Created using remix-ide: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://remix.ethereum.org/#version=soljson-v0.4.17+commit.bdeb9e52.js&optimize=false&gist=

test_external_function.sol

pragma solidity >=0.4.16 <0.6.0;

contract Oracle{
    struct Request{
        bytes data;
        function(uint) external callback;
    }
    Request[] requests;
    event NewRequest(uint);
    event Reply(uint);
    function query(bytes memory data, function(uint) external callabck)
    public {
        requests.push(Request(data,callabck));
        NewRequest(requests.length-1);
        reply(requests.length-1, 100);
    }
    function reply(uint requestID, uint response) public{
        requests[requestID].callback(response);
        Reply(requestID);
    }
}

contract OracleUser{
    Oracle constant oracle = Oracle(0x1c0ec5e2c07b55effe5a2058a3c5b5e581d9dee8); // known contract
    uint exchangeRate;
    function buySomething() public{
        oracle.query("USD", this.oracleResponse);
    }
    function oracleResponse(uint response) public{
        require(msg.sender == address(oracle));
            //,"Only oracle can call this.");
        exchangeRate = response;
    }
    function getExchangeRate() public returns(uint){
        return exchangeRate;
    }
}

test_function_type.sol

//Write your own contracts here. Currently compiles using solc v0.4.15+commit.bbb8e64f.
pragma solidity >=0.4.16 <0.6.0;

contract Selector{
  function f() public view returns(bytes4){
    return this.f.selector;
  }
}

test_internal_function.sol

pragma solidity >=0.4.16 <0.6.0;

library ArrayUtils {
    // internal functions can be used in internal library functions because
    // they will be part of the same code context
    function map(uint[] memory self, function (uint) pure returns (uint) f)
    internal pure returns (uint[] memory r){
        r = new uint[](self.length);
        for(uint i=0; i<self.length; i++){
            r[i] = f(self[i]);
        }
    }
    function reduce(uint[] memory self, function (uint, uint) pure returns (uint) f)
    internal pure returns (uint r){
        r = self[0];
        for(uint i=1; i<self.length; i++){
            r = f(r, self[i]);
        }
    }
    function range(uint length) 
    internal pure returns (uint[] memory r){
        r = new uint[](length);
        for(uint i=0; i<r.length; i++){
            r[i] = i;
        }
    }
}

contract Pyramid{
    using ArrayUtils for *;
    
    function pyramid(uint l) public pure returns(uint){
        return ArrayUtils.range(l).map(square).reduce(sum);
    }
    function square(uint x) internal pure returns(uint){
        return x*x;
    }
    function sum(uint x, uint y) internal pure returns(uint){
        return x+y;
    }
}