DRAFT: Proposed syntax and initial documentation for error type.

docs/Solidity.g4

@@ -8,7 +8,8 @@
 grammar Solidity;
 
 sourceUnit
-  : (pragmaDirective | importDirective | structDefinition | enumDefinition | contractDefinition)* EOF ;
+  : (pragmaDirective | importDirective | structDefinition | enumDefinition | contractDefinition |
+     errorDefinition)* EOF ;
 
 pragmaDirective
   : 'pragma' pragmaName pragmaValue ';' ;
@@ -51,6 +52,7 @@ contractPart
   | modifierDefinition
   | functionDefinition
   | eventDefinition
+  | errorDefinition
   | enumDefinition ;
 
 stateVariableDeclaration
@@ -92,6 +94,10 @@ modifierInvocation
 eventDefinition
   : 'event' identifier eventParameterList AnonymousKeyword? ';' ;
 
+errorDefinition    : 'error' identifier errorParameterList ';' ;
+errorParameterList : '(' (errorParameter (',' errorParameter)*)? ')' ;
+errorParameter     : typeName identifier? ;
+
 enumDefinition
   : 'enum' identifier '{' enumValue? (',' enumValue)* '}' ;
 
@@ -167,6 +173,9 @@ tryStatement : 'try' expression returnParameters? block catchClause+ ;
 // leave this as is, befitting with the Solidity docs.
 catchClause : 'catch' ( identifier? parameterList )? block ;
 
+catchClauseNEW : 'catch' errorTypeName parameterList block ;
+errorTypeName : identifier ;
+
 whileStatement
   : 'while' '(' expression ')' statement ;
 

docs/error-type.md

@@ -0,0 +1,92 @@
+# Solidity `error` type
+
+## Motivation
+
+We want to support typed errors in a syntactically equivalent and convenient way like
+the programmer can define `event` types.
+
+Example:
+
+```solidity
+contract C {
+	event HighestBidIncreased(address bidder, uint amount);
+	error BiddingFailed(string reason);
+
+	function bid() public payable {
+		try attemptBid() {
+			emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
+		} catch BiddingFailed(reason) {
+			// handle error with given reason
+		}
+    }
+
+	function attemptBid() {
+		// require() should now support a custom secondary parameter to throw custom errors upon failed expectations.
+		require(biddingPossible, BiddingFailed("Bidding impossible"));
+	}
+}
+```
+
+## Syntax
+
+```bnf
+errorDefinition
+  : 'error' identifier eventParameterList AnonymousKeyword? ';' ;
+```
+
+## Semantic
+
+Errors can be instantiated wherever a variable of a type can be declared.
+Errors can NOT be instantiated like any arbitrary type, but instead only be used
+within `require` and `revert`.
+
+Error types are ALWAYS publicly visible, i.e. they can not be made private.
+
+## Catching Errors
+
+With the try-catch statement, you can already catch `Error(string)`. This will be extended
+to catch errors by other structured types that were declared via the `error` keyword.
+
+The fallback catch will also apply to error types that weren't matched.
+
+Example:
+
+```solidity
+contract C {
+	event HighestBidIncreased(address bidder, uint amount);
+
+	error BiddingFailed(string reason);
+	error SomeOtherError(string text, uint value);
+
+	function bid() public payable {
+		try attemptBid() {
+			emit HighestBidIncreased(msg.sender, msg.value); // Triggering event
+		} catch BiddingFailed(string reason) {
+			// handle error with given reason
+		} catch SomeOtherError(string reason, uint value) {
+			// handle other error with an additional value
+		} catch Error(string message) {
+			// handle classic error messages
+		} catch (bytes data) {
+			// parametrized default
+		}
+	}
+}
+```
+
+## The current `Error(string message)` type.
+
+Currently, exceptions can be caught via `catch Error(string text)`. This behaviour will be retained
+by providing the default error type `Error`.
+
+## Code Generation
+
+Whatever ABI encoder is selected, this one will be used to encode/decode the error in
+require/revert/catch.
+
+
+
+
+
+
+