query erc20 contract using go-ethereum

query-erc20.md

Commands

solc --abi erc20.sol
abigen --abi=erc20_sol_ERC20.abi --pkg=token --out=erc20.go

erc20.sol

pragma solidity ^0.4.24;

contract ERC20 {
    string public constant name = "";
    string public constant symbol = "";
    uint8 public constant decimals = 0;

    function totalSupply() public constant returns (uint);
    function balanceOf(address tokenOwner) public constant returns (uint balance);
    function allowance(address tokenOwner, address spender) public constant returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

contract_read_erc20.go

package main

import (
    "fmt"
    "log"
    "math"
    "math/big"

    "github.com/ethereum/go-ethereum/accounts/abi/bind"
    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/ethclient"

    token "./contracts_erc20" // for demo
)

func main() {
    client, err := ethclient.Dial("https://mainnet.infura.io")
    if err != nil {
        log.Fatal(err)
    }

    // Golem (GNT) Address
    tokenAddress := common.HexToAddress("0xa74476443119A942dE498590Fe1f2454d7D4aC0d")
    instance, err := token.NewToken(tokenAddress, client)
    if err != nil {
        log.Fatal(err)
    }

    address := common.HexToAddress("0x0536806df512d6cdde913cf95c9886f65b1d3462")
    bal, err := instance.BalanceOf(&bind.CallOpts{}, address)
    if err != nil {
        log.Fatal(err)
    }

    name, err := instance.Name(&bind.CallOpts{})
    if err != nil {
        log.Fatal(err)
    }

    symbol, err := instance.Symbol(&bind.CallOpts{})
    if err != nil {
        log.Fatal(err)
    }

    decimals, err := instance.Decimals(&bind.CallOpts{})
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("name: %s\n", name)         // "name: Golem Network"
    fmt.Printf("symbol: %s\n", symbol)     // "symbol: GNT"
    fmt.Printf("decimals: %v\n", decimals) // "decimals: 18"

    fmt.Printf("wei: %s\n", bal) // "wei: 74605500647408739782407023"

    fbal := new(big.Float)
    fbal.SetString(bal.String())
    value := new(big.Float).Quo(fbal, big.NewFloat(math.Pow10(int(decimals))))

    fmt.Printf("balance: %f", value) // "balance: 74605500.647409"
}