There is only one data type in Cairo: a field element, or felt.

A field element is best considered an unsigned 252-bit integer (uint252) for practical purposes.

Short strings up to 31 ASCII characters can be used, but they are represented internally as felts.

%lang starknet
%builtins range_check

func echo_world(
        user_number : felt
    ) -> (
        user_number_echoed : felt,
        short_string : felt,
        mangled_string : felt,
        hello_string : felt,
    # Short strings are ASCII encoded humbers.
    # They are identified by single quotation marks.
    # They are actually just numbers (felts), not true strings.
    # 'ab' = a:61 b:62 = 0x6162 = 24930
    let short_string = 'ab'
    # Adding to a string does not make sense.
    # 24930 + 1 = 24931 = 0x6163 = a:61 c:63 = 'ac'
    let mangled_string = short_string + 1

    # h 68 e 65 l 6c l 6c o 6f
    # 0x68656c6c6f = 448378203247
    let hello_string = 'hello'
    return (user_number, short_string, mangled_string, hello_string)

Things to note:

  • The new string representation is an ASCII-lookup conversion of two hex characters to one ASCII character. Adding strings does not perform concatenation, it performs addition of the underlying felts.

Save as contracts/data_types.cairo



nile compile

Or compile this specific contract

nile compile contracts/data_types.cairo


Make a new file called and populate it:

import pytest
import asyncio
from starkware.starknet.testing.starknet import Starknet

# Enables modules.
def event_loop():
    return asyncio.new_event_loop()

# Reusable to save testing time.
async def contract_factory():
    starknet = await Starknet.empty()
    contract = await starknet.deploy("contracts/data_types.cairo")
    return starknet, contract

async def test_contract(contract_factory):
    starknet, contract = contract_factory

    # Read from contract
    response = await contract.echo_world(10).call()
    assert response.result == (10, 24930, 24931, 448378203247)

Run the test

pytest tests/

Local Deployment

Deploy to the local devnet.

nile deploy data_types --alias data_types



nile call data_types echo_world 10


10 24930 24931 448378203247

Public deployment

Will default to the Goerli/alpha testnet until mainnet is available.

nile deploy data_types --alias data_types --network mainnet
🚀 Deploying data_types
🌕 artifacts/data_types.json successfully deployed to 0x076ee2784726bf5a29edbcd8fdce5524416f0a06260ef83547d0151af1215d90
📦 Registering deployment as data_types in mainnet.deployments.txt

Deployments can be viewed in the voyager explorer