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Basic: Zome functions

In this chapter you will learn

  • what zome functions are
  • how you create a zome function
  • how to get data in and out of your zome
  • about some commons errors you encounter while coding

Zome Functions

Before you talk about a zome function you need to understand what a zome is. A zome is a "basic unit of modularity inside a DNA". You can think of it as a package inside your codebase. Let's take a look.

Open the zomes folder developers/1.basic/0.zome-functions/exercise/zomes in the developers-exercise git repo you cloned.

It contains just one folder: exercise. But you can rename the folder or add more zomes folders, so you can separate different concerns in different folders. Most exercises in the gym will have only one zome, named 'exercise'. This is just to keep things simple.


All your zome(s) together form a DNA. Just like the chromosomes in a cell in your body form one DNA. This DNA is compiled into a WASM binary. This means that when you want call a function in your zome from your UI or your integration test, you are calling an actual program. This is the point where you need zome functions.

Zome function

A zome function is the bridge between the inside and the outside world. The inside world is your Rust code your zomes, entries in the DHT, ... The outside world is your UI or integration test that calls your code with the help of the conductor. So basically a zome function is a function that can be called from the outside.

Getting ready

You need to figure out

  • how to mark a function so it can be called from the outside
  • how data can be send in and out the zome

In case you forgot, Zomes are written in Rust. Don't worry if you are new to the language, we will gladly help you grow comfortable with it. Look in the Rust section or join us at the Holochain forum.

We went ahead and added some standard Rust code for you in the zomes/exercise/src/lib.rs file:

pub struct SomeExternalInput {
    first_name: String,
    last_name: String,

pub struct SomeExternalOutput(String);

pub fn hello_world(_:()) -> ExternResult<SomeExternalOutput> {
    let message: String = String::from("Hello world");
    let output: SomeExternalOutput = SomeExternalOutput(message);

pub fn say_my_name(external_input:SomeExternalInput) -> ExternResult<SomeExternalOutput> {
    let message: String = format!("Your name is {} {}", 
    let output: SomeExternalOutput = SomeExternalOutput(message);

pub fn get_agent_id(_:()) -> ExternResult<AgentInfo> {

SomeExternalInput and SomeExternalOutput are just simple structs. A struct in Rust is similar to classes in other languages. SomeExternalInput has 2 fields: first_name and last_name. SomeExternalOutput is a struct wrapped around a simple String. You will soon see why you need to wrap a String in a struct when you want to use them as input/output. And why you cannot send simple Strings in and outside of your zome.

Next you have 3 public functions. If you are new to Rust, then _:() might seem weird. The input parameter has a name _ and the type is an Object (). But it just means: "I accept anything, because I will not be using it".

Finally you see the return type of each function. Functions that return a Result are very common in Rust. ExternResult is an enhanced Result type, specially adapted for use in zomes. In stead of a standard Err, you need a WasmError as the error type in your result. Because, as we mentioned above, your zome is compiled into a WASM binary. So every error you want to report to the outside world is in fact a WasmError.

Import HDK

Becoming an expert starts by making every possible error. So go ahead and make some errors.

  1. Open a terminal in developer-exercises folder.
  2. Enter the nix-shell by running: nix-shell
    You should run this in the folder containing the default.nix file
  3. Go to folder with the exercise 1.basic/0.zome-functions/exercise
  4. Compile your code by running the following script: ./run_build.sh.
  5. Inspect the Rust compiler error

You should see the following error:

error[E0412]: cannot find type `ExternResult` in this scope
  --> zomes/exercise/src/lib.rs:10:29
10 | pub fn hello_world(_:()) -> ExternResult<SomeExternalOutput> {
   |                             ^^^^^^^^^^^^ not found in this scope
help: consider importing this type alias
3  | use hdk::prelude::ExternResult;

The compiler tells you:

  1. it cannot find ExternResult
  2. suggests you might want to import it using: use hdk::prelude::ExternResult; The Rust compiler makes this lovely suggestion because we added hdk as a dependency in cargo.toml

The Holochain team built a Rust library hdk, which stands for Holochain Development Kit and contains all important Holochain functions, types and attributes you will want to use in your zome. Since you will be using more than one HDK function or type, it is best to add the following statement to the top of the lib.rs file.

use hdk::prelude::*;

Go ahead and take a quick look at that prelude file to get a taste of what the HDK provides.

External inputs

Now that you have fixed the import statement, you will discover that building your zome succeeds. Let's see if we can break something else.
We wrote an integration test for you, to test if everything works correctly. The best way to write integration/scenario tests in typescript is to use Tryorama, a holochain typescript library, to write these tests. Take a look in the 1.basic/0.zome-functions/exercise/tests/src/index.ts file. Run the test to see what fails.

  1. Check that your are still in a nix-shell. The beginning of the command line should have nix-shell in it
  2. Check that your are still in the folder 1.basic/0.zome-functions/exercise.
  3. Compile your code: ./run_build.sh.
  4. Run the tests by running our the following script: ./run_test.sh.
  5. Inspect the error.

You should see the following error:

tryorama] error: Test error: {
  type: 'error',
  data: {
    type: 'ribosome_error',
    data: "Attempted to call a zome function that doesn't exist: Zome: exercise Fn hello_world"
not ok 1 Test threw an exception. See output for details.

It simply means that our test code .call("exercise", "hello_world", null);, which calls the hello_world function in the zome exercise, cannot find that function. So making a function public in your zome is not enough. You need to add an attribute on the top of a public function to make it possible for this function to be called from outside the zome. The attribute you need to add is #[hdk_extern]. Only public functions with this attribute can be called from outside a zome. Adding the attribute makes it a zome function.

  1. Add #[hkd_extern] on top of the 3 public functions.
  2. Compile: ./run_build.sh.
  3. Test: ./run_test.sh.
  4. Inspect the error.
error[E0277]: the trait bound `SomeExternalOutput: hdk::prelude::Serialize` is not satisfied
  --> zomes/exercise/src/lib.rs:11:1
11 | #[hdk_extern]
   | ^^^^^^^^^^^^^ the trait `hdk::prelude::Serialize` is not implemented for `SomeExternalOutput`

The compiler is complaining that our SomeExternalOutput struct is not implementing the trait hdk::prelude::Serialize. You will also see #[hdk_extern] mentioned. #[hdk_extern] is a Rust macro and it requires that the function it is added to, has one input parameter and that this parameter can be serialized. Because zome functions live on the boundary of your WASM binary, they need to be able to serialize and deserialize all data structs that go in or out the zome.
Luckily this is all standard Rust stuff. Annotate the 2 structs SomeExternalInput and SomeExternalOutput with the following attribute #[derive(Serialize, Deserialize, Debug)]. This attribute makes sure the data in the structs can be sent to this zome over a network, from a GUI, or – like in our case – from a test script.
It is also the reason why you cannot use a simple String as the input or output for a zome function. You cannot add #[derive(Serialize, Deserialize, Debug)] to a String, only to a struct.

Side note
Requirements for a zome function:

  • needs to be a public function
  • needs to return a ExternResult<T>
  • needs to be decorated with #[hdk_extern]
  • takes exactly 1 input param

To test this last requirement remove the input param external_input or _:() in one of the functions. Or add a second param. And look at what the Rust compiler tells you.


To finish this exercise add the attributes to the structs.

  1. Add #[derive(Serialize, Deserialize, Debug)] on top of the 2 structs.
  2. Compile: ./run_build.sh.
  3. Test: ./run_test.sh.
  4. Inspect the error.

Agent info

The hello_world and say_my_name are very simple toy functions. In get_agent_info on the other hand you call a real hdk function agent_info(). AgentInfo is the current agent’s original pubkey/address that they joined the network with and their most recent pubkey/address. Your agent info or Agent ID is one of the four genesis events that are created add the beginning of your source-chain by the subconscious part of your holochain application. When you install a holochain app an Agent ID is created. When a DNA, composed of one or more zomes, is instantiated and Agent ID is created they form a cell. Zomes, DNA, cells might sound confusing at first. Stick with it because the design principles of holochain are deeply rooted in nature. And everything in nature that is slow and consumes too much power, does not survive ...

So now it is up to you to finish this exercise. Add all the things we just explained to your code and finish by implementing the say_greeting function, building your zome and running the test. You only need to add a few lines in this first exercise, but know that when the test succeeds, you:

  • made your first zome module in Rust
  • compiled your zome into a DNA in WASM, a state of the art binary format
  • ran a test script, written in Typescript, executed in Nodejs
  • initiated an actual real holochain conductor
  • which instantiated your DNA into a real, actual Holochain cell
  • and executed a test script that asked your cell to make a real entry with the words "Hello, World!"

You will in fact have created your very first decentralized, agent centric, boundary pushing Holochain app. A real hApp!

  1. Go to the developer-exercises.
  2. Enter the nix-shell: nix-shell. you should run this in the folder containing the default.nix file
    starting the nix-shell for the very first time might take a long time, somewhere between 20 to 80 minutes, after that I will take just a few seconds
  3. Go to folder with the exercise basic/0.zome-functions/exercise.
  4. Inside zome/exercise/src/lib.rs:
    • Define a new struct for your entry: 'Greeting'.
    • Implement the function with unimplemented!().
  5. Compile your code: ./run_build.sh.
  6. Run the test: ./run_tests.sh.
  7. Don't stop until the test runs green.


If you encounter an error check here if you can find something that looks like your error. If not head to the forum.holochain.org and ask for help.

  • You forgot to add the #[hdk_extern] attribute on the say_greeting function.
got an error for test 'say a greeting': {
  type: 'error',
  data: {
    type: 'ribosome_error',
    data: "Attempted to call a zome function that doesn't exist: Zome: exercise Fn say_greeting"
  • You forgot to compile the zome. Run CARGO_TARGET_DIR=target cargo build --release --target wasm32-unknown-unknown in the developer-exercises/basic/0.zome-functions/exercise folder.
thread 'holochain-tokio-thread' panicked at 'TODO: DnaError
(ZomeNotFound("Zome \'exercise\' not found"))',

This is the error you see when you call a zome function that has no params

error: custom attribute panicked
  --> zomes/exercise/src/lib.rs:12:1
12 | #[hdk_extern]
   | ^^^^^^^^^^^^^
   = help: message: internal error: entered unreachable code

This is the error you see when you call a zome function that has more than one params

error[E0061]: this function takes 2 arguments but 1 argument was supplied
  --> zomes/exercise/src/lib.rs:20:1
20 | #[hdk_extern]
   | ^^^^^^^^^^^^^
   | |
   | supplied 1 argument
   | expected 2 arguments
21 | pub fn say_my_name(a:SomeExternalInput, b:SomeExternalInput) -> ExternResult<SomeExternalOutput> {
   | ------------------------------------------------------------------------------------------------ defined here
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

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