Error: invalid solidity type!: (library.enum)

Question

Consider the following sample Truffle project:

.
├── contracts
│   ├── Foo.sol
│   └── Migrations.sol
├── migrations
│   ├── 1_initial_migration.js
│   └── 2_foo.js
├── test
│   └── Foo.test.js
└── truffle.js

Contents of contracts/Foo.sol:

pragma solidity ^0.4.21;
contract Foo {
  enum Bar {zero, one, two, three}
  function getOne() public pure returns (Bar) {
    return Bar.one;
  }
}

Contents of test/Foo.test.js:

let Foo = artifacts.require("./Foo.sol");
contract("", async (accounts) => {
    it("", async () => {
        let instance = await Foo.deployed();
        let one = await instance.getOne();
        assert.equal(one, 1);
    });
});

Let's test our project:

$ truffle test
Compiling ./contracts/Foo.sol...
Compiling ./contracts/Migrations.sol...


  Contract: 
    ✓ 


  1 passing (27ms)

As you can see, everything is perfect. Now, just for the sake of science, let's turn our contract into a library in contracts/Foo.sol:

pragma solidity ^0.4.21;
library Foo {
  enum Bar {zero, one, two, three}
  function getOne() public pure returns (Bar) {
    return Bar.one;
  }
}

And let's run the test again:

$ truffle test
Compiling ./contracts/Foo.sol...
Compiling ./contracts/Migrations.sol...


  Contract: 
    1) 
    > No events were emitted


  0 passing (32ms)
  1 failing

  1) Contract:  :
     Error: invalid solidity type!: Foo.Bar
      at SolidityCoder._requireType (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/solidity/coder.js:61:1)
      at /home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/solidity/coder.js:231:1
      at Array.map (<anonymous>)
      at SolidityCoder.getSolidityTypes (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/solidity/coder.js:230:1)
      at SolidityCoder.decodeParams (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/solidity/coder.js:203:1)
      at SolidityFunction.unpackOutput (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/web3/function.js:113:1)
      at Object.callback (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/web3/function.js:145:1)
      at /home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/web3/method.js:142:1
      at /home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/web3/requestmanager.js:89:1
      at /home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/truffle-provider/wrapper.js:134:1
      at XMLHttpRequest.request.onreadystatechange (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/web3/lib/web3/httpprovider.js:128:1)
      at XMLHttpRequestEventTarget.dispatchEvent (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/xhr2/lib/xhr2.js:64:1)
      at XMLHttpRequest._setReadyState (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/xhr2/lib/xhr2.js:354:1)
      at XMLHttpRequest._onHttpResponseEnd (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/xhr2/lib/xhr2.js:509:1)
      at IncomingMessage.<anonymous> (/home/emilio/.nvm/versions/node/v9.8.0/lib/node_modules/truffle/build/webpack:/~/xhr2/lib/xhr2.js:469:1)
      at endReadableNT (_stream_readable.js:1101:12)
      at process._tickCallback (internal/process/next_tick.js:114:19)

Why?

First I said to myself: sure, it's because libraries can't have state variables. But then I realized an enum is not a state variable; it's a type definition. The documentation uses structs extensively, which I would say are similar in nature to enums. Also, although not shown in this example, when referencing the library enum from a contract, it compiles well, which might suggest it's a web3js issue.

So what's going on? Is this by design? What's the best practice here? Define the enum in every contract that uses the library? Turn the enum into a struct? What am I missing?

Reproducibility info

For the sake of reproducibility I include the (rather trivial) contents of migrations/2_foo.js:

let Foo = artifacts.require("./Foo.sol");
module.exports = (deployer) => {
    deployer.deploy(Foo);
};

And truffle.js:

module.exports = {
    networks: {
        development: {
            host: "127.0.0.1",
            port: 8545,
            network_id: "*" // Match any network id
        }
    }
};

The remaining files, as you know, are created by truffle init.

I also run ganache-cli in a separate terminal as my testnet.

Here are the versions of the development tools I'm using:

$ truffle version
Truffle v4.1.5 (core: 4.1.5)
Solidity v0.4.21 (solc-js)
$ ganache-cli version
Ganache CLI v6.1.0 (ganache-core: 2.1.0)

Answer 1

If you cast it to an integer type it will work properly.

function getOne() public pure returns (uint) {
  return uint(Bar.one);
}

I don't know why its not working without casting it explicitly. This is another working example but requires a contract

pragma solidity 0.4.21;

library Foo {
  enum Bar {zero, one, two, three}
  function getOne() public pure returns (Bar) {
    return Bar.one;
  }
}

contract Bar {
    function getOne() public pure returns(Foo.Bar) {
        return Foo.getOne();
    }
}

edit:

this a comment on the solidity documentation.

// Since enum types are not part of the ABI, the signature of "getChoice"
// will automatically be changed to "getChoice() returns (uint8)"
// for all matters external to Solidity. The integer type used is just
// large enough to hold all enum values, i.e. if you have more values,
// `uint16` will be used and so on.

somehow when the ABI is created for the library is generated wrongly:

"abi": [
  {
    "constant": true,
    "inputs": [],
    "name": "getOne",
    "outputs": [
      {
        "name": "",
        "type": "Foo.Bar"
      }
    ],
    "payable": false,
    "stateMutability": "pure",
    "type": "function"
  }
],

while when its created for a contract it work fine:

"abi": [
  {
    "constant": true,
    "inputs": [],
    "name": "getOne",
    "outputs": [
      {
        "name": "",
        "type": "uint8"
      }
    ],
    "payable": false,
    "stateMutability": "pure",
    "type": "function"
  }
],

so I might say it looks like a bug of the compiler or is like this by design, I can't answer to this question. But since libraries are meant to be used for internal calls an not external it might a design choice.

Anyway my suggestion is to use a contract to perform this kind of call. You might have functions that requires storage and the only way to access it is to use a contract. Libraries are useful to reuse code and perform common task across contracts.