Ergo Example

Hello World

Each Ergo files is a module and can contain declarations (e.g., functions, contracts). The following is the “Hello World!” contract in Ergo:

namespace org.accordproject.helloworld

contract HelloWorld over TemplateModel {
  // Simple Clause
  clause greet(request : Request) : Response {
    return Response{ output: "Hello " ++ contract.name ++ " " ++ request.input }
  }
}

It declares that the file belongs to the org.accordproject.helloworld namespace and contains a single HelloWorld contract with one greet clause.

It also declares that the contract HelloWorld is parameterized over a given TemplateModel.

The TemplateModel as well as the Request and Response are types which are specified using the Hyperledger CTO format.

The clause takes a Request as input and returns a Response.

The code for the clause just constructs a new Response with a property output which is a string containing the property name of from the contract (contract) and the property input from the request (request).

Eat Apples!

A more complete example is the healthy eating clause inspired by a not so serious terms of services contract.

For this example, let us look first at the template for that legal clause written in natural language:

Eating healthy clause between [{employee}] (the Employee) and [{company}] (the Company).
The canteen only sells apple products. Apples, apple juice, apple flapjacks, toffee
apples. Employee gets fired if caught eating anything without apples in it.
THE EMPLOYEE, IF ALLERGIC TO APPLES, SHALL ALWAYS BE HUNGRY.
Apple products at the canteen are subject to a [{tax}]% tax.

The text specify the terms for the legal clause, and includes a few variables such as employee, company and tax.

The second component of a smart legal template is the model, which is expressed using the Hyperledger Composer Modeling Language. The model describe the variables of the contract, as well as additional information required to execute the contract logic. In our example, this includes the input request for the clause (Food), the response to that request (Outcome) and possible events emitted during the clause execution (Bill).

namespace org.accordproject.canteen

@AccordTemplateModel("eat-apples")
concept CanteenContract {
  o String employee
  o String company
  o Double tax
}

transaction Food {
  o String produce
  o Double price
}

transaction Outcome {
  o String notice
}

event Bill {
  o String billTo
  o Double amount
}

The last component of a smart legal template is the Ergo logic. In our example it is a single clause eathealthy which can be used to process a Food request.

namespace org.accordproject.canteen

contract EatApples over CanteenContract {
  clause eathealthy(request : Food) : Outcome {
    enforce request.produce = "apple"
    else return Outcome{ notice : "You're fired!" };

    emit Bill{
      billTo: contract.employee,
      amount: request.price * (1.0 + contract.tax / 100.0)
    };
    return Outcome{ notice : "Very healthy!" }
  }
}

As in the "Hello World!" example, this is a smart legal contract with a single clause, but it illustrate a few new ideas.

The enforce expression is used to check conditions that must be true for normal execution of the clause. In this example, the enforce makes sure the food is an apple and if not returns a new outcome indicating termination of employment.

If the condition is true, the contract proceeds by emitting a bill for the purchase of the apple. The employee to be billed is obtained from the contract (contract.employee). The total amount is calculated by adding the tax, which is obtained from the contract (contract.tax), to the purchase price, which is obtained from the request (request.price). The calculation is done using a simple arithmetic expression (request.price * (1.0 + contract.tax / 100.0)).