#internet-computer #idl #dfinity #binary-format


Candid is an interface description language (IDL) for interacting with canisters running on the Internet Computer

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#1384 in Magic Beans

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Candid is an interface description language (IDL) for interacting with canisters (also known as services or actors) running on the Internet Computer.

The Candid crate is a serialization/deserialization library for Candid. You can seamlessly convert between Rust values and Candid in both binary and text format.


See the docs here.



Candid is an interface description language (IDL) for interacting with canisters (also known as services or actors) running on the Internet Computer.

There are three common ways that you might find yourself needing to work with Candid in Rust.

  • As a typed Rust data structure. When you write canisters or frontend in Rust, you want to have a seamless way of converting data between Rust and Candid.
  • As an untyped Candid value. When you write generic tools for the Internet Computer without knowing the type of the Candid data.
  • As text data. When you get the data from CLI or read from a file, you can use the provided parser to send/receive messages.

Candid provides efficient, flexible and safe ways of converting data between each of these representations.

Operating on native Rust values

We are using a builder pattern to encode/decode Candid messages, see candid::ser::IDLBuilder for serialization and candid::de::IDLDeserialize for deserialization.

// Serialize 10 numbers to Candid binary format
let mut ser = candid::ser::IDLBuilder::new();
for i in 0..10 {
let bytes: Vec<u8> = ser.serialize_to_vec()?;

// Deserialize Candid message and verify the values match
let mut de = candid::de::IDLDeserialize::new(&bytes)?;
let mut i = 0;
while !de.is_done() {
  let x = de.get_value::<i32>()?;
  assert_eq!(x, i);
  i += 1;

Candid provides functions for encoding/decoding a Candid message in a type-safe way.

use candid::{encode_args, decode_args};
// Serialize two values [(42, "text")] and (42u32, "text")
let bytes: Vec<u8> = encode_args((&[(42, "text")], &(42u32, "text")))?;
// Deserialize the first value as type Vec<(i32, &str)>,
// and the second value as type (u32, String)
let (a, b): (Vec<(i32, &str)>, (u32, String)) = decode_args(&bytes)?;

assert_eq!(a, [(42, "text")]);
assert_eq!(b, (42u32, "text".to_string()));

We also provide macros for encoding/decoding Candid message in a convenient way.

use candid::{Encode, Decode};
// Serialize two values [(42, "text")] and (42u32, "text")
let bytes: Vec<u8> = Encode!(&[(42, "text")], &(42u32, "text"))?;
// Deserialize the first value as type Vec<(i32, &str)>,
// and the second value as type (u32, String)
let (a, b) = Decode!(&bytes, Vec<(i32, &str)>, (u32, String))?;

assert_eq!(a, [(42, "text")]);
assert_eq!(b, (42u32, "text".to_string()));

The Encode! macro takes a sequence of Rust values, and returns a binary format Vec<u8> that can be sent over the wire. The Decode! macro takes the binary message and a sequence of Rust types that you want to decode into, and returns a tuple of Rust values of the given types.

Note that a fixed Candid message may be decoded in multiple Rust types. For example, we can decode a Candid text type into either String or &str in Rust.

Operating on user defined struct/enum

We use trait CandidType for serialization. Deserialization requires both CandidType and Serde's Deserialize trait. Any type that implements these two traits can be used for serialization and deserialization. This includes built-in Rust standard library types like Vec<T> and Result<T, E>, as well as any structs or enums annotated with #[derive(CandidType, Deserialize)].

We do not use Serde's Serialize trait because Candid requires serializing types along with the values. This is difficult to achieve in Serialize, especially for enum types. Besides serialization, CandidType trait also converts Rust type to Candid type defined as candid::types::Type.

#[cfg(feature = "serde_bytes")]
use candid::{Encode, Decode, CandidType, Deserialize};
#[derive(CandidType, Deserialize)]
enum List {
    #[serde(rename = "nil")]
    #[serde(with = "serde_bytes")]
    Cons(i32, Box<List>),
let list = List::Cons(42, Box::new(List::Nil));

let bytes = Encode!(&list)?;
let res = Decode!(&bytes, List)?;
assert_eq!(res, list);

We support serde's rename attributes for each field, namely #[serde(rename = "foo")] and #[serde(rename(serialize = "foo", deserialize = "foo"))]. This is useful when interoperating between Rust and Motoko canisters involving variant types, because they use different naming conventions for field names.

We support #[serde(with = "serde_bytes")] for efficient handling of &[u8] and Vec<u8>. You can also use the wrapper type serde_bytes::ByteBuf and serde_bytes::Bytes.

Note that if you are deriving Deserialize trait from Candid, you need to import serde as a dependency in your project, as the derived implementation will refer to the serde crate.

Operating on big integers

To support big integer types Candid::Int and Candid::Nat, we use the num_bigint crate. We provide interface to convert i64, u64, &str and &[u8] to big integers. You can also use i128 and u128 to represent Candid int and nat types respectively (decoding will fail if the number is more than 128 bits).

#[cfg(feature = "bignum")]
use candid::{Int, Nat, Encode, Decode};
let x = "-10000000000000000000".parse::<Int>()?;
let bytes = Encode!(&Nat::from(1024u32), &x)?;
let (a, b) = Decode!(&bytes, Nat, Int)?;
let (c, d) = Decode!(&bytes, u128, i128)?;
assert_eq!(a + 1u8, 1025u32);
assert_eq!(b, Int::parse(b"-10000000000000000000")?);

Operating on reference types

The type of function and service references cannot be derived automatically. We provide two macros define_function! and define_service! to help defining the reference types. To specify reference types in the macro, you need to use the corresponding Rust types, instead of the Candid types.

#[cfg(feature = "bignum")]
use candid::{define_function, define_service, func, Encode, Decode, Principal};
let principal = Principal::from_text("aaaaa-aa").unwrap();

define_function!(pub CustomFunc : (u8, &str) -> (u128));
let func = CustomFunc::new(principal, "method_name".to_string());
assert_eq!(func, Decode!(&Encode!(&func)?, CustomFunc)?);

define_service!(MyService : {
  "f": CustomFunc::ty();
  "g": func!((candid::Int) -> (candid::Nat, CustomFunc) query)
let serv = MyService::new(principal);
assert_eq!(serv, Decode!(&Encode!(&serv)?, MyService)?);

Operating on untyped Candid values

Any valid Candid value can be manipulated in an recursive enum representation candid::parser::value::IDLValue. We use ser.value_arg(v) and de.get_value::<IDLValue>() for encoding and decoding the value. The use of Rust value and IDLValue can be intermixed.

#[cfg(feature = "value")]
use candid::types::value::IDLValue;
// Serialize Rust value Some(42u8) and IDLValue "hello"
let bytes = candid::ser::IDLBuilder::new()

// Deserialize the first Rust value into IDLValue,
// and the second IDLValue into Rust value
let mut de = candid::de::IDLDeserialize::new(&bytes)?;
let x = de.get_value::<IDLValue>()?;
let y = de.get_value::<&str>()?;

assert_eq!(x, IDLValue::Opt(Box::new(IDLValue::Nat8(42))));
assert_eq!(y, "hello");

Building the library as a JS/Wasm package

With the help of wasm-bindgen and wasm-pack, we can build the library as a Wasm binary and run in the browser. This is useful for client-side UIs and parsing did files in JavaScript.

Create a new project with the following Cargo.toml.

crate-type = ["cdylib"]

wasm-bindgen = "0.2"
candid = "0.9.0"
candid_parser = "0.1.0"

lto = true
opt-level = 'z'

Expose the methods in lib.rs

use candid::TypeEnv;
use candid_parser::{check_prog, IDLProg};
use wasm_bindgen::prelude::*;
pub fn did_to_js(prog: String) -> Option<String> {
  let ast = prog.parse::<IDLProg>().ok()?;
  let mut env = TypeEnv::new();
  let actor = check_prog(&mut env, &ast).ok()?;
  Some(candid_parser::bindings::javascript::compile(&env, &actor))


cargo install wasm-pack
wasm-pack build --target bundler
wasm-opt --strip-debug -Oz pkg/didc_bg.wasm -o pkg/didc_bg.wasm


const didc = import("pkg/didc");
didc.then((mod) => {
  const service = "service : {}";
  const js = mod.did_to_js(service);


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