Fixes #18

doc/source/language/flow.rst

@@ -61,6 +61,32 @@ Checks if the expression supplied returns true. If it does, ``IF`` executes the
         // syntax error - ELSE without IF.
         IF X > 10 { PRINT "Large". }. ELSE { PRINT "Small". }.
 
+.. index:: CHOOSE
+.. _choose:
+
+CHOOSE (Ternary operator)
+-------------------------
+
+An expression that evalualtes to one of two choices depending on a
+conditional check:
+
+   CHOOSE expression1 IF condition ELSE expression2
+
+Note this is NOT a statement.  This is an expression that can be embedded
+inside other statements, like so:
+
+   SET X TO CHOOSE expression1 IF condition ELSE expression2.
+   PRINT CHOOSE "High" IF altitude > 20000 ELSE "Low".
+
+The reason to use the ``CHOOSE`` operator instead of an
+IF/ELSE statement is that IF/ELSE won't return a value, while
+this does, and thus this can be embedded inside other expressions.
+
+(This is similar to the ``?`` operator in languages like "C" and its
+derivatives, except it puts the "true" choice first, then the
+conditional check, then the "false" choice.)
+
+
 .. index:: LOCK
 .. _lock:
 

kerboscript_tests/ternary.ks

@@ -0,0 +1,38 @@
+print "THESE are tests of the ternary CHOOSE operator.".
+
+Print "Basic case - Next line should print 'A':".
+print choose "A" if true else "B".
+
+Print "Basic case - Next line should print 'B':".
+print choose "A" if false else "B".
+
+
+print "Testing nested CHOOSEs.  Next line should be 'ABCDE*****':".
+set str to "".
+for i in range(0,10) {
+  set str to str + (choose "A" if i = 0 else choose "B" if i = 1 else choose "C" if i = 2 else choose "D" if i = 3 else choose "E" if i = 4 else "*").
+
+}
+print str.
+
+
+print "Weird case - nesting choose in the boolean.".
+set x to true.
+set y to false.
+set a to 1.
+print "  Next 2 lines should be 'A':".
+print "  " + (choose "A" if (choose x if a=1 else y) else "B").
+print "  " + (choose "A" if choose x if a=1 else y else "B"). // same without parens should work.
+print "  Next 2 lines should be 'B':".
+print "  " + (choose "A" if (choose x if a=2 else y) else "B").
+print "  " + (choose "A" if choose x if a=2 else y else "B"). // same without parens should work.
+
+print "Complex case - selecting delegate with choose.".
+set a to 1.
+set del1 to choose { print "trueDel". } if a = 1 else { print "falseDel.". }.
+set a to 2.
+set del2 to choose { print "trueDel". } if a = 1 else { print "falseDel.". }.
+print "Next line should say 'trueDel':".
+del1:call().
+print "Next line should say 'falseDel':".
+del2:call().

src/kOS.Safe/Compilation/KS/Compiler.cs

@@ -282,6 +282,7 @@ private void IterateUserFunctions(ParseNode node, Action<ParseNode> action)
                 case TokenType.COMPARATOR:
                 case TokenType.AND:
                 case TokenType.OR:
+                case TokenType.CHOOSE:
                 case TokenType.directive:
                     doChildren = false;
                     break;                
@@ -1149,6 +1150,9 @@ private void VisitNode(ParseNode node)
                 case TokenType.expr:
                     VisitExpr(node);
                     break;
+                case TokenType.ternary_expr:
+                    VisitTernary(node);
+                    break;
                 case TokenType.or_expr:
                 case TokenType.and_expr:
                     VisitShortCircuitBoolean(node);
@@ -1352,6 +1356,33 @@ private void VisitExpr(ParseNode node)
             }
         }
 
+        private void VisitTernary(ParseNode node)
+        {
+            NodeStartHousekeeping(node);
+
+            // Syntax pattern is:
+            // [0] = keyword CHOOSE
+            // [1] = expression returned if true
+            // [2] = keyword IF
+            // [3] = expression with boolean value
+            // [4] = keyword ELSE
+            // [5] = expression returned if false
+
+            VisitNode(node.Nodes[3]); // eval the boolean clause, put on stack.
+
+            Opcode bypassTrue = AddOpcode(new OpcodeBranchIfFalse());
+
+            VisitNode(node.Nodes[1]); // expression if true.
+            Opcode bypassFalse = AddOpcode(new OpcodeBranchJump());
+
+            bypassTrue.DestinationLabel = GetNextLabel(false);
+
+            VisitNode(node.Nodes[5]); // expression if false.
+
+            bypassFalse.DestinationLabel = GetNextLabel(false);
+            addBranchDestination = true;
+        }
+
         /// <summary>
         /// Handles the short-circuit logic of boolean OR and boolean AND
         /// chains.  It is like VisitExpressionChain (see elsewhere) but

src/kOS.Safe/Compilation/KS/ParseTree.cs

@@ -319,6 +319,9 @@ internal object Eval(ParseTree tree, params object[] paramlist)
                 case TokenType.expr:
                     Value = Evalexpr(tree, paramlist);
                     break;
+                case TokenType.ternary_expr:
+                    Value = Evalternary_expr(tree, paramlist);
+                    break;
                 case TokenType.or_expr:
                     Value = Evalor_expr(tree, paramlist);
                     break;
@@ -711,6 +714,13 @@ protected virtual object Evalexpr(ParseTree tree, params object[] paramlist)
             return null;
         }
 
+        protected virtual object Evalternary_expr(ParseTree tree, params object[] paramlist)
+        {
+            foreach (var node in Nodes)
+                node.Eval(tree, paramlist);
+            return null;
+        }
+
         protected virtual object Evalor_expr(ParseTree tree, params object[] paramlist)
         {
             foreach (var node in Nodes)

src/kOS.Safe/Compilation/KS/Parser.cs

@@ -1749,8 +1749,9 @@ private void Parsereturn_stmt(ParseNode parent) // NonTerminalSymbol: return_stm
             }
 
              // Concat Rule
-            tok = scanner.LookAhead(TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Option Rule
-            if (tok.Type == TokenType.PLUSMINUS
+            tok = scanner.LookAhead(TokenType.CHOOSE, TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Option Rule
+            if (tok.Type == TokenType.CHOOSE
+                || tok.Type == TokenType.PLUSMINUS
                 || tok.Type == TokenType.NOT
                 || tok.Type == TokenType.DEFINED
                 || tok.Type == TokenType.INTEGER
@@ -2674,9 +2675,12 @@ private void Parseexpr(ParseNode parent) // NonTerminalSymbol: expr
             ParseNode node = parent.CreateNode(scanner.GetToken(TokenType.expr), "expr");
             parent.Nodes.Add(node);
 
-            tok = scanner.LookAhead(TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Choice Rule
+            tok = scanner.LookAhead(TokenType.CHOOSE, TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Choice Rule
             switch (tok.Type)
             { // Choice Rule
+                case TokenType.CHOOSE:
+                    Parseternary_expr(node); // NonTerminal Rule: ternary_expr
+                    break;
                 case TokenType.PLUSMINUS:
                 case TokenType.NOT:
                 case TokenType.DEFINED:
@@ -2693,13 +2697,63 @@ private void Parseexpr(ParseNode parent) // NonTerminalSymbol: expr
                     Parseinstruction_block(node); // NonTerminal Rule: instruction_block
                     break;
                 default:
-                    tree.Errors.Add(new ParseError("Unexpected token '" + tok.Text.Replace("\n", "") + "' found. Expected PLUSMINUS, NOT, DEFINED, INTEGER, DOUBLE, TRUEFALSE, IDENTIFIER, FILEIDENT, BRACKETOPEN, STRING, or CURLYOPEN.", 0x0002, tok));
+                    tree.Errors.Add(new ParseError("Unexpected token '" + tok.Text.Replace("\n", "") + "' found. Expected CHOOSE, PLUSMINUS, NOT, DEFINED, INTEGER, DOUBLE, TRUEFALSE, IDENTIFIER, FILEIDENT, BRACKETOPEN, STRING, or CURLYOPEN.", 0x0002, tok));
                     break;
             } // Choice Rule
 
             parent.Token.UpdateRange(node.Token);
         } // NonTerminalSymbol: expr
 
+        private void Parseternary_expr(ParseNode parent) // NonTerminalSymbol: ternary_expr
+        {
+            Token tok;
+            ParseNode n;
+            ParseNode node = parent.CreateNode(scanner.GetToken(TokenType.ternary_expr), "ternary_expr");
+            parent.Nodes.Add(node);
+
+
+             // Concat Rule
+            tok = scanner.Scan(TokenType.CHOOSE); // Terminal Rule: CHOOSE
+            n = node.CreateNode(tok, tok.ToString() );
+            node.Token.UpdateRange(tok);
+            node.Nodes.Add(n);
+            if (tok.Type != TokenType.CHOOSE) {
+                tree.Errors.Add(new ParseError("Unexpected token '" + tok.Text.Replace("\n", "") + "' found. Expected " + TokenType.CHOOSE.ToString(), 0x1001, tok));
+                return;
+            }
+
+             // Concat Rule
+            Parseexpr(node); // NonTerminal Rule: expr
+
+             // Concat Rule
+            tok = scanner.Scan(TokenType.IF); // Terminal Rule: IF
+            n = node.CreateNode(tok, tok.ToString() );
+            node.Token.UpdateRange(tok);
+            node.Nodes.Add(n);
+            if (tok.Type != TokenType.IF) {
+                tree.Errors.Add(new ParseError("Unexpected token '" + tok.Text.Replace("\n", "") + "' found. Expected " + TokenType.IF.ToString(), 0x1001, tok));
+                return;
+            }
+
+             // Concat Rule
+            Parseexpr(node); // NonTerminal Rule: expr
+
+             // Concat Rule
+            tok = scanner.Scan(TokenType.ELSE); // Terminal Rule: ELSE
+            n = node.CreateNode(tok, tok.ToString() );
+            node.Token.UpdateRange(tok);
+            node.Nodes.Add(n);
+            if (tok.Type != TokenType.ELSE) {
+                tree.Errors.Add(new ParseError("Unexpected token '" + tok.Text.Replace("\n", "") + "' found. Expected " + TokenType.ELSE.ToString(), 0x1001, tok));
+                return;
+            }
+
+             // Concat Rule
+            Parseexpr(node); // NonTerminal Rule: expr
+
+            parent.Token.UpdateRange(node.Token);
+        } // NonTerminalSymbol: ternary_expr
+
         private void Parseor_expr(ParseNode parent) // NonTerminalSymbol: or_expr
         {
             Token tok;
@@ -3103,8 +3157,9 @@ private void Parsefunction_trailer(ParseNode parent) // NonTerminalSymbol: funct
                     }
 
                      // Concat Rule
-                    tok = scanner.LookAhead(TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Option Rule
-                    if (tok.Type == TokenType.PLUSMINUS
+                    tok = scanner.LookAhead(TokenType.CHOOSE, TokenType.PLUSMINUS, TokenType.NOT, TokenType.DEFINED, TokenType.INTEGER, TokenType.DOUBLE, TokenType.TRUEFALSE, TokenType.IDENTIFIER, TokenType.FILEIDENT, TokenType.BRACKETOPEN, TokenType.STRING, TokenType.CURLYOPEN); // Option Rule
+                    if (tok.Type == TokenType.CHOOSE
+                        || tok.Type == TokenType.PLUSMINUS
                         || tok.Type == TokenType.NOT
                         || tok.Type == TokenType.DEFINED
                         || tok.Type == TokenType.INTEGER