feat: function calls

src/AstGen.zig

@@ -280,6 +280,13 @@ const GenIr = struct {
             self.instructions.items[self.instructions_top..];
     }
 
+    fn endsWithNoReturn(self: *GenIr) bool {
+        if (self.isEmpty()) return false;
+        const last_inst_index = self.instructions.items[self.instructions.items.len - 1];
+        const last_inst = self.astgen.instructions.items[@intFromEnum(last_inst_index)];
+        return last_inst.isNoReturn();
+    }
+
     fn makeSubBlock(self: *GenIr) GenIr {
         return .{
             .astgen = self.astgen,
@@ -730,7 +737,7 @@ fn expr(gi: *GenIr, scope: *Scope, optional_node: ?*const Ast.Node) InnerError!I
         .logical_greater_or_equal_expr => return binaryOp(gi, scope, node, .cmp_gte),
         .logical_lesser_expr => return binaryOp(gi, scope, node, .cmp_lt),
         .logical_lesser_or_equal_expr => return binaryOp(gi, scope, node, .cmp_lte),
-        .call_expr => unreachable,
+        .call_expr => return callExpr(gi, scope, node, .call),
         .choice_expr => unreachable,
         .choice_start_expr => unreachable,
         .choice_option_expr => unreachable,
@@ -1171,7 +1178,9 @@ fn callExpr(
     const scratch_top = astgen.scratch.items.len;
     defer astgen.scratch.shrinkRetainingCapacity(scratch_top);
 
-    const arguments: ?[]*Ast.Node = if (args_node) |n| n.data.list.items.? else null;
+    // FIXME: List nodes should not have optional slices.
+    // This hack is an abomination.
+    const arguments: ?[]*Ast.Node = if (args_node) |n| if (n.data.list.items) |items| items else null else null;
     const args_count = if (arguments) |args| args.len else 0;
 
     try astgen.scratch.resize(gpa, scratch_top + args_count);
@@ -1275,10 +1284,20 @@ fn divertExpr(gi: *GenIr, scope: *Scope, node: *const Ast.Node) !void {
 }
 
 fn divertStmt(gi: *GenIr, scope: *Scope, node: *const Ast.Node) !void {
+    // TODO: Revisit this.
     const data = node.data.bin;
     return divertExpr(gi, scope, data.lhs.?);
 }
 
+fn returnStmt(gi: *GenIr, scope: *Scope, node: *const Ast.Node) !void {
+    // TODO: Revisit this.
+    const ret_arg = if (node.data.bin.lhs) |lhs| blk: {
+        const arg_inst = try expr(gi, scope, lhs);
+        break :blk arg_inst;
+    } else .none;
+    _ = try gi.addUnaryNode(.ret, ret_arg);
+}
+
 fn tempDecl(gi: *GenIr, scope: *Scope, decl_node: *const Ast.Node) !void {
     const astgen = gi.astgen;
     const identifier_node = decl_node.data.bin.lhs.?;
@@ -1327,20 +1346,23 @@ fn blockInner(gi: *GenIr, parent_scope: *Scope, stmt_list: []*Ast.Node) !void {
     var child_scope = parent_scope.makeChild();
     defer child_scope.deinit();
 
-    for (stmt_list) |inner_node| {
-        _ = switch (inner_node.tag) {
-            .var_decl => try varDecl(gi, &child_scope, inner_node),
-            .const_decl => try varDecl(gi, &child_scope, inner_node),
-            .temp_decl => try tempDecl(gi, &child_scope, inner_node),
-            .assign_stmt => try assignStmt(gi, &child_scope, inner_node),
-            .content_stmt => try contentStmt(gi, &child_scope, inner_node),
-            .choice_stmt => try choiceStmt(gi, &child_scope, inner_node),
-            .expr_stmt => try exprStmt(gi, &child_scope, inner_node),
-            .divert_stmt => try divertStmt(gi, &child_scope, inner_node),
+    for (stmt_list) |node| {
+        _ = switch (node.tag) {
+            .var_decl => try varDecl(gi, &child_scope, node),
+            .const_decl => try varDecl(gi, &child_scope, node),
+            .temp_decl => try tempDecl(gi, &child_scope, node),
+            .assign_stmt => try assignStmt(gi, &child_scope, node),
+            .content_stmt => try contentStmt(gi, &child_scope, node),
+            .choice_stmt => try choiceStmt(gi, &child_scope, node),
+            .expr_stmt => try exprStmt(gi, &child_scope, node),
+            .divert_stmt => try divertStmt(gi, &child_scope, node),
+            .return_stmt => try returnStmt(gi, &child_scope, node),
             inline else => |e| @panic("Unexpected node: " ++ @tagName(e)),
         };
     }
+    if (!gi.endsWithNoReturn()) {
         _ = try gi.addUnaryNode(.implicit_ret, .none);
+    }
 }
 
 fn blockStmt(block: *GenIr, scope: *Scope, stmt_node: *const Ast.Node) InnerError!void {
@@ -1415,9 +1437,10 @@ fn stitchDeclInner(
         }
     }
     if (body_node) |body| {
-        try blockStmt(&decl_block, scope, body);
+        const body_list = body.data.list.items.?;
+        try blockInner(&decl_block, scope, body_list);
     } else {
-        _ = try decl_block.addUnaryNode(.implicit_ret, .none);
+        try blockInner(&decl_block, scope, &.{});
     }
 
     const decl_str = try astgen.strFromNode(identifier_node);
@@ -1440,7 +1463,65 @@ fn stitchDecl(gi: *GenIr, parent_scope: *Scope, decl_node: *const Ast.Node) Inne
     return stitchDeclInner(gi, &decl_scope, decl_node, prototype_node.?, body_node);
 }
 
-fn functionDecl(_: *GenIr, _: *Scope, _: *const Ast.Node) InnerError!void {}
+fn functionDeclInner(
+    gi: *GenIr,
+    scope: *Scope,
+    node: *const Ast.Node,
+    prototype_node: *const Ast.Node,
+    body_node: ?*const Ast.Node,
+) InnerError!void {
+    const astgen = gi.astgen;
+    const prototype_data = prototype_node.data.bin;
+    const identifier_node = prototype_data.lhs.?;
+    const decl_inst = try gi.addAsIndex(.{
+        .tag = .declaration,
+        .data = .{ .payload = undefined },
+    });
+
+    var decl_block = gi.makeSubBlock();
+    defer decl_block.unstack();
+
+    const stitch_inst = try decl_block.makePayloadNode(.decl_function);
+
+    if (prototype_data.rhs) |args_node| {
+        const args_list = args_node.data.list.items.?;
+        for (args_list) |arg| {
+            assert(arg.tag == .parameter_decl);
+            const arg_str = try astgen.strFromNode(arg);
+            const arg_inst = try decl_block.addStrTok(.param, arg_str.index, arg.loc.start);
+
+            // TODO: Maybe make decl accept a ref?
+            try scope.insert(arg_str.index, .{
+                .decl_node = arg,
+                .inst_index = arg_inst.toIndex().?,
+            });
+        }
+    }
+    if (body_node) |body| {
+        try blockStmt(&decl_block, scope, body);
+    } else {
+        _ = try decl_block.addUnaryNode(.implicit_ret, .none);
+    }
+
+    const decl_str = try astgen.strFromNode(identifier_node);
+    try setDeclStitchPayload(stitch_inst, &decl_block);
+    try setDeclaration(decl_inst, .{
+        .name = decl_str.index,
+        .value = stitch_inst,
+        .gi = gi,
+        .node = node,
+    });
+}
+
+fn functionDecl(gi: *GenIr, parent_scope: *Scope, decl_node: *const Ast.Node) InnerError!void {
+    const knot_data = decl_node.data.bin;
+    const prototype_node = knot_data.lhs;
+    const body_node = knot_data.rhs;
+    var decl_scope = parent_scope.makeChild();
+    defer decl_scope.deinit();
+
+    return functionDeclInner(gi, &decl_scope, decl_node, prototype_node.?, body_node);
+}
 
 fn knotDecl(gi: *GenIr, parent_scope: *Scope, decl_node: *const Ast.Node) InnerError!void {
     const astgen = gi.astgen;

src/Ir.zig

@@ -147,10 +147,16 @@ pub const Inst = struct {
 
     pub const Tag = enum {
         file,
+        /// Uses the `payload` union field.
         declaration,
+        /// Uses the `payload` union field.
         decl_var,
+        /// Uses the `payload` union field.
         decl_knot,
+        /// Uses the `payload` union field.
         decl_stitch,
+        /// Uses the `payload` union field.
+        decl_function,
         /// Uses the `str_tok` union field.
         decl_ref,
         alloc,
@@ -199,6 +205,9 @@ pub const Inst = struct {
         content_push,
         content_flush,
         choice_br,
+        // Uses the `un` union field.
+        ret,
+        // Uses the `un` union field.
         implicit_ret,
         call,
         divert,
@@ -264,6 +273,11 @@ pub const Inst = struct {
         body_len: u32,
     };
 
+    pub const Function = struct {
+        /// Number of instructions for the function's body block.
+        body_len: u32,
+    };
+
     pub const Field = struct {
         lhs: Ref,
         field_name_start: NullTerminatedString,
@@ -324,4 +338,56 @@ pub const Inst = struct {
             byte_offset: u32,
         };
     };
+
+    pub fn isNoReturn(inst: Inst) bool {
+        return switch (inst.tag) {
+            .file,
+            .declaration,
+            .decl_var,
+            .decl_knot,
+            .decl_stitch,
+            .decl_function,
+            .decl_ref,
+            .alloc,
+            .load,
+            .store,
+            .add,
+            .sub,
+            .mul,
+            .div,
+            .mod,
+            .neg,
+            .not,
+            .bool_and,
+            .bool_or,
+            .cmp_eq,
+            .cmp_neq,
+            .cmp_gt,
+            .cmp_gte,
+            .cmp_lt,
+            .cmp_lte,
+            .int,
+            .float,
+            .str,
+            .block,
+            .condbr,
+            .switch_br,
+            .content_push,
+            .content_flush,
+            .choice_br,
+            .call,
+            .divert,
+            .field_ptr,
+            .field_call,
+            .field_divert,
+            .param,
+            => false,
+            .ret,
+            .implicit_ret,
+            .@"break",
+            .done,
+            .exit,
+            => true,
+        };
+    }
 };

src/Sema.zig

@@ -29,6 +29,7 @@ pub const ValueInfo = union(enum) {
     variable: InternPool.Index,
     knot: InternPool.Index,
     stitch: InternPool.Index,
+    function: InternPool.Index,
     temp: u32,
 };
 
@@ -294,7 +295,11 @@ pub const Builder = struct {
                 const local_index = try self.getOrPutConstantIndex(index);
                 try self.addConstOp(.load_const, @intCast(local_index));
             },
-            .variable, .knot, .stitch => |index| {
+            .variable,
+            .knot,
+            .stitch,
+            .function,
+            => |index| {
                 const local_index = try self.getOrPutConstantIndex(index);
                 try self.addConstOp(.load_global, @intCast(local_index));
             },
@@ -536,7 +541,7 @@ fn irDeclRef(
     const ident = try sema.lookupIdentifier(builder, ip_index, src_loc);
     if (inline_block) {
         switch (ident.tag) {
-            .knot, .stitch => unreachable,
+            .knot, .stitch, .function => unreachable,
             .var_const => return sema.resolveGlobalDecl(builder, ip_index, src_loc),
             .var_mut => return sema.fail(
                 src_loc,
@@ -548,6 +553,7 @@ fn irDeclRef(
         switch (ident.tag) {
             .knot => return .{ .knot = ip_index },
             .stitch => return .{ .stitch = ip_index },
+            .function => return .{ .function = ip_index },
             .var_mut => return .{ .variable = ip_index },
             .var_const => return .{ .variable = ip_index },
         }
@@ -577,8 +583,9 @@ fn irStore(sema: *Sema, builder: *Builder, inst: Ir.Inst.Index) InnerError!void
             try builder.addConstOp(.store_global, @intCast(local_index));
         },
         .temp => |temp| try builder.addConstOp(.store, @intCast(temp)),
-        .stitch => |_| return sema.fail(src, "could not assign to stitch", .{}),
         .knot => |_| return sema.fail(src, "could not assign to knot", .{}),
+        .stitch => |_| return sema.fail(src, "could not assign to stitch", .{}),
+        .function => |_| return sema.fail(src, "could not assign to function", .{}),
     }
 
     try builder.addByteOp(.pop);
@@ -757,12 +764,66 @@ fn irChoiceBr(sema: *Sema, builder: *Builder, inst: Ir.Inst.Index) InnerError!vo
     }
 }
 
+fn irRet(sema: *Sema, builder: *Builder, inst: Ir.Inst.Index) InnerError!void {
+    const data = sema.ir.instructions[@intFromEnum(inst)].data.un;
+    const value = sema.resolveInst(data.lhs);
+    if (value != .none) try builder.ensureLoad(value);
+    try builder.addByteOp(.ret);
+}
+
 fn irImplicitRet(_: *Sema, builder: *Builder, _: Ir.Inst.Index) InnerError!void {
     try builder.addByteOp(.exit);
 }
 
-fn irCall(_: *Sema, _: *Builder, _: Ir.Inst.Index) !ValueInfo {
-    return .none;
+fn irCall(
+    sema: *Sema,
+    builder: *Builder,
+    inst: Ir.Inst.Index,
+    comptime kind: enum { direct, field },
+) !ValueInfo {
+    const ExtraType = switch (kind) {
+        .direct => Ir.Inst.Call,
+        .field => Ir.Inst.FieldCall,
+    };
+    const data = sema.ir.instructions[@intFromEnum(inst)].data.payload;
+    const extra = sema.ir.extraData(ExtraType, data.extra_index);
+    const body = sema.ir.extra[extra.end..];
+    const callee_src: SrcLoc = .{ .src_offset = data.src_offset };
+    switch (kind) {
+        .direct => {
+            const callee = sema.resolveInst(extra.data.callee);
+            _ = try analyzeCallTarget(sema, builder, callee_src, callee);
+        },
+        .field => {
+            const callee = sema.resolveInst(extra.data.obj_ptr);
+            const target = try analyzeCallTarget(sema, builder, callee_src, callee);
+            const ip_index = try sema.module.intern_pool.getOrPutStr(
+                sema.gpa,
+                extra.data.field_name_start,
+            );
+            const e = try sema.lookupInNamespace(target.namespace.?, ip_index, callee_src);
+            switch (e.tag) {
+                .function => {
+                    const local_index = try builder.getOrPutConstantIndex(ip_index);
+                    _ = try builder.addConstOp(.load_attr, @intCast(local_index));
+                },
+                else => return sema.fail(callee_src, "invalid call target", .{}),
+            }
+        },
+    }
+
+    const args_len = extra.data.args_len;
+    var arg_start: u32 = args_len;
+    var i: u32 = 0;
+    while (i < args_len) : (i += 1) {
+        const arg_end = sema.ir.extra[extra.end + i];
+        defer arg_start = arg_end;
+        const arg_body = body[arg_start..arg_end];
+        const arg_value = try analyzeBodyInner(sema, builder, @ptrCast(arg_body), false);
+        if (arg_value != .none) try builder.ensureLoad(arg_value);
+    }
+    try builder.addConstOp(.call, @intCast(args_len));
+    return .stack;
 }
 
 fn irDivert(
@@ -809,13 +870,8 @@ fn irDivert(
         const arg_end = sema.ir.extra[extra.end + i];
         defer arg_start = arg_end;
         const arg_body = body[arg_start..arg_end];
-        _ = try analyzeBodyInner(sema, builder, @ptrCast(arg_body), false);
-        // FIXME: hack
-        {
-            const last_inst: Ir.Inst.Index = @enumFromInt(arg_body[arg_body.len - 1]);
-            const val = sema.resolveInst(last_inst.toRef());
-            try builder.ensureLoad(val);
-        }
+        const arg_value = try analyzeBodyInner(sema, builder, @ptrCast(arg_body), false);
+        if (arg_value != .none) try builder.ensureLoad(arg_value);
     }
     try builder.addConstOp(.divert, @intCast(args_len));
 }
@@ -864,6 +920,21 @@ fn analyzeArithmeticArg(
     }
 }
 
+fn analyzeCallTarget(
+    sema: *Sema,
+    builder: *Builder,
+    src: SrcLoc,
+    callee: ValueInfo,
+) !Module.Namespace.Decl {
+    switch (callee) {
+        .function => |ip_index| {
+            try builder.ensureLoad(callee);
+            return sema.lookupIdentifier(builder, ip_index, src);
+        },
+        else => return sema.fail(src, "invalid call target", .{}),
+    }
+}
+
 fn analyzeDivertTarget(
     sema: *Sema,
     builder: *Builder,
@@ -894,6 +965,7 @@ fn analyzeBodyInner(
             .decl_var => unreachable, // never present inside block bodies
             .decl_knot => unreachable, // never present inside block bodies
             .decl_stitch => unreachable, // never present inside block bodies
+            .decl_function => unreachable, // never present inside block bodies
             .alloc => try irAlloc(sema, builder, inst),
             .store => {
                 try irStore(sema, builder, inst);
@@ -919,6 +991,10 @@ fn analyzeBodyInner(
             .cmp_gt => try irBinaryOp(sema, builder, inst, .cmp_gt),
             .cmp_gte => try irBinaryOp(sema, builder, inst, .cmp_gte),
             .decl_ref => try irDeclRef(sema, builder, inst, inline_block),
+            .ret => {
+                try irRet(sema, builder, inst);
+                continue;
+            },
             .implicit_ret => {
                 try irImplicitRet(sema, builder, inst);
                 continue;
@@ -948,12 +1024,12 @@ fn analyzeBodyInner(
                 try irSwitchBr(sema, builder, inst);
                 continue;
             },
-            .call => try irCall(sema, builder, inst),
+            .call => try irCall(sema, builder, inst, .direct),
+            .field_call => try irCall(sema, builder, inst, .field),
             .divert => {
                 try irDivert(sema, builder, inst, .direct);
                 continue;
             },
-            .field_call => try irCall(sema, builder, inst),
             .field_divert => {
                 try irDivert(sema, builder, inst, .field);
                 continue;
@@ -968,6 +1044,7 @@ fn analyzeBodyInner(
     return result;
 }
 
+// TODO: No return allowed.
 pub fn analyzeStitch(
     sema: *Sema,
     builder: *Builder,
@@ -979,6 +1056,19 @@ pub fn analyzeStitch(
     _ = try analyzeBodyInner(sema, builder, body, false);
 }
 
+// TODO: No diverts allowed.
+pub fn analyzeFunction(
+    sema: *Sema,
+    builder: *Builder,
+    inst: Ir.Inst.Index,
+) !void {
+    const data = sema.ir.instructions[@intFromEnum(inst)].data.payload;
+    const extra = sema.ir.extraData(Ir.Inst.Function, data.extra_index);
+    const body = sema.ir.bodySlice(extra.end, extra.data.body_len);
+    _ = try analyzeBodyInner(sema, builder, body, false);
+}
+
+// TODO: No return allowed.
 pub fn analyzeKnot(
     sema: *Sema,
     builder: *Builder,
@@ -1088,7 +1178,6 @@ fn scanTopLevelDecl(
                     .namespace = child_namespace,
                 };
             }
-
             try sema.module.queueWorkItem(.{
                 .tag = .stitch,
                 .decl_name = decl_name,
@@ -1096,6 +1185,27 @@ fn scanTopLevelDecl(
                 .namespace = child_namespace,
             });
         },
+        .decl_function => {
+            const child_namespace = try sema.module.createNamespace(namespace);
+            const gop = try namespace.decls.getOrPut(sema.arena, decl_name);
+            if (gop.found_existing) {
+                return sema.fail(src_loc, "duplicate identifier", .{});
+            } else {
+                gop.value_ptr.* = .{
+                    .tag = .function,
+                    .decl_inst = extra.value,
+                    .args_count = 0,
+                    .namespace = child_namespace,
+                };
+            }
+            try sema.module.queueWorkItem(.{
+                .tag = .function,
+                .decl_name = decl_name,
+                .inst_index = extra.value,
+                .namespace = child_namespace,
+            });
+        },
+
         else => unreachable,
     }
 }

src/Story.zig

@@ -14,13 +14,16 @@ dump_writer: ?*std.Io.Writer = null,
 is_exited: bool = false,
 can_advance: bool = false,
 choice_index: usize = 0,
+stack_top: usize = 0,
+call_stack_top: usize = 0,
 current_choices: std.ArrayListUnmanaged(Choice) = .empty,
 code_chunks: std.ArrayListUnmanaged(*Object.Code) = .empty,
-constants_pool: std.ArrayListUnmanaged(Value) = .empty,
 globals: std.StringHashMapUnmanaged(?Value) = .empty,
-stack: std.ArrayListUnmanaged(?Value) = .empty,
-call_stack: std.ArrayListUnmanaged(CallFrame) = .empty,
-stack_max: usize = 128,
+stack: []Value = &.{},
+call_stack: []CallFrame = &.{},
+/// Global constants pool.
+constants_pool: []const Value = &.{},
+/// Linked list of all tracked runtime objects.
 gc_objects: std.SinglyLinkedList = .{},
 // FIXME: This was a hack to keep string bytes alive.
 string_bytes: []const u8 = &.{},
@@ -28,6 +31,7 @@ string_bytes: []const u8 = &.{},
 pub const default_knot_name: [:0]const u8 = "$__main__$";
 
 pub const Value = union(enum) {
+    nil,
     bool: bool,
     int: i64,
     float: f64,
@@ -35,6 +39,7 @@ pub const Value = union(enum) {
 
     pub fn tagBytes(v: Value) []const u8 {
         return switch (v) {
+            .nil => "Nil",
             .bool => "Bool",
             .int => "Int",
             .float => "Float",
@@ -57,7 +62,7 @@ pub const Value = union(enum) {
 
     pub fn isTruthy(v: Value) bool {
         return switch (v) {
-            //.nil => false,
+            .nil => false,
             .bool => |b| b,
             .int => |i| i != 0,
             .float => |f| f != 0.0,
@@ -133,6 +138,7 @@ pub const Value = union(enum) {
 
     pub fn eql(lhs: Value, rhs: Value) bool {
         return switch (lhs) {
+            .nil => rhs == .nil,
             .bool => |l| rhs == .bool and l == rhs.bool,
             .int => |l| switch (rhs) {
                 .int => |r| l == r,
@@ -183,15 +189,15 @@ pub const Value = union(enum) {
 
     pub fn negate(lhs: Value) !Value {
         switch (lhs) {
-            .bool => return error.TypeError,
+            .nil, .bool, .object => return error.TypeError,
             .int => |int| return .{ .int = -int },
             .float => |float| return .{ .float = -float },
-            .object => return error.TypeError,
         }
     }
 
     pub fn format(value: Value, writer: *std.Io.Writer) error{WriteFailed}!void {
         switch (value) {
+            .nil => try writer.writeAll(value.tagBytes()),
             .bool => |boolean| try writer.writeAll(if (boolean) "true" else "false"),
             .int => |int| try writer.print("{d}", .{int}),
             .float => |float| {
@@ -222,9 +228,10 @@ pub const Value = union(enum) {
 };
 
 pub const CallFrame = struct {
+    callee: *Object.Knot,
+    caller_top: usize,
     ip: usize,
     sp: usize,
-    callee: *Object.Knot,
 };
 
 pub const Choice = struct {
@@ -304,62 +311,52 @@ pub fn deinit(story: *Story) void {
     }
 
     story.current_choices.deinit(gpa);
-    story.constants_pool.deinit(gpa);
     story.globals.deinit(gpa);
-    story.stack.deinit(gpa);
-    story.call_stack.deinit(gpa);
-    gpa.free(story.string_bytes);
-}
 
-fn isCallStackEmpty(vm: *const Story) bool {
-    return vm.call_stack.items.len == 0;
+    gpa.free(story.string_bytes);
+    gpa.free(story.constants_pool);
+    gpa.free(story.stack);
+    gpa.free(story.call_stack);
 }
 
 fn currentFrame(vm: *Story) *CallFrame {
-    return &vm.call_stack.items[vm.call_stack.items.len - 1];
+    assert(vm.call_stack_top > 0);
+    return &vm.call_stack[vm.call_stack_top - 1];
 }
 
 fn peekStack(vm: *Story, offset: usize) ?Value {
-    const stack_top = vm.stack.items.len;
-    if (stack_top <= offset) return null;
-
-    return vm.stack.items[stack_top - offset - 1];
+    if (vm.stack_top <= offset) return null;
+    return vm.stack[vm.stack_top - offset - 1];
 }
 
 fn pushStack(vm: *Story, value: Value) !void {
-    const gpa = vm.allocator;
-    const stack_top = vm.stack.items.len;
-    const max_stack_top = vm.stack_max;
-    if (stack_top >= max_stack_top) return error.StackOverflow;
-
-    return vm.stack.append(gpa, value);
+    if (vm.stack_top >= vm.stack.len) return error.StackOverflow;
+    vm.stack[vm.stack_top] = value;
+    vm.stack_top += 1;
 }
 
 fn popStack(vm: *Story) ?Value {
-    return vm.stack.pop() orelse unreachable;
+    if (vm.stack_top == 0) return null;
+
+    const stack_top = vm.stack_top;
+    vm.stack_top -= 1;
+    return vm.stack[stack_top];
 }
 
 fn getConstant(story: *Story, frame: *CallFrame, offset: u8) !Value {
     if (offset >= frame.callee.code.constants.len) return error.InvalidArgument;
-
     const constant_index = frame.callee.code.constants[offset];
-    return story.constants_pool.items[constant_index];
+    return story.constants_pool[constant_index];
 }
 
 fn getLocal(vm: *Story, frame: *CallFrame, offset: u8) ?Value {
-    const stack_top = vm.stack.items.len;
-    const stack_offset = frame.sp + offset;
-    assert(stack_top > stack_offset);
-
-    return vm.stack.items[stack_offset];
+    assert(vm.stack_top > frame.sp + offset);
+    return vm.stack[frame.sp + offset];
 }
 
 fn setLocal(vm: *Story, frame: *CallFrame, offset: u8, value: Value) void {
-    const stack_top = vm.stack.items.len;
-    const stack_offset = frame.sp + offset;
-    assert(stack_top > stack_offset);
-
-    vm.stack.items[stack_offset] = value;
+    assert(vm.stack_top > frame.sp + offset);
+    vm.stack[frame.sp + offset] = value;
 }
 
 // TODO: This should probably check the constants table first.
@@ -396,15 +393,14 @@ fn setGlobal(vm: *Story, key: Value, value: Value) !void {
 
 fn execute(vm: *Story) !std.ArrayListUnmanaged(u8) {
     const gpa = vm.allocator;
+    if (vm.call_stack_top == 0) return .empty;
     errdefer vm.can_advance = false;
 
-    if (vm.isCallStackEmpty()) return .empty;
-
     var stream_writer = std.Io.Writer.Allocating.init(gpa);
     defer stream_writer.deinit();
 
+    var frame = vm.currentFrame();
     while (true) {
-        const frame = vm.currentFrame();
         const code = std.mem.bytesAsSlice(Opcode, frame.callee.code.bytecode);
         if (vm.dump_writer) |w| {
             Dumper.trace(vm, w, frame) catch {};
@@ -419,14 +415,27 @@ fn execute(vm: *Story) !std.ArrayListUnmanaged(u8) {
                 vm.can_advance = false;
                 return .empty;
             },
+            .ret => {
+                const return_value = vm.stack[vm.stack_top - 1];
+
+                vm.call_stack_top -= 1;
+                const completed_frame = vm.call_stack[vm.call_stack_top];
+
+                vm.stack_top = completed_frame.caller_top;
+
+                vm.stack[vm.stack_top] = return_value;
+                vm.stack_top += 1;
+
+                if (vm.call_stack_top == 0) return error.UnexpectedReturn;
+
+                frame = &vm.call_stack[vm.call_stack_top - 1];
+            },
             .true => {
-                const value: Value = .{ .bool = true };
-                try vm.pushStack(value);
+                try vm.pushStack(.{ .bool = true });
                 frame.ip += 1;
             },
             .false => {
-                const value: Value = .{ .bool = false };
-                try vm.pushStack(value);
+                try vm.pushStack(.{ .bool = false });
                 frame.ip += 1;
             },
             .pop => {
@@ -519,6 +528,43 @@ fn execute(vm: *Story) !std.ArrayListUnmanaged(u8) {
                     return error.InvalidArgument;
                 }
             },
+            .call => {
+                const arg_offset: u8 = @intFromEnum(code[frame.ip + 1]);
+                frame.ip += 2;
+
+                if (peekStack(vm, arg_offset)) |value| {
+                    switch (value) {
+                        .object => |object| switch (object.tag) {
+                            .knot => try call(vm, @ptrCast(object)),
+                            else => unreachable,
+                        },
+                        else => unreachable,
+                    }
+                } else {
+                    return error.InvalidArgument;
+                }
+
+                // Re-fetch — we're now in the callee's frame
+                frame = &vm.call_stack[vm.call_stack_top - 1];
+            },
+            .divert => {
+                const arg_offset: u8 = @intFromEnum(code[frame.ip + 1]);
+                frame.ip += 2;
+
+                if (peekStack(vm, arg_offset)) |value| {
+                    switch (value) {
+                        .object => |object| switch (object.tag) {
+                            .knot => try divert(vm, @ptrCast(object)),
+                            else => unreachable,
+                        },
+                        else => unreachable,
+                    }
+                } else {
+                    return error.InvalidArgument;
+                }
+
+                frame = &vm.call_stack[vm.call_stack_top - 1];
+            },
             .load_const => {
                 const index: u8 = @intFromEnum(code[frame.ip + 1]);
                 const value = try vm.getConstant(frame, index);
@@ -611,16 +657,6 @@ fn execute(vm: *Story) !std.ArrayListUnmanaged(u8) {
 
                 frame.ip = branch_dispatch.dest_offset;
             },
-            .divert => {
-                const arg_offset: u8 = @intFromEnum(code[frame.ip + 1]);
-                frame.ip += 2;
-
-                if (peekStack(vm, arg_offset)) |value| {
-                    try divertToValue(vm, value);
-                } else {
-                    return error.InvalidArgument;
-                }
-            },
             .load_attr => {
                 const arg_offset: u8 = @intFromEnum(code[frame.ip + 1]);
                 frame.ip += 2;
@@ -667,38 +703,62 @@ pub fn getKnot(vm: *Story, name: []const u8) ?*Object.Knot {
     return knot;
 }
 
-// TODO(Brett): Add arguments?
-fn divertToKnot(vm: *Story, knot: *Object.Knot) !void {
-    const gpa = vm.allocator;
-    const stack_ptr = vm.stack.items.len - knot.code.args_count;
-    const stack_needed = knot.code.stack_size;
+fn call(vm: *Story, knot: *Object.Knot) !void {
+    if (vm.call_stack_top >= vm.call_stack.len)
+        return error.CallStackOverflow;
 
-    try vm.stack.ensureUnusedCapacity(gpa, stack_needed);
-    try vm.call_stack.ensureUnusedCapacity(gpa, 1);
+    const locals_count = knot.code.locals_count;
+    const args_count = knot.code.args_count;
+    const sp = vm.stack_top - args_count;
+    const caller_top = if (vm.call_stack_top == 0)
+        sp
+    else
+        sp - 1;
 
-    vm.call_stack.appendAssumeCapacity(.{
+    const frame_top = sp + args_count + locals_count;
+    if (frame_top > vm.stack.len) return error.StackOverflow;
+    for (vm.stack[sp + args_count .. frame_top]) |*slot| slot.* = .nil;
+
+    vm.stack_top = frame_top;
+    vm.call_stack[vm.call_stack_top] = .{
         .callee = knot,
         .ip = 0,
-        .sp = stack_ptr,
-    });
-    vm.stack.appendNTimesAssumeCapacity(null, stack_needed);
-    vm.can_advance = true;
+        .sp = sp,
+        .caller_top = caller_top,
+    };
+    vm.call_stack_top += 1;
 }
 
-fn divertToValue(vm: *Story, value: Value) !void {
-    switch (value) {
-        .object => |object| switch (object.tag) {
-            .knot => try divertToKnot(vm, @ptrCast(object)),
-            else => return error.TypeError,
-        },
-        else => return error.TypeError,
+// Diverts are essentially tail calls.
+fn divert(vm: *Story, knot: *Object.Knot) !void {
+    const args_count = knot.code.args_count;
+    const locals_count = knot.code.locals_count;
+    if (vm.call_stack_top == 0) return vm.call(knot);
+
+    const args_start = vm.stack_top - args_count;
+    const current_frame = &vm.call_stack[vm.call_stack_top - 1];
+    const sp = current_frame.sp;
+    const caller_top = current_frame.caller_top;
+
+    if (args_count > 0) {
+        std.mem.copyForwards(
+            Value,
+            vm.stack[sp .. sp + args_count],
+            vm.stack[args_start .. args_start + args_count],
+        );
     }
-}
 
-fn divert(vm: *Story, knot_name: []const u8) !void {
-    return if (getKnot(vm, knot_name)) |knot| {
-        return divertToKnot(vm, knot);
-    } else return error.InvalidPath;
+    const frame_top = sp + args_count + locals_count;
+    if (frame_top > vm.stack.len) return error.StackOverflow;
+    for (vm.stack[sp + args_count .. frame_top]) |*slot| slot.* = .nil;
+    vm.stack_top = frame_top;
+
+    current_frame.* = .{
+        .callee = knot,
+        .ip = 0,
+        .sp = sp,
+        .caller_top = caller_top,
+    };
 }
 
 pub const LoadOptions = struct {
@@ -746,15 +806,25 @@ pub fn loadFromString(
         return error.LoadFailed;
     }
 
+    const stack_size = 128;
+    const eval_stack_ptr = try gpa.alloc(Value, stack_size);
+    errdefer gpa.free(eval_stack_ptr);
+
+    const call_stack_ptr = try gpa.alloc(CallFrame, stack_size);
+    errdefer gpa.free(call_stack_ptr);
+
     var story: Story = .{
         .allocator = gpa,
         .can_advance = false,
         .dump_writer = if (options.dump_trace) options.dump_writer else null,
+        .stack = eval_stack_ptr,
+        .call_stack = call_stack_ptr,
     };
     errdefer story.deinit();
+
     try comp.setupStoryRuntime(gpa, &story);
     if (story.getKnot(Story.default_knot_name)) |knot| {
-        try story.divertToKnot(knot);
+        try story.divert(knot);
         story.can_advance = true;
     }
     return story;

src/Story/Dumper.zig

@@ -22,7 +22,6 @@ fn dumpByteInst(
     op: Opcode,
 ) !usize {
     const code = knot.code;
-    const constants_pool = &self.story.constants_pool;
 
     assert(code.bytecode.len > offset + 1);
     const arg = code.bytecode[offset + 1];
@@ -31,9 +30,8 @@ fn dumpByteInst(
         try w.writeAll(" (");
         if (code.constants.len > arg) {
             const constant_index = code.constants[arg];
-            if (constants_pool.items.len > constant_index) {
-                const global_constant = &constants_pool.items[constant_index];
-                try self.dumpValue(w, global_constant);
+            if (self.story.constants_pool.len > constant_index) {
+                try self.dumpValue(w, &self.story.constants_pool[constant_index]);
             } else {
                 try w.writeAll("invalid!");
             }
@@ -55,14 +53,11 @@ fn dumpGlobalInst(
     op: Opcode,
 ) !usize {
     const code = knot.code;
-    const constants_pool = &self.story.constants_pool;
-
     assert(code.bytecode.len > offset + 1);
     const arg = code.bytecode[offset + 1];
     assert(code.constants.len > arg);
     const constant_index = code.constants[arg];
-    const global_constant = constants_pool.items[constant_index];
-    switch (global_constant) {
+    switch (self.story.constants_pool[constant_index]) {
         .object => |object| switch (object.tag) {
             .string => {
                 const global_name: *Object.String = @ptrCast(object);
@@ -280,6 +275,10 @@ pub fn dumpKnot(self: *Dumper, w: *std.Io.Writer, knot: *const Object.Knot) !voi
 
 pub fn dumpValue(_: Dumper, w: *std.Io.Writer, value: *const Value) !void {
     switch (value.*) {
+        .nil => try w.print(
+            "<{s}>",
+            .{value.tagBytes()},
+        ),
         .bool => |_| try w.print(
             "<{s} value={f}>",
             .{ value.tagBytes(), value },
@@ -313,8 +312,8 @@ pub fn dump(story: *Story, writer: *std.Io.Writer) !void {
     try writer.writeAll("=== Constants ===\n");
 
     var i: usize = 0;
-    while (i < story.constants_pool.items.len) : (i += 1) {
-        const global_constant = &story.constants_pool.items[i];
+    while (i < story.constants_pool.len) : (i += 1) {
+        const global_constant = &story.constants_pool[i];
         try story_dumper.dumpValue(writer, global_constant);
         try writer.writeAll("\n");
     }
@@ -350,31 +349,14 @@ pub fn dump(story: *Story, writer: *std.Io.Writer) !void {
     }
 }
 
-pub fn trace(story: *Story, writer: *std.Io.Writer, frame: *Story.CallFrame) !void {
-    var dumper: Dumper = .{ .story = story };
-    const stack = &story.stack;
-    const stack_top = story.stack.items.len;
-
+pub fn trace(vm: *Story, writer: *std.Io.Writer, frame: *Story.CallFrame) !void {
+    var dumper: Dumper = .{ .story = vm };
     try writer.print("\tStack => stack_pointer={d}, objects=[", .{frame.sp});
 
-    if (stack_top > 0) {
-        // FIXME: There has to be a better way to do this.
-        if (stack_top > 1) {
-            var i: usize = frame.sp;
-            while (i < stack.items.len - 1) : (i += 1) {
-                if (stack.items[i]) |*value| {
-                    try dumper.dumpValue(writer, value);
-                } else {
-                    try writer.writeAll("null");
-                }
-                try writer.writeAll(", ");
-            }
-        }
-        if (stack.items[stack.items.len - 1]) |*object| {
-            try dumper.dumpValue(writer, object);
-        } else {
-            try writer.writeAll("null");
-        }
+    const window = vm.stack[frame.sp..vm.stack_top];
+    for (window, 0..) |*slot, i| {
+        if (i > 0) try writer.writeAll(", ");
+        try dumper.dumpValue(writer, slot);
     }
 
     try writer.writeAll("]\n");

src/Story/Object.zig

@@ -95,7 +95,7 @@ pub const String = struct {
         const print_buffer_len = 64;
         var print_buffer: [print_buffer_len]u8 = undefined;
         switch (value) {
-            .bool, .int, .float => {
+            .nil, .bool, .int, .float => {
                 const bytes = try std.fmt.bufPrint(&print_buffer, "{f}", .{value});
                 return .create(story, .{ .bytes = bytes });
             },

src/Story/runtime_tests.zig

@@ -18,10 +18,18 @@ test "fixture - I002 (Fogg comforts Passepartout)" {
     try testRuntimeFixture("I002");
 }
 
+test "fixture - I004 (Print number as English)" {
+    try testRuntimeFixture("I004");
+}
+
 test "fixture - I005 (Const variable)" {
     try testRuntimeFixture("I005");
 }
 
+test "fixture - I006 (Multiple constant references)" {
+    try testRuntimeFixture("I006");
+}
+
 test "fixture - I007 (Set non existant variable)" {
     try testRuntimeFixture("I007");
 }
@@ -35,6 +43,10 @@ test "fixture - I011 (Temporaries at global scope)" {
 //    try testRuntimeFixture("I012");
 //}
 
+test "fixture - I014 (Variable swap recurse)" {
+    try testRuntimeFixture("I014");
+}
+
 test "fixture - I016 (Empty)" {
     try testRuntimeFixture("I016");
 }
@@ -59,10 +71,26 @@ test "fixture - I023 (Whitespace)" {
     try testRuntimeFixture("I023");
 }
 
+test "fixture - I033 (Newline consistency, the first)" {
+    try testRuntimeFixture("I033");
+}
+
+test "fixture - I034 (Newline consistency, the second)" {
+    try testRuntimeFixture("I034");
+}
+
 test "fixture - I035 (Newline consistency, the third)" {
     try testRuntimeFixture("I035");
 }
 
+test "fixture - I036 (Newlines with string eval)" {
+    try testRuntimeFixture("I036");
+}
+
+test "fixture - I037 (Newline at start of multiline conditional)" {
+    try testRuntimeFixture("I037");
+}
+
 test "fixture - I042 (Weave options)" {
     try testRuntimeFixture("I042");
 }
@@ -75,18 +103,35 @@ test "fixture - I064 (Done stops thread)" {
     try testRuntimeFixture("I064");
 }
 
+// This one is named oddly...
+test "fixture - I075 (Clean callstack reset on path choice)" {
+    try testRuntimeFixture("I075");
+}
+
 test "fixture - I078 (Choice with brackets only)" {
     try testRuntimeFixture("I078");
 }
 
+test "fixture - I117 (Factorial recursive)" {
+    try testRuntimeFixture("I117");
+}
+
 test "fixture - I118 (Literal unary)" {
     try testRuntimeFixture("I118");
 }
 
+test "fixture - I119 (Basic string literals)" {
+    try testRuntimeFixture("I119");
+}
+
 test "fixture - I121 (Arithmetic)" {
     try testRuntimeFixture("I121");
 }
 
+test "fixture - I124 (Evaluating ink functions from game 2)" {
+    try testRuntimeFixture("I124");
+}
+
 test "fixture - I133 (Float printing precision)" {
     try testRuntimeFixture("I133");
 }

src/Story/testdata/I004/story.ink

@@ -0,0 +1,53 @@
+You have {print_num(58)} coins.
+
+=== function print_num(x)
+{
+    - x >= 1000:
+        {print_num(x / 1000)} thousand { x mod 1000 > 0:{print_num(x mod 1000)}}
+    - x >= 100:
+        {print_num(x / 100)} hundred { x mod 100 > 0:and {print_num(x mod 100)}}
+    - x == 0:
+        zero
+    - else:
+        { x >= 20:
+            { x / 10:
+                - 2: twenty
+                - 3: thirty
+                - 4: forty
+                - 5: fifty
+                - 6: sixty
+                - 7: seventy
+                - 8: eighty
+                - 9: ninety
+            }
+            { x mod 10 > 0:
+                <>-<>
+            }
+        }
+        { x < 10 || x > 20:
+            { x mod 10:
+                - 1: one
+                - 2: two
+                - 3: three
+                - 4: four
+                - 5: five
+                - 6: six
+                - 7: seven
+                - 8: eight
+                - 9: nine
+            }
+        - else:
+            { x:
+                - 10: ten
+                - 11: eleven
+                - 12: twelve
+                - 13: thirteen
+                - 14: fourteen
+                - 15: fifteen
+                - 16: sixteen
+                - 17: seventeen
+                - 18: eighteen
+                - 19: nineteen
+            }
+        }
+}

src/Story/testdata/I004/transcript.txt

@@ -0,0 +1 @@
+You have fifty-eight coins.

src/Story/testdata/I006/story.ink

@@ -0,0 +1,3 @@
+CONST CONST_STR = "ConstantString"
+VAR varStr = CONST_STR
+{varStr == CONST_STR:success}

src/Story/testdata/I006/transcript.txt

@@ -0,0 +1 @@
+success

src/Story/testdata/I014/story.ink

@@ -0,0 +1,9 @@
+~ f(1, 1)
+== function f(x, y) ==
+{ x == 1 and y == 1:
+  ~ x = 2
+  ~ f(y, x)
+- else:
+  {x} {y}
+}
+~ return

src/Story/testdata/I014/transcript.txt

@@ -0,0 +1 @@
+1 2

src/Story/testdata/I033/story.ink

@@ -0,0 +1,4 @@
+hello -> world
+== world
+world
+-> END

src/Story/testdata/I033/transcript.txt

@@ -0,0 +1 @@
+hello world

src/Story/testdata/I034/input.txt

@@ -0,0 +1 @@
+1

src/Story/testdata/I034/story.ink

@@ -0,0 +1,4 @@
+* hello -> world
+== world
+world
+-> END

src/Story/testdata/I034/transcript.txt

@@ -0,0 +1,2 @@
+1: hello
+?> hello world

src/Story/testdata/I036/story.ink

@@ -0,0 +1,9 @@
+A
+~temp someTemp = string()
+B
+A
+{string()}
+B
+=== function string()
+    ~ return "{3}"
+}

src/Story/testdata/I036/transcript.txt

@@ -0,0 +1,5 @@
+A
+B
+A
+3
+B

src/Story/testdata/I037/story.ink

@@ -0,0 +1,6 @@
+{isTrue():
+    x
+}
+=== function isTrue()
+    X
+    ~ return true

src/Story/testdata/I037/transcript.txt

@@ -0,0 +1,2 @@
+X
+x

src/Story/testdata/I044/story.ink

@@ -0,0 +1,7 @@
+A
+{f():X}
+C
+=== function f()
+{ true:
+    ~ return false
+}

src/Story/testdata/I044/transcript.txt

@@ -0,0 +1,2 @@
+A
+C

src/Story/testdata/I045/story.ink

@@ -0,0 +1,6 @@
+A {f():B}
+X
+=== function f() ===
+{true:
+    ~ return false
+}

src/Story/testdata/I045/transcript.txt

@@ -0,0 +1,2 @@
+A
+X

src/Story/testdata/I046/story.ink

@@ -0,0 +1,7 @@
+A line.
+{ f():
+    Another line.
+}
+== function f ==
+{false:nothing}
+~ return true

src/Story/testdata/I046/transcript.txt

@@ -0,0 +1,2 @@
+A line.
+Another line.

src/Story/testdata/I047/story.ink

@@ -0,0 +1,6 @@
+I have {five()} eggs.
+== function five ==
+{false:
+    Don't print this
+}
+five

src/Story/testdata/I047/transcript.txt

@@ -0,0 +1 @@
+I have five eggs.

src/Story/testdata/I048/story.ink

@@ -0,0 +1,2 @@
+Some <>
+content<> with glue.

src/Story/testdata/I048/transcript.txt

@@ -0,0 +1 @@
+Some content with glue.

src/Story/testdata/I055/story.ink

@@ -0,0 +1,6 @@
+-> hurry_home
+=== hurry_home ===
+We hurried home to Savile Row -> as_fast_as_we_could
+=== as_fast_as_we_could ===
+as fast as we could.
+-> DONE

src/Story/testdata/I055/transcript.txt

@@ -0,0 +1 @@
+We hurried home to Savile Row as fast as we could.

src/Story/testdata/I061/story.ink

@@ -0,0 +1,8 @@
+=== intro
+= top
+    { main: -> done }
+    -> END
+= main
+    -> top
+= done
+    -> END

src/Story/testdata/I075/story.ink

@@ -0,0 +1,7 @@
+{RunAThing()}
+== function RunAThing ==
+The first line.
+The second line.
+== SomewhereElse ==
+{"somewhere else"}
+->END

src/Story/testdata/I075/transcript.txt

@@ -0,0 +1,2 @@
+The first line.
+The second line.

src/Story/testdata/I076/story.ink

@@ -0,0 +1,8 @@
+    { six() + two() }
+    -> END
+=== function six
+    ~ return four() + two()
+=== function four
+    ~ return two() + two()
+=== function two
+    ~ return 2

src/Story/testdata/I076/transcript.txt

@@ -0,0 +1 @@
+8

src/Story/testdata/I082/input.txt

@@ -0,0 +1 @@
+1

src/Story/testdata/I082/story.ink

@@ -0,0 +1 @@
+* choice -> DONE

src/Story/testdata/I082/transcript.txt

@@ -0,0 +1,2 @@
+1: choice
+?> choice

src/Story/testdata/I085/input.txt

@@ -0,0 +1 @@
+1

src/Story/testdata/I085/story.ink

@@ -0,0 +1,4 @@
+* 'Hello {name()}[, your name is {name()}.'],' I said, knowing full well that his name was {name()}.
+-> DONE
+== function name ==
+Joe

src/Story/testdata/I085/transcript.txt

@@ -0,0 +1,2 @@
+1: 'Hello Joe, your name is Joe.'
+?> 'Hello Joe,' I said, knowing full well that his name was Joe.

src/Story/testdata/I087/input.txt

@@ -0,0 +1 @@
+1

src/Story/testdata/I087/story.ink

@@ -0,0 +1,7 @@
+-> knot
+== knot
+   *   option text[]. {true: Conditional bit.} -> next
+   -> DONE
+== next
+    Next.
+    -> DONE

src/Story/testdata/I087/transcript.txt

@@ -0,0 +1,2 @@
+1: option text
+?> option text. Conditional bit. Next.

src/Story/testdata/I094/story.ink

@@ -0,0 +1,55 @@
+. {print_num(4)} .
+. {print_num(15)} .
+. {print_num(37)} .
+. {print_num(101)} .
+. {print_num(222)} .
+. {print_num(1234)} .
+=== function print_num(x) ===
+{
+    - x >= 1000:
+        {print_num(x / 1000)} thousand { x mod 1000 > 0:{print_num(x mod 1000)}}
+    - x >= 100:
+        {print_num(x / 100)} hundred { x mod 100 > 0:and {print_num(x mod 100)}}
+    - x == 0:
+        zero
+    - else:
+        { x >= 20:
+            { x / 10:
+                - 2: twenty
+                - 3: thirty
+                - 4: forty
+                - 5: fifty
+                - 6: sixty
+                - 7: seventy
+                - 8: eighty
+                - 9: ninety
+            }
+            { x mod 10 > 0:<>-<>}
+        }
+        { x < 10 || x > 20:
+            { x mod 10:
+                - 1: one
+                - 2: two
+                - 3: three
+                - 4: four
+                - 5: five
+                - 6: six
+                - 7: seven
+                - 8: eight
+                - 9: nine
+            }
+        - else:
+            { x:
+                - 10: ten
+                - 11: eleven
+                - 12: twelve
+                - 13: thirteen
+                - 14: fourteen
+                - 15: fifteen
+                - 16: sixteen
+                - 17: seventeen
+                - 18: eighteen
+                - 19: nineteen
+            }
+        }
+}

src/Story/testdata/I094/transcript.txt

@@ -0,0 +1,6 @@
+. four .
+. fifteen .
+. thirty-seven .
+. one hundred and one .
+. two hundred and twenty-two .
+. one thousand two hundred and thirty-four .

src/Story/testdata/I095/story.ink

@@ -0,0 +1,8 @@
+{true:
+    a
+} <> b
+{true:
+    a
+} <> { true:
+    b
+}

src/Story/testdata/I095/transcript.txt

@@ -0,0 +1,2 @@
+a b
+a b

src/Story/testdata/I097/story.ink

@@ -0,0 +1,7 @@
+~ func ()
+text 2
+~ temp tempVar = func ()
+text 2
+== function func ()
+    text1
+    ~ return true

src/Story/testdata/I097/transcript.txt

@@ -0,0 +1,4 @@
+text1
+text 2
+text1
+text 2

src/Story/testdata/I112/story.ink

@@ -0,0 +1,4 @@
+{ 1:
+    - 2: x
+    - 3: y
+}

src/Story/testdata/I113/story.ink

@@ -0,0 +1,20 @@
+VAR x = 3
+{
+    - x == 1: one
+    - x == 2: two
+    - else: other
+}
+{
+    - x == 1: one
+    - x == 2: two
+    - other
+}
+{ x == 4:
+  - The main clause
+  - else: other
+}
+{ x == 4:
+  The main clause
+- else:
+  other
+}

src/Story/testdata/I113/transcript.txt

@@ -0,0 +1,4 @@
+other
+other
+other
+other

src/Story/testdata/I115/story.ink

@@ -0,0 +1,5 @@
+{ 3:
+    - 3:
+    - 4:
+        txt
+}

src/Story/testdata/I116/story.ink

@@ -0,0 +1,4 @@
+{
+- false:
+   beep
+}

src/Story/testdata/I117/story.ink

@@ -0,0 +1,7 @@
+{ factorial(5) }
+== function factorial(n) ==
+ { n == 1:
+    ~ return 1
+ - else:
+    ~ return (n * factorial(n-1))
+ }

src/Story/testdata/I117/transcript.txt

@@ -0,0 +1 @@
+120

src/Story/testdata/I119/story.ink

@@ -0,0 +1,3 @@
+VAR x = "Hello world 1"
+{x}
+Hello {"world"} 2.

src/Story/testdata/I119/transcript.txt

@@ -0,0 +1,2 @@
+Hello world 1
+Hello world 2.

src/Story/testdata/I124/story.ink

@@ -0,0 +1,12 @@
+One
+Two
+Three
+== function func1 ==
+This is a function
+~ return 5
+== function func2 ==
+This is a function without a return value
+~ return
+== function add(x,y) ==
+x = {x}, y = {y}
+~ return x + y

src/Story/testdata/I124/transcript.txt

@@ -0,0 +1,3 @@
+One
+Two
+Three

src/Story/testdata/I127/input.txt

@@ -0,0 +1 @@
+1

src/Story/testdata/I127/story.ink

@@ -0,0 +1,10 @@
+VAR testVar = 5
+VAR testVar2 = 10
+Hello world!
+~ testVar = 15
+~ testVar2 = 100
+Hello world 2!
+* choice
+    ~ testVar = 25
+    ~ testVar2 = 200
+    -> END

src/Story/testdata/I127/transcript.txt

@@ -0,0 +1,5 @@
+Hello world!
+Hello world 2!
+
+1: choice
+?> choice

src/compile.zig

@@ -164,6 +164,7 @@ pub const InternPool = struct {
     }
 };
 
+// TODO: Revisit this. We might not need this at all.
 pub const WorkItem = struct {
     tag: Tag,
     next: ?*WorkItem = null,
@@ -174,6 +175,7 @@ pub const WorkItem = struct {
     pub const Tag = enum {
         knot,
         stitch,
+        function,
     };
 };
 
@@ -235,6 +237,7 @@ pub const Module = struct {
             pub const Tag = enum {
                 knot,
                 stitch,
+                function,
                 var_mut,
                 var_const,
             };
@@ -315,6 +318,16 @@ pub const Module = struct {
                         .code_index = @enumFromInt(chunk_index),
                     });
                 },
+                .function => {
+                    try sema.analyzeFunction(&builder, work_unit.inst_index);
+                    try builder.finalize();
+
+                    try mod.stitches.append(gpa, .{
+                        .knot_index = null,
+                        .name_index = work_unit.decl_name,
+                        .code_index = @enumFromInt(chunk_index),
+                    });
+                },
             }
         }
     }
@@ -412,11 +425,13 @@ pub const Module = struct {
     pub fn setupStoryRuntime(mod: *Module, gpa: std.mem.Allocator, story: *Story) !void {
         assert(mod.errors.items.len == 0);
         const constants_len = mod.intern_pool.values.items.len;
+        var constants_pool: std.ArrayListUnmanaged(Value) = .empty;
+        try constants_pool.ensureUnusedCapacity(gpa, constants_len);
+        defer constants_pool.deinit(gpa);
 
-        try story.constants_pool.ensureUnusedCapacity(gpa, constants_len);
         for (mod.intern_pool.values.items) |value| {
             const obj = try mod.makeValueFromInterned(story, value);
-            story.constants_pool.appendAssumeCapacity(obj);
+            constants_pool.appendAssumeCapacity(obj);
         }
         for (mod.globals.items) |global| {
             const key_bytes = mod.intern_pool.getStrBytes(mod.ir, global.key);
@@ -459,8 +474,13 @@ pub const Module = struct {
                 const parent_knot_value = story.globals.get(parent_knot_name).?;
                 const parent_knot_obj: *Object.Knot = @ptrCast(parent_knot_value.?.object);
                 try parent_knot_obj.members.put(gpa, name_bytes, &stitch_obj.base);
+            } else {
+                const value: Value = .{ .object = &stitch_obj.base };
+                try story.globals.put(gpa, name_bytes, value);
             }
         }
+
+        story.constants_pool = try constants_pool.toOwnedSlice(gpa);
         story.string_bytes = mod.ir.string_bytes;
         mod.ir.string_bytes = &.{};
     }

src/print_ir.zig

@@ -293,7 +293,10 @@ pub const Writer = struct {
             .declaration => try self.writeDeclarationInst(w, inst),
             .decl_var => try self.writeVarDeclInst(w, inst),
             .decl_knot => try self.writeKnotDeclInst(w, inst),
+            // TODO: Revisit this.
             .decl_stitch => try self.writeVarDeclInst(w, inst),
+            // TODO: Revisit this.
+            .decl_function => try self.writeVarDeclInst(w, inst),
             .decl_ref => try self.writeStrTokInst(w, inst),
             .condbr => try self.writeCondbrInst(w, inst),
             .@"break" => try self.writeBreakInst(w, inst),
@@ -323,6 +326,7 @@ pub const Writer = struct {
             .content_push => try self.writeUnaryInst(w, inst),
             .content_flush => try self.writeUnaryInst(w, inst),
             .choice_br => try self.writeChoiceBrInst(w, inst),
+            .ret => try self.writeUnaryInst(w, inst),
             .implicit_ret => try self.writeUnaryInst(w, inst),
             .call => try self.writeCallInst(w, inst, .direct),
             .divert => try self.writeCallInst(w, inst, .direct),