod (hash k) @(n-buckets m))]
      (Bucket.get (nth (buckets m) idx) k)))
Fig. 10: Getting things from a map.

From the hashmap’s point of view, retrieving values is simple: it performs the same hashing operation that we used in Figure 8 to locate the correct bucket, and tells it to get the value under the key.

Next we need to implement Bucket.get.

(defmodule Bucket
  (defn get [b k]
    (let-do [e (zero)
             l (length (entries b))
             es (entries b)]
      (for [i 0 l]
        (when (= (Entry.key (nth es i)) k)
            (set! e (Entry.value (nth es i)))
Fig. 11: Getting things from a bucket.

This is by far the most complicated function we’ve defined in this blog post, but it is still fairly manageable. We initialize the element we want to return using zero, which will generate a zero value. This will be the default if we don’t find anything. Then we iterate over the entries in the bucket and, if we find any entry whose key member matches the given key, set the element we want to return to that entries value and return.

This is it! Those 40-ish lines of code are enough to specify a simple hashmap type!

But is it any good?


There are a few things that make this hashmap implementation suboptimal. ALthough the Carp community was very excited when I first wrote this simple little library, I immediately cautioned against using it as our default hashmap implementation. This is for a few reasons that I want to share with you, and maybe you can rectify some of its flaws and make it better!

Some of these flaws are easily remediable, and might be great exercises for you if you’re looking to dive into Carp! Others are a bit trickier to get rid of, as they are concerned with the basic functionality and type shape of the hashmap implementation.


Today we have implemented a simple hashmap. While the implementation itself does not reflect the state of the art in fast, memory-efficient hashmaps, it has most of the basic functionality you’d want. It is also type-generic without any annotations, and Carp’s borrow checker makes sure that it is memory-safe and its performance deterministic.

I hope I was able to show you the beauty of Carp’s generic types. They are really quite powerful, and an important step towards the expressiveness we want from Carp.

See you soon!


1. To prevent the modulus operation from screwing up the uniformity of our hash function, we should ensure to always use a power of two as the number of buckets.