Adding Constants

source/language.d

@@ -31,14 +31,16 @@ final class AssignStatement : Statement
 	Type type;
 	VarExpression dest;
 	Expression expr;
+	bool isConst;
 
-	this (int lineId_, Type type_, VarExpression dest_, Expression expr_)
+	this (int lineId_, Type type_, VarExpression dest_, Expression expr_, bool isConst_ = false)
 	{
 		lineId = lineId_;
 		type = type_;
 		dest = dest_;
 		expr = expr_;
 		complexity = 1 + dest.complexity + expr.complexity;
+		isConst = isConst_;
 	}
 }
 

source/parser.d

@@ -499,9 +499,21 @@ final class StatementParser
 		    "indent does not match");
 		t.popFront ();
 
+
+		bool isConst = false;
+		if (!line.tokens.empty && line.tokens.front == "const")
+		{
+			isConst = true;
+			line.tokens.popFront();
+		}
+
 		auto var = cast (VarExpression) (parseExpression (line));
 		check (var !is null, line,
-		    "assign statement detected but left side not parsed");
+			"assign statement detected but left side not parsed");
+		if (isConst && var.index !is null)
+		{
+			throw new Exception("cannot declare array element as constant");
+		}
 
 		check (!line.tokens.empty, line,
 		    "expected assignment operator, found end of line");
@@ -528,7 +540,7 @@ final class StatementParser
 		check (line.tokens.empty, line,
 		    "extra token at end of line: " ~ line.tokens.front);
 
-		return new AssignStatement (line.lineId, type, var, cur);
+		return new AssignStatement (line.lineId, type, var, cur, isConst);
 	}
 
 	Statement parseStatement (string prevIndent)

source/runner.d

@@ -110,7 +110,7 @@ class Runner
 		}
 	}
 
-	long * varAddress (string name, bool canCreate = false)
+	long * varAddress (string name, bool canCreate = false, bool isConst = false)
 	{
 		foreach_reverse (ref cur; state)
 		{
@@ -119,7 +119,7 @@ class Runner
 				if (cur.vars[name].isConst)
 				{
 					throw new Exception ("variable " ~ name ~
-					    " is constant");
+						" is constant");
 				}
 				return &(cur.vars[name].value);
 			}
@@ -128,11 +128,11 @@ class Runner
 		{
 			throw new Exception ("no such variable: " ~ name);
 		}
-		state.back.vars[name] = Var (0);
+		state.back.vars[name] = Var (0, isConst);
 		return &(state.back.vars[name].value);
 	}
 
-	long * arrayAddress (string name, long index)
+	long * arrayAddress (string name, long index, bool isConst = false)
 	{
 		foreach_reverse (ref cur; state)
 		{
@@ -141,7 +141,7 @@ class Runner
 				if (cur.arrays[name].isConst)
 				{
 					throw new Exception ("array " ~ name ~
-					    " is constant");
+						" is constant");
 				}
 				if (index < 0 ||
 				    cur.arrays[name].contents.length <= index)
@@ -306,17 +306,17 @@ class Runner
 		assert (false);
 	}
 
-	long * getAddr (VarExpression dest, bool canCreate)
+	long * getAddr (VarExpression dest, bool canCreate, bool isConst = false)
 	{
 		long * res;
 		if (dest.index is null)
 		{
-			res = varAddress (dest.name, canCreate);
+			res = varAddress (dest.name, canCreate, isConst);
 		}
 		else
 		{
 			auto indexValue = evalExpression (dest.index);
-			res = arrayAddress (dest.name, indexValue);
+			res = arrayAddress (dest.name, indexValue, isConst);
 		}
 		return res;
 	}
@@ -372,7 +372,7 @@ class Runner
 			return;
 		}
 
-		auto addr = getAddr (cur.dest, true);
+		auto addr = getAddr (cur.dest, true, cur.isConst);
 
 		auto value = control.queues[otherId][id].pop ();
 		doAssign (cur, addr, value);
@@ -409,11 +409,17 @@ class Runner
 		{
 			if (cur.dest.name in curState.arrays)
 			{
+				if(cur.isConst)
+					throw new Exception
+						("array: array cannot be constant");
 				curState.arrays[cur.dest.name] =
 				    Array (new long [values[0].to !(size_t)]);
 				return;
 			}
 		}
+		if(cur.isConst)
+			throw new Exception
+				("array: array cannot be constant");
 		state.back.arrays[cur.dest.name] =
 		    Array (new long [values[0].to !(size_t)]);
 	}
@@ -438,8 +444,37 @@ class Runner
 				return;
 			}
 
+			if(isConst && type == Type.assign)
+			{
+				auto varExpr = cast(VarExpression)(expr);	
+            	if (varExpr !is null && varExpr.name == dest.name && varExpr.index is null)
+            	{
+					throw new Exception
+						("cannot initialize constant " ~ dest.name ~ " with itself");
+            	}
+			}
+
 			auto value = evalExpression (expr);
-			auto addr = getAddr (dest, type == Type.assign);
+			auto addr = getAddr (dest, type == Type.assign, isConst);
+
+
+
+			if(isConst && type == Type.assign)
+			{
+				auto varExpr = cast(VarExpression)(expr);
+				foreach_reverse(ref curState; state)
+				{
+
+					if(dest.name in curState.vars)
+					{
+						if(dest.name in curState.vars && !curState.vars[dest.name].isConst)
+							throw new Exception
+								("redefinition of variable " ~ dest.name ~ " as constant");
+						curState.vars[dest.name].isConst = true;
+                    	break;
+					}
+				}
+			}
 			doAssign (cur0, addr, value);
 			delay = complexity;
 			return;

syntax.txt

@@ -21,6 +21,7 @@ function <name> ( <parameter-list> ) : \n <statement-list>
 
 <assignment> is
 <variable> <assign-op> <expression0>
+const <variable> <assign-op> <expression0>
 
 <variable> is
 <name>
@@ -120,4 +121,4 @@ if <expression0> : \n <statement-list> \n <elif-block>
 <elif-block> is
 elif <expression0> : \n <statement-list>
 elif <expression0> : \n <statement-list> \n else \n <statement-list>
-elif <expression0> : \n <statement-list> \n <elif-block>
+elif <expression0> : \n <statement-list> \n <elif-block>