Update documentation.

docs/common-patterns.rst

@@ -147,7 +147,10 @@ restrictions highly readable.
         // a certain address.
         modifier onlyBy(address _account)
         {
-            require(msg.sender == _account);
+            require(
+                msg.sender == _account,
+                "Sender not authorized."
+            );
             // Do not forget the "_;"! It will
             // be replaced by the actual function
             // body when the modifier is used.
@@ -164,7 +167,10 @@ restrictions highly readable.
         }
 
         modifier onlyAfter(uint _time) {
-            require(now >= _time);
+            require(
+                now >= _time,
+                "Function called too early."
+            );
             _;
         }
 
@@ -186,7 +192,10 @@ restrictions highly readable.
         // This was dangerous before Solidity version 0.4.0,
         // where it was possible to skip the part after `_;`.
         modifier costs(uint _amount) {
-            require(msg.value >= _amount);
+            require(
+                msg.value >= _amount,
+                "Not enough Ether provided."
+            );
             _;
             if (msg.value > _amount)
                 msg.sender.send(msg.value - _amount);
@@ -290,7 +299,10 @@ function finishes.
         uint public creationTime = now;
 
         modifier atStage(Stages _stage) {
-            require(stage == _stage);
+            require(
+                stage == _stage,
+                "Function cannot be called at this time."
+            );
             _;
         }
 

docs/contracts.rst

@@ -315,7 +315,10 @@ inheritable properties of contracts and may be overridden by derived contracts.
         // function is executed and otherwise, an exception is
         // thrown.
         modifier onlyOwner {
-            require(msg.sender == owner);
+            require(
+                msg.sender == owner,
+                "Only owner can call this function."
+            );
             _;
         }
     }
@@ -360,7 +363,10 @@ inheritable properties of contracts and may be overridden by derived contracts.
     contract Mutex {
         bool locked;
         modifier noReentrancy() {
-            require(!locked);
+            require(
+                !locked,
+                "Reentrant call."
+            );
             locked = true;
             _;
             locked = false;

docs/control-structures.rst

@@ -472,13 +472,16 @@ of an exception instead of "bubbling up".
 Catching exceptions is not yet possible.
 
 In the following example, you can see how ``require`` can be used to easily check conditions on inputs
-and how ``assert`` can be used for internal error checking::
+and how ``assert`` can be used for internal error checking. Note that you can optionally provide
+a message string for require, but not for assert.
+
+::
 
     pragma solidity ^0.4.0;
 
     contract Sharer {
         function sendHalf(address addr) public payable returns (uint balance) {
-            require(msg.value % 2 == 0); // Only allow even numbers
+            require(msg.value % 2 == 0, "Even value required.");
             uint balanceBeforeTransfer = this.balance;
             addr.transfer(msg.value / 2);
             // Since transfer throws an exception on failure and
@@ -517,7 +520,7 @@ did not occur. Because we want to retain the atomicity of transactions, the safe
 (or at least call) without effect. Note that ``assert``-style exceptions consume all gas available to the call, while
 ``require``-style exceptions will not consume any gas starting from the Metropolis release.
 
-The following example shows how an error string can be used together with revert:
+The following example shows how an error string can be used together with revert and require:
 
 ::
 
@@ -527,13 +530,18 @@ The following example shows how an error string can be used together with revert
         function buy(uint amount) payable {
             if (amount > msg.value / 2 ether)
                 revert("Not enough Ether provided.");
+            // Alternative way to do it:
+            require(
+                amount <= msg.value / 2 ether,
+                "Not enough Ether provided."
+            );
             // Perform the purchase.
         }
     }
 
 The provided string will be abi-encoded together with a uint value that will always be zero.
 This means that if a string ``x`` is provided, ``(0, x)`` will be encoded as if a function with arguments
-``(uint256, string)`` would be called. This zero is used as a version identifier. Future extensions
+``(uint256, string)`` was called. This zero is used as a version identifier. Future extensions
 of this feature might provide actual exception payload of dynamic type and this number could be used
 to encode the type or also as a simple numeric error code. Until this is specified, a zero will
 be used in all cases.

docs/miscellaneous.rst

@@ -333,8 +333,9 @@ Global Variables
 - ``tx.origin`` (``address``): sender of the transaction (full call chain)
 - ``assert(bool condition)``: abort execution and revert state changes if condition is ``false`` (use for internal error)
 - ``require(bool condition)``: abort execution and revert state changes if condition is ``false`` (use for malformed input or error in external component)
+- ``require(bool condition, string message)``: abort execution and revert state changes if condition is ``false`` (use for malformed input or error in external component). Also provide error message.
 - ``revert()``: abort execution and revert state changes
-- ``revert(string)``: abort execution and revert state changes providing an explanatory string
+- ``revert(string message)``: abort execution and revert state changes providing an explanatory string
 - ``blockhash(uint blockNumber) returns (bytes32)``: hash of the given block - only works for 256 most recent blocks
 - ``keccak256(...) returns (bytes32)``: compute the Ethereum-SHA-3 (Keccak-256) hash of the :ref:`(tightly packed) arguments <abi_packed_mode>`
 - ``sha3(...) returns (bytes32)``: an alias to ``keccak256``

docs/solidity-by-example.rst

@@ -87,17 +87,25 @@ of votes.
         // Give `voter` the right to vote on this ballot.
         // May only be called by `chairperson`.
         function giveRightToVote(address voter) public {
-            // If the argument of `require` evaluates to `false`,
-            // it terminates and reverts all changes to
-            // the state and to Ether balances. 
-            // This consumes all gas in old EVM versions, but not anymore.
-            // It is often a good idea to use this if functions are
-            // called incorrectly.
+            // If the first argument of `require` evaluates
+            // to `false`, execution terminates and all
+            // changes to the state and to Ether balances
+            // are reverted. 
+            // This used to consume all gas in old EVM versions, but
+            // not anymore.
+            // It is often a good idea to use `require` to check if
+            // functions are called correctly.
+            // As a second argument, you can also provide an
+            // explanation about what went wrong.
             require(
-                (msg.sender == chairperson) &&
-                !voters[voter].voted &&
-                (voters[voter].weight == 0)
+                msg.sender == chairperson,
+                "Only chairperson can give right to vote."
             );
+            require(
+                !voters[voter].voted,
+                "The voter already voted."
+            );
+            require(voters[voter].weight == 0);
             voters[voter].weight = 1;
         }
 
@@ -105,10 +113,9 @@ of votes.
         function delegate(address to) public {
             // assigns reference
             Voter storage sender = voters[msg.sender];
-            require(!sender.voted);
+            require(!sender.voted, "You already voted.");
 
-            // Self-delegation is not allowed.
-            require(to != msg.sender);
+            require(to != msg.sender, "Self-delegation is disallowed.");
 
             // Forward the delegation as long as
             // `to` also delegated.
@@ -122,7 +129,7 @@ of votes.
                 to = voters[to].delegate;
 
                 // We found a loop in the delegation, not allowed.
-                require(to != msg.sender);
+                require(to != msg.sender, "Found loop in delegation.");
             }
 
             // Since `sender` is a reference, this
@@ -145,7 +152,7 @@ of votes.
         /// to proposal `proposals[proposal].name`.
         function vote(uint proposal) public {
             Voter storage sender = voters[msg.sender];
-            require(!sender.voted);
+            require(!sender.voted, "Already voted.");
             sender.voted = true;
             sender.vote = proposal;
 
@@ -270,11 +277,17 @@ activate themselves.
 
             // Revert the call if the bidding
             // period is over.
-            require(now <= auctionEnd);
+            require(
+                now <= auctionEnd,
+                "Auction already ended."
+            );
 
             // If the bid is not higher, send the
             // money back.
-            require(msg.value > highestBid);
+            require(
+                msg.value > highestBid,
+                "There already is a higher bid."
+            );
 
             if (highestBid != 0) {
                 // Sending back the money by simply using
@@ -324,8 +337,8 @@ activate themselves.
             // external contracts.
 
             // 1. Conditions
-            require(now >= auctionEnd); // auction did not yet end
-            require(!ended); // this function has already been called
+            require(now >= auctionEnd, "Auction not yet ended.");
+            require(!ended, "auctionEnd has already been called.");
 
             // 2. Effects
             ended = true;
@@ -543,7 +556,7 @@ Safe Remote Purchase
         function Purchase() public payable {
             seller = msg.sender;
             value = msg.value / 2;
-            require((2 * value) == msg.value);
+            require((2 * value) == msg.value, "Value has to be even.");
         }
 
         modifier condition(bool _condition) {
@@ -552,17 +565,26 @@ Safe Remote Purchase
         }
 
         modifier onlyBuyer() {
-            require(msg.sender == buyer);
+            require(
+                msg.sender == buyer,
+                "Only buyer can call this."
+            );
             _;
         }
 
         modifier onlySeller() {
-            require(msg.sender == seller);
+            require(
+                msg.sender == seller,
+                "Only seller can call this."
+            );
             _;
         }
 
         modifier inState(State _state) {
-            require(state == _state);
+            require(
+                state == _state,
+                "Invalid state."
+            );
             _;
         }
 

docs/structure-of-a-contract.rst

@@ -68,7 +68,10 @@ Function modifiers can be used to amend the semantics of functions in a declarat
         address public seller;
 
         modifier onlySeller() { // Modifier
-            require(msg.sender == seller);
+            require(
+                msg.sender == seller,
+                "Only seller can call this."
+            );
             _;
         }
 

docs/types.rst

@@ -495,7 +495,10 @@ Another example that uses external function types::
         oracle.query("USD", this.oracleResponse);
       }
       function oracleResponse(bytes response) public {
-        require(msg.sender == address(oracle));
+        require(
+            msg.sender == address(oracle),
+            "Only oracle can call this."
+        );
         // Use the data
       }
     }

docs/units-and-global-variables.rst

@@ -99,6 +99,8 @@ Error Handling
     throws if the condition is not met - to be used for internal errors.
 ``require(bool condition)``:
     throws if the condition is not met - to be used for errors in inputs or external components.
+``require(bool condition, string message)``:
+    throws if the condition is not met - to be used for errors in inputs or external components. Also provides an error message.
 ``revert()``:
     abort execution and revert state changes
 ``revert(string reason)``: