Aeternity is a public blockchain with its own crypto token AE. This blockchain goes beyond the Smart Contracts concept brought by Ethereum and adds new components into its protocol, such as state channels, oracles and human-readable names (AENS) for contract and user addresses. The main Aeternity software is built in Erlang, which is known for its high-availability and scalability characteristics. The Aeternity team has many experienced Erlang developers, which should help to build robust blockchain technology.
One of the interesting properties of the Aeternity platform is support of the Ethereum Virtual Machine, so that Solidity contracts can be deployed to Aeternity Network as well. Besides that, there are two new languages, called Sophia and Varna. In this article, we are going to use the Sophia language to implement our smart contracts.
Certification using Smart Contract
Let’s imagine some real life scenario, where trust would be guaranteed by blockchain technology using smart contracts.
Credentials and certificates are good examples to demonstrate the function of oracles, smart contracts and the public blockchain in general. In the area of certification, a person gets their certificate by doing trainings and taking exams. Every training may bring them a particular amount of points. These points eventually allow participating in an exam, which can result in a particular certification, in case they pass.
We model our system as a pair of the following contracts:
- Examinee: stores achieved certification levels and trainings. It encapsulates business logic for granting access to the next level of certification
- ExamVerifier: calculates the grade based on the exam points and potentially on external information. It will be our oracle entity
We assume that there must be an owner of this application. Such an owner deploys an Examinee contract for every new student who wants to start their certification path.
ExamVerifier can be deployed only one time and may exists as a singleton object. However, a particular category of certification may require another ExamVerifier instance. We leave this as an open question to the smart contract deployer. We will look at the deployment process later.
Blockchain oracle(s) is not the company behind the SQL database and Java, but actually a source of information from the physical world. The Oxford Dictionary of English defines „oracle“ as follows:
a person or thing regarded as an infallible authority on something: for instance, „he reigned supreme as the Colonial Office’s oracle on Africa“
An oracle can be considered as an external expert or adviser. Oracles help blockchain contracts to facilitate particular business logic. They report some information, which may lead to a new state of a smart contract. Anybody can make a query to an oracle and expect a result. An example of an oracle query is „What will the weather be like tomorrow: sunny or rainy?“. Eventually, the oracle will report: sunny. Importantly, oracles do not predict the future, they report facts. There can be many oracles, so that the smart contract can ask the same question multiple times, and then decide which answer to trust. How to use oracles is up to the smart contract logic. IoT devices, such as sensors, can be another example of oracles. An oracle as an entity could be modelled as a smart contract as well. However, having them as part of the protocol, like in Aeternity, makes it easier to design an application on top of a blockchain. A good explanation of oracles can be found at the Aeternity blog.
Code in Sophia
Before we look at Sophia code, let us highlight a couple of points about this language to make reading the code easier.
- Sophia is very similar to the Reason language and based on OCaml
- it uses indentations for code blocks like Python instead of curly braces like C
- it requires special constructs to mutate the contract state
- variable types are set on the right hand side; the colon sign is used to separate variable name and type
- it has type aliases, generic types, pattern matching, closures
In general, it is much easier and nicer to write code in Sophia, than in Solidity.
Let’s start with with a simple contract.
- o – is a reference to a newly registered Aeternity oracle object, which will be used inside the current contract and by the Examinee contract as well
- exams – a hash map to store examinee addresses with their query objects
- qfee – an oracle fee for making a query. This is the amount of AE tokens that needs to be paid by examinee contract
The ExamVerifier smart contract wraps an oracle object, which is created as part of the contract constructor. An Aeternity oracle can be registered and used off-chain as well. Aeternity Node provides a REST API for that. The current implementation of a wrapping oracle inside the contract is just our design choice.
Business Use Case of Oracles
In general, our oracle owner can be a certification institute, which has off-chain access to examinee data. This organization may have business incentives to take and respond to queries.
An examinee goes to any certification provider nearby to take an exam. Upon result submission to the blockchain, the examinee contract asks the certification institute for the exam grade. This request includes a fee, which will be given to the certification institute. This fee will be set in advance by the certification provider in the examinee contract, so that the examinee just needs to pay once and off-chain for their exam submission.
Every function above has a short description of its logic. There are functions to mutate state and trigger remote contracts. The code is quite straightforward so far. To summarize the first part of this contract, we have the following ‚write‘ functions:
We also need a couple of functions like:
canRequestHomework – returns true, if the examinee can take the next certification level, which includes some hypothetical homework, otherwise returns false
retrieveExamGrade – to look up an answer of the oracle contract from the blockchain state and update Examinee contract state
Examinee contract, continued
We have designed the
foundationLevel certificate as an
option of the
foundationLevel record type. Sophia does not have
null/undefined values, and that is really great. Instead, it has variant types and the default variant type for undefined values
Before we implement the remaining functions, let us define some helpers for option type:
These functions should be familiar to functional programmers. Basically, we have created a monad for the existing option type. Now, it will be easier to implement our functions, which need to deal with variant types like option. Get ready for functional programming:
getOrElse would lead to clumsy code with nested pattern matching like:
It is important to mention that Sophia is a strongly typed language with type inference. Type inference helps to reduce amount of code, which needs to be written. For example:
will be inferred to
record foundationLevel, which is defined in the contract.
At this point, we are ready to deploy both contracts to the Aeternity blockchain.
Aeternity offers the Forgae framework for development of decentralized applications. It helps to set up a project template, compile, unit test and deploy smart contracts. It comes with a CLI and a local Aeternity node.
First, we need to launch the local node:
then, we can compile all existing contracts with:
and finally, deploy to the local node
We have skipped a big part related to project setup with Forgae, as well as the process for writing deployment scripts, unit tests and web-application code. Most of these topics we will cover in the next blogpost about Aeternity Smart Contract development.
Aeternity is a promising blockchain, which brings a great language to implement better quality smart contracts than in Solidity. Smart contract developers and multiple blockchain companies realised that language support to build correct and robust code is very important. At the end of the day, these contracts move money, which can be stolen – which already happened in the case of the DAO hack in the Ethereum blockchain. Functional programming is the best available tool today for developers to write correct business code.
The Aeternity team is developing a number of tools to support developers in building decentralized applications on top of the Aeternity blockchain. There is still a lot to do in terms of developer experience and tooling. However, existing frameworks like Forgae are a great boost in developers productivity. It took a while for Ethereum community to build the Truffle framework, which is similar to Aeternity Forgae.
Further reading and links
- Sophia Language Documentation
- Sophia Introduction and Tutorial
- Monad in Functional Programming
- Smartphone image source