solidity_HOF.sol

/*
browser/dType_V2.sol:52:29: TypeError: Invalid type for argument in function call. Invalid implicit conversion from bytes24 to function (struct typeALib.TypeA memory) pure external returns (struct typeALib.TypeA memory) requested.
        return typeALib.map(signature, typeAarr);
                            ^-------^
*/

pragma solidity ^0.5.0;
pragma experimental ABIEncoderV2;

library typeALib {

    struct TypeA {
        uint256 balance;
        address token;
    }

    function map(
        function(TypeA memory) external pure returns (TypeA memory) callback,
        TypeA[] memory typeAarr
    )
        view
        public
        returns (TypeA[] memory result)
    {
        uint length = typeAarr.length;
        result = new TypeA[](length);

        for (uint i = 0; i < length; i++) {
            result[i] = callback(typeAarr[i]);
        }

        return result;
    }
}

contract typeAAFunctions {
    using typeALib for typeALib.TypeA;

    function double(typeALib.TypeA memory typeA)
        pure
        public
        returns(typeALib.TypeA memory result)
    {
        typeA.balance *= 2;
        return typeA;
    }

    function doubleBalances(typeALib.TypeA[] memory typeAarr)
        view
        public
        returns(typeALib.TypeA[] memory result)
    {
        bytes memory sign = abi.encodePacked(address(this), this.double.selector);
        bytes24 signature;
        assembly {
            signature := mload(add(sign, 32))
        }
        return typeALib.map(signature, typeAarr);
    }
    
    function doubleBalances2(typeALib.TypeA[] memory typeAarr)
        view
        public
        returns(typeALib.TypeA[] memory result)
    {
        return typeALib.map(this.double, typeAarr);
    }
}