Add lookup functions for prefix and postfix operators

Author: DougGregor

Sources/SwiftOperators/OperatorTable.swift

@@ -89,6 +89,18 @@ extension OperatorTable {
     return infixOperators[operatorName]
   }
 
+  /// Returns the ``Operator`` corresponding to the given prefix operator, or
+  /// `nil` if it is not defined in the operator table.
+  public func prefixOperator(named operatorName: OperatorName) -> Operator? {
+    return prefixOperators[operatorName]
+  }
+
+  /// Returns the ``Operator`` corresponding to the given prefix operator, or
+  /// `nil` if it is not defined in the operator table.
+  public func postfixOperator(named operatorName: OperatorName) -> Operator? {
+    return postfixOperators[operatorName]
+  }
+
   /// Look for the precedence group corresponding to the given operator.
   func lookupOperatorPrecedenceGroupName(
     _ operatorName: OperatorName,

Tests/SwiftOperatorsTest/OperatorTableTests.swift

@@ -405,4 +405,22 @@ public class OperatorPrecedenceTests: XCTestCase {
     }
     XCTAssertNil(opPrecedence.infixOperator(named: "^*^"))
   }
+
+  func testUnaryOperatorLookup() throws {
+    let opPrecedence = OperatorTable.standardOperators
+    do {
+      let op = try XCTUnwrap(opPrecedence.prefixOperator(named: "-"))
+      XCTAssertEqual(op.kind, .prefix)
+      XCTAssertEqual(op.name, "-")
+      XCTAssertNil(op.precedenceGroup)
+    }
+
+    do {
+      let op = try XCTUnwrap(opPrecedence.postfixOperator(named: "..."))
+      XCTAssertEqual(op.kind, .postfix)
+      XCTAssertEqual(op.name, "...")
+      XCTAssertNil(op.precedenceGroup)
+    }
+
+  }
 }