#internet-computer #idl #dfinity #parser #candid


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

25 releases (5 breaking)

0.6.7 Sep 22, 2020
0.5.4 Sep 3, 2020
0.5.0 Jul 30, 2020

#129 in Encoding

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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 strcuture. 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;
# Ok::<(), candid::Error>(())

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()));
# Ok::<(), candid::Error>(())

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()));
# Ok::<(), candid::Error>(())

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, and Serde's Deserialize trait for deserialization. Any type that implements these two traits can be used for serialization and deserialization respectively. 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.

use candid::{Encode, Decode, CandidType, Deserialize};
#[derive(CandidType, Deserialize)]
# #[derive(Debug, PartialEq)]
enum List {
    #[serde(rename = "nil")]
    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);
# Ok::<(), candid::Error>(())

We also 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.

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.

use candid::{Int, Nat, Encode, Decode};
let x = "-10000000000000000000".parse::<Int>()?;
let bytes = Encode!(&Nat::from(1024), &x)?;
let (a, b) = Decode!(&bytes, Nat, Int)?;
assert_eq!(a + 1, 1025);
assert_eq!(b, Int::parse(b"-10000000000000000000")?);
# Ok::<(), candid::Error>(())

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.

use candid::parser::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");
# Ok::<(), candid::Error>(())

We provide a data structure candid::IDLArgs to represent a sequence of IDLValues, and use to_bytes() and from_bytes() to encode and decode Candid messages. We also provide a parser to parse Candid values in text format.

use candid::IDLArgs;
// Candid values represented in text format
let text_value = r#"
     (42, opt true, vec {1;2;3},
      opt record {label="text"; 42="haha"})

// Parse text format into IDLArgs for serialization
let args: IDLArgs = text_value.parse()?;
let encoded: Vec<u8> = args.to_bytes()?;

// Deserialize into IDLArgs
let decoded: IDLArgs = IDLArgs::from_bytes(&encoded)?;
assert_eq!(encoded, decoded.to_bytes()?);

// Convert IDLArgs to text format
let output: String = decoded.to_string();
let parsed_args: IDLArgs = output.parse()?;
assert_eq!(args, parsed_args);
# Ok::<(), candid::Error>(())

Note that when parsing Candid values, we assume the number literals are always of type Int. This can be changed by providing the type of the method arguments, which can usually be obtained by parsing a Candid file in the following section.

Operating on Candid AST

We provide a parser and type checker for Candid files specifying the service interface.

use candid::{IDLProg, TypeEnv, check_prog, types::Type};
let did_file = r#"
    type List = opt record { head: int; tail: List };
    type byte = nat8;
    service : {
      f : (byte, int, nat, int8) -> (List);
      g : (List) -> (int) query;

// Parse did file into an AST
let ast: IDLProg = did_file.parse()?;

// Pretty-print AST
let pretty: String = candid::parser::types::to_pretty(&ast, 80);

// Type checking
let mut env = TypeEnv::new();
let actor: Type = check_prog(&mut env, &ast)?.unwrap();
let method = env.get_method(&actor, "g").unwrap();
assert_eq!(method.is_query(), true);
assert_eq!(method.args, vec![Type::Var("List".to_string())]);
# Ok::<(), candid::Error>(())

Serializing untyped Candid values with type annotations.

With type signatures from the Candid file, candid::IDLArgs uses to_bytes_with_types function to serialize arguments directed by the Candid types. This is useful when serializing different number types and recursive types. There is no need to use types for deserialization as the types are available in the Candid message.

use candid::{IDLArgs, parser::value::IDLValue};
# use candid::{IDLProg, TypeEnv, check_prog};
# let did_file = r#"
#    type List = opt record { head: int; tail: List };
#    type byte = nat8;
#    service : {
#      f : (byte, int, nat, int8) -> (List);
#      g : (List) -> (int) query;
#    }
# "#;
# let ast = did_file.parse::<IDLProg>()?;
# let mut env = TypeEnv::new();
# let actor = check_prog(&mut env, &ast)?.unwrap();
// Get method type f : (byte, int, nat, int8) -> (List)
let method = env.get_method(&actor, "f").unwrap();
let args = "(42, 42, 42, 42)".parse::<IDLArgs>()?;
// Serialize arguments with candid types
let encoded = args.to_bytes_with_types(&env, &method.args)?;
let decoded = IDLArgs::from_bytes(&encoded)?;
# Ok::<(), candid::Error>(())


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