Writing a smart contract in Golang (Go)

Prerequisites

Sample contract functionality

The sample contract serves as a demonstration of general QVM logic.

It is capable of:

• registering users into the database
• storing the last registered user's name in the database
• echoing the previous user's name to STDOUT
• incrementing the total registered user count
• storing above counter in the database

Constructor logic

While some programming languages do not have explicit support for constructors you are free to implement your own logic for contract initialization after it gets deployed.

Your binary will be invoked with a single “construct” argument.

Golang (Go) smart contract sample

package main

import (
    "fmt"
    "os"
    "strconv"
)

var maxUser int = 100

// $0 construct
func constructor() {
    if os.Getenv("DB_QVM_INITIALIZED") == "true" {
        panic("contract is already initialized")
    }
    os.Stdout.WriteString(fmt.Sprintf("DBW=QVM_INIT_MAXUSER=%s\n", fmt.Sprintf("%d", maxUser)))
}

func initialize() {
    if os.Getenv("DB_QVM_INITIALIZED") != "true" {
        panic("contract is not initialized")
    }
    maxUser, _ = strconv.Atoi(os.Getenv("DB_QVM_INIT_MAXUSER"))
}

func main() {
    if len(os.Args) == 2 && os.Args[1] == "construct" {
        constructor()
        os.Exit(0)
    } else {
        initialize()
    }

    // THIS SAMPLE ONLY SUPPORTS THE "register" FUNCTION
    if len(os.Args) == 3 && os.Args[1] == "register" {

        // GET THE CURRENT USER'S NAME
        previousName := os.Getenv("DB_USER_CURRENT")

        // OR DEFAULT TO "unknown" IF THIS IS THE FIRST CALL
        if len(previousName) == 0 {
            previousName = "unknown"
        }

        // GET THE TOTAL USER COUNT
        totalUserCount, _ := strconv.Atoi(os.Getenv("DB_TOTALUSERS"))
        if totalUserCount+1 > maxUser {
            panic("exceeded max user")
        }

        // WRITE PREVIOUS USER NAME TO STDOUT
        os.Stdout.WriteString(fmt.Sprintf("OUT=prevname: %s\n", previousName))

        // UPDATE CURRENT USER NAME BY WRITING IT TO DB
        os.Stdout.WriteString(fmt.Sprintf("DBW=USER_CURRENT=%s\n", os.Args[2]))

        // STORE USER NAME UNDER A STORAGE SLOT FOR PERSISTENCE (CURRENT GETS OVERWRITTEN ON EACH CALL)
        os.Stdout.WriteString(fmt.Sprintf("DBW=USER_%d=%s\n", totalUserCount, os.Args[2]))

        // INCREMENT THE TOTAL USER COUNT
        os.Stdout.WriteString(fmt.Sprintf("DBW=TOTALUSERS=%d\n", totalUserCount+1))
        os.Exit(0)
    }
    if len(os.Args) >= 2 {
        os.Stderr.WriteString(fmt.Sprintf("Wrong CMD: %s\n", os.Args[1]))
        os.Exit(1)
    }
    os.Stderr.WriteString("Wrong args!\n")
    os.Exit(1)
}

Save the contract

Open a text editor and save above sample contract as main.go file.