add PlusEqual and MinusEqual for variable assignment

examples/hello.v

@@ -55,6 +55,7 @@ fn main(a int) int {
 	str := 'hello' 
 	len := str.len
 	str.len = str.len + 1
+	x += 1
 	// str.len += 1
 }
 

src/parser.zig

@@ -688,7 +688,7 @@ pub const Parser = struct {
     fn parseAssignment(self: *@This()) anyerror!*Expr {
         // there can be two assignments coming out of this function:
         //  - a mutable/immutable variable declaration with :=
-        //  - an assignment to a variable with =
+        //  - an assignment to a variable with =, +=, -=
 
         // one is a statement, other is an expression. since the normal result
         // of this is an Expr, we wrap variable assignments in an Expr as well.
@@ -701,7 +701,9 @@ pub const Parser = struct {
 
         var expr = try self.parseOr();
 
-        if (self.check(.ColonEqual) or self.check(.Equal)) {
+        if (self.compareAnyOf(&[_]TokenType{
+            .ColonEqual, .Equal, .PlusEqual, .MinusEqual,
+        })) {
             return try self.finishAssignment(expr, mutable);
         }
 
@@ -713,22 +715,61 @@ pub const Parser = struct {
         _ = try self.nextToken();
         var value = try self.parseAssignment();
 
+        // TODO convert binary's op field from Token to
+        // something else, maybe enum'd
+
         switch (expr.*) {
             .Variable => {
                 switch (op.ttype) {
                     .ColonEqual => return try self.mkVarDecl(expr.Variable, value, mutable),
                     .Equal => return try self.mkAssign(expr.Variable, value),
+
+                    .PlusEqual => {
+                        var plus_op = try self.mkToken(.Plus, "+", op.line);
+                        return try self.mkAssign(
+                            expr.Variable,
+                            try self.mkBinary(expr, plus_op, value),
+                        );
+                    },
+
+                    .MinusEqual => {
+                        var new_op = try self.mkToken(.Minus, "-", op.line);
+                        return try self.mkAssign(
+                            expr.Variable,
+                            try self.mkBinary(expr, new_op, value),
+                        );
+                    },
+
                     else => unreachable,
                 }
             },
 
             .Get => |get| {
-                if (op.ttype == .ColonEqual) {
-                    self.doError("can not initialize struct field");
-                    return Result.CompileError;
-                }
+                switch (op.ttype) {
+                    .ColonEqual => {
+                        self.doError("can not initialize struct field");
+                        return Result.CompileError;
+                    },
 
-                return try self.mkSet(get.struc, get.name, value);
+                    //.PlusEqual => {
+                    //    var plus_op = try self.mkToken(.Plus, "+", op.line);
+                    //    return try self.mkAssign(
+                    //        expr.Variable,
+                    //        try self.mkBinary(expr, plus_op, value),
+                    //    );
+                    //},
+
+                    //.MinusEqual => {
+                    //    var new_op = try self.mkToken(.Minus, "-", op.line);
+                    //    return try self.mkSet(
+                    //        get.struc, get.name,
+                    //        try self.mkBinary(expr, new_op, value),
+                    //    );
+                    //},
+
+                    .Equal => return try self.mkSet(get.struc, get.name, value),
+                    else => unreachable,
+                }
             },
 
             else => |expr_typ| {