How to use solidity library with truffle

Question

I'm using truffle V5.1.13 and when I run truffle test the res variable contains a transaction receipt rather than the expected 2. I notice if I remove everything that has to do with the library things work as expected.

Why is this happening? How can I use my library?

Contract code:

pragma solidity ^0.5.1;

library TestLib{
    struct DataSet {
        uint a;
    }

    function assign(DataSet storage self) internal {
        self.a = 2;
    }

    function getA(DataSet storage self) internal view returns(uint) {
        return self.a;
    }
}

contract TestContract {
    using TestLib for TestLib.DataSet;
    TestLib.DataSet set;

    function getBalanceInEth() public returns(uint) {
        set.assign();
        return set.getA();
    }
}

Test code:

    const TestContract = artifacts.require('TestContract')

    contract('TestContract', async accounts => {
        let TestContractInstance

        beforeEach('setup contract for each test', async function() {
           TestContractInstance = await TestContract.deployed()
        })

        it('Should return 2', async () => {
           let res = await TestContractInstance.getBalanceInEth()
           console.log(res)
           assert.equal(res, 2)
        })
    })

Migration Code:

const TestContract = artifacts.require('TestContract')
const TestLib = artifacts.require('TestLib')
module.exports = function(deployer) {
  deployer.deploy(TestLib)
  deployer.link(TestLib, TestContract)
  deployer.deploy(TestContract)
}

console.log(res) gives:

{
  tx: '0x1f05b5fa7ed635aaa619cc548a0a37ff9d9d57398d85ecb3f8000ed7e124373c',
  receipt: {
    transactionHash: '0x1f05b5fa7ed635aaa619cc548a0a37ff9d9d57398d85ecb3f8000ed7e124373c',
    transactionIndex: 0,
    blockHash: '0xe6bc488fce167970b781588d1c70c0b4a452834c78daf4f052c1a045575fd959',
    blockNumber: 3,
    from: '0x627306090abab3a6e1400e9345bc60c78a8bef57',
    to: '0xf12b5dd4ead5f743c6baa640b0216200e89b60da',
    gasUsed: 42158,
    cumulativeGasUsed: 42158,
    contractAddress: null,
    logs: [],
    status: true,
    logsBloom: '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
    rawLogs: []
  },
  logs: []
}

Answer 1

A possible solution to this problem is to change the contract code to the following:

contract TestContract {
    using TestLib for TestLib.DataSet;
    TestLib.DataSet set;

    function setBalanceInEth() public {
        set.assign();
    }

    function getBalanceInEth() public view returns(uint) {
        return set.getA();
    }
}

And change the test code accordingly:

const TestContract = artifacts.require('TestContract')

contract('TestContract', async (accounts) => {
  let TestContractInstance

  beforeEach('setup contract for each test', async function() {
    TestContractInstance = await TestContract.deployed()
  })

  it('Should return 2', async () => {
    await TestContractInstance.setBalanceInEth()
    let res = await TestContractInstance.getBalanceInEth()
    console.log(res)
    assert.equal(res, 2)
  })
})