[rlox] Implement Globals (and cleanup a...lot)

rlox/src/bc.rs

@@ -1,4 +1,4 @@
-use crate::gc::{GcHandle, Object, ObjectType};
+use crate::gc::{GcHandle, Object};
 use std::collections::LinkedList;
 use std::convert::From;
 use std::fmt::Debug;
@@ -22,7 +22,10 @@ pub enum Op {
     Less,
 
     Print,
-    Pop
+    Pop,
+
+    DefineGlobal { offset: u8 },
+    GetGlobal { offset: u8 },
 }
 
 #[derive(Clone, Debug, PartialEq)]
@@ -87,11 +90,7 @@ impl Display for Value {
                     None => write!(f, "{}", stringified),
                 }
             }
-            Value::Obj(object) => match object.get_otype() {
+            Value::Obj(object) => write!(f, "{}", object),
-                ObjectType::String => {
-                    write!(f, "{}", object)
-                }
-            },
         }
     }
 }
@@ -138,6 +137,11 @@ impl Chunk {
         self
     }
 
+    pub fn add_constant_value(&mut self, value: Value) -> &mut Self {
+        self.constants.push(value);
+        self
+    }
+
     pub fn add_constant(&mut self, value: Value, line: usize) -> &mut Self {
         self.constants.push(value);
         self.add_op(
@@ -216,18 +220,18 @@ impl fmt::Debug for TraceInfo<'_> {
 
 mod tests {
 
+
     #[test]
     fn string_value_equality() {
         use crate::bc::Value;
-        use crate::gc::allocate_string;
+        use crate::gc::GC;
 
         let s1 = "bla5";
         let s2 = "bla6";
 
-        unsafe {
+        let o1 = GC::new_string(s1);
-            let o1 = allocate_string(s1).unwrap();
+        let o2 = GC::new_string(s2);
-            let o2 = allocate_string(s2).unwrap();
+        let o3 = GC::new_string(s2);
-            let o3 = allocate_string(s2).unwrap();
         let v1 = Value::from(o1.get_object());
         let v2 = Value::from(o2.get_object());
         let v3 = Value::from(o3.get_object());
@@ -238,4 +242,3 @@ mod tests {
         assert_eq!(v2, v4);
     }
 }
-}

rlox/src/gc.rs

@@ -1,46 +1,126 @@
+#![allow(unused, dead_code)]
+
+use core::hash;
 use std::{
     alloc::{alloc, dealloc, Layout, LayoutError},
     fmt::{self, Display},
 };
 
+/// Api
+
+pub struct GC {}
+
+impl GC {
+    pub fn new_string(content: &str) -> GcHandle {
+        unsafe { allocate_string(content) }.unwrap()
+    }
+
+    pub fn new_concat_string(first: ObjString, second: ObjString) -> GcHandle {
+        unsafe { concat_string(first, second) }.unwrap()
+    }
+
+    pub fn free(handle: GcHandle) {
+        unsafe { deallocate_object(handle.object) }
+    }
+}
+
+/// Markers
 #[derive(PartialEq, Eq, Clone, Copy)]
 #[repr(usize)]
 pub enum ObjectType {
     String,
 }
 
-#[repr(C)]
+pub(crate) trait IsObject {
-struct ObjectHeader {
+    fn otype() -> ObjectType;
-    otype: ObjectType,
+    fn from_object(object: Object) -> Self;
+    fn upcast(self) -> Object;
 }
 
-#[repr(C)]
+
-struct ObjStringHeader {
+/// Object Hierarchy / Layout stuff
-    object_header: ObjectHeader,
+///
-    len: usize,
+/// Object
+///    |
+/// ObjString
+///
+/// Object:    --ptr-to-->   [ [<otype>], .... data ....   ]
+/// ObjString: --ptr-to-->   [[[<otype>], len], ...data... ]
+///                           ^-StringHeader-^
+///                          ^----------StringAlloc--------^
+///
+/// GcHandle owns the underlying memory and must not be dropped before the corresponding Objects are.
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct GcHandle {
+    object: Object,
 }
 
-#[repr(C)]
+impl Drop for GcHandle {
-struct ObjString {
+    fn drop(&mut self) {
-    header: ObjStringHeader,
+        unsafe { deallocate_object(self.object) };
-    data: [u8],
+    }
 }
 
-const fn data_offset() -> usize {
+impl GcHandle {
-    std::mem::size_of::<ObjStringHeader>()
+    pub fn get_object(&self) -> Object {
+        return self.object;
+    }
 }
 
 #[derive(Copy, Clone)]
 pub struct Object {
-    ptr: *mut ObjectHeader,
+    ptr: *mut Header,
 }
 
+#[derive(Copy, Clone, Eq)]
+pub struct ObjString {
+    ptr: *mut StringHeader,
+}
+
+impl IsObject for ObjString {
+    fn otype() -> ObjectType {
+        ObjectType::String
+    }
+
+    fn from_object(object: Object) -> ObjString {
+        ObjString { ptr: object.ptr as *mut StringHeader }
+    }
+
+    fn upcast(self) -> Object {
+        Object { ptr: self.ptr as *mut Header }
+    }
+}
+
+#[repr(C)]
+struct Header {
+    otype: ObjectType,
+}
+
+#[repr(C)]
+struct StringAlloc {
+    header: StringHeader,
+    data: [u8],
+}
+
+#[repr(C)]
+struct StringHeader {
+    object_header: Header,
+    len: usize,
+}
+
+
+const fn data_offset() -> usize {
+    std::mem::size_of::<StringHeader>()
+}
+
+
+/// Pretty-print Object
 impl Display for Object {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self.get_otype() {
-            ObjectType::String => {
+            ObjectType::String =>
-                write!(f, "{}", ObjString::as_str(self.ptr as *mut ObjStringHeader))
+                fmt::Display::fmt(&self.downcast::<ObjString>().unwrap(), f)
-            }
         }
     }
 }
@@ -49,16 +129,22 @@ impl Object {
     pub fn get_otype(&self) -> ObjectType {
         unsafe { (*self.ptr).otype }
     }
+
+    pub(crate) fn downcast<T: IsObject>(self) -> Option<T> {
+        if self.get_otype() == T::otype() {
+            Some(T::from_object(self))
+        } else {
+            None
+        }
+    }
 }
 
 impl fmt::Debug for Object {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self.get_otype() {
             ObjectType::String => {
-                let string = self.ptr as *mut ObjStringHeader;
+                let string = self.downcast::<ObjString>().unwrap().as_str();
-                let data = ObjString::as_str(string);
+                write!(f, "STR {} {:?}", string.len(), &string[..8.min(string.len())])
-
-                write!(f, "STR {} {:?}", data.len(), &data[..8.min(data.len())],)
             }
         }
     }
@@ -77,27 +163,58 @@ impl PartialEq for Object {
 
             match (*self.ptr).otype {
                 ObjectType::String => {
-                    let header = self.ptr as *mut ObjStringHeader;
+                    self.downcast::<ObjString>() == other.downcast::<ObjString>()
-                    let other_header = other.ptr as *mut ObjStringHeader;
+                }
+            }
+        }
+    }
+}
 
-                    if (*header).len != (*other_header).len {
+impl PartialEq for ObjString {
+    fn eq(&self, other: &Self) -> bool {
+        unsafe {
+            if (*self.ptr).len != (*other.ptr).len {
                 return false;
             }
 
-                    let slice = ObjString::as_str(header);
+            self.as_slice() == other.as_slice()
-                    let other_slice = ObjString::as_str(other_header);
-
-                    slice == other_slice
-                }
-            }
         }
     }
 }
 
 impl ObjString {
+    fn as_slice<'a>(&self) -> &'a [u8] {
+        let length = unsafe { (*self.ptr).len };
+        let (layout_, offset)  = StringAlloc::layout(length).unwrap();
+        unsafe {
+            std::slice::from_raw_parts(
+                (self.ptr as *mut u8).offset(offset as isize),
+                length
+            )
+        }
+    }
+
+    fn as_str<'a>(&self) -> &'a str {
+        unsafe { std::str::from_utf8_unchecked(self.as_slice()) }
+    }
+}
+
+impl fmt::Display for ObjString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self.as_str(), f)
+    }
+}
+
+impl std::hash::Hash for ObjString {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        std::hash::Hash::hash::<H>(self.as_str(), state);
+    }
+}
+
+impl StringAlloc {
     fn layout(length: usize) -> Result<(Layout, usize), LayoutError> {
-        let (layout, offset) = Layout::for_value(&ObjStringHeader {
+        let (layout, offset) = Layout::for_value(&StringHeader {
-            object_header: ObjectHeader {
+            object_header: Header {
                 otype: ObjectType::String,
             },
             len: length,
@@ -106,47 +223,34 @@ impl ObjString {
 
         Ok((layout.pad_to_align(), offset))
     }
-
-    fn as_bytes<'a>(ptr: *const ObjStringHeader) -> &'a [u8] {
-        unsafe {
-            std::slice::from_raw_parts((ptr as *mut u8).offset(data_offset() as isize), (*ptr).len)
-        }
 }
 
-    fn as_str<'a>(ptr: *const ObjStringHeader) -> &'a str {
+unsafe fn allocate_string_obj<'a>(
-        unsafe { std::str::from_utf8_unchecked(ObjString::as_bytes(ptr)) }
-    }
-}
-
-pub unsafe fn allocate_string_obj<'a>(
     length: usize,
 ) -> Result<(GcHandle, &'a mut [u8]), LayoutError> {
-    let (layout, offset) = ObjString::layout(length)?;
+    let (layout, offset) = StringAlloc::layout(length)?;
     let allocation = alloc(layout);
     let data_ptr = allocation.offset(offset as isize);
-    let header = allocation as *mut ObjStringHeader;
+    let header = allocation as *mut StringHeader;
     (*header).len = length;
     (*header).object_header.otype = ObjectType::String;
     let object = Object {
-        ptr: header as *mut ObjectHeader,
+        ptr: header as *mut Header,
     };
     let str = std::slice::from_raw_parts_mut(data_ptr, length);
     Ok((GcHandle { object }, str))
 }
 
-pub unsafe fn allocate_string(content: &str) -> Result<GcHandle, LayoutError> {
+unsafe fn allocate_string(content: &str) -> Result<GcHandle, LayoutError> {
     let (gc_handle, slice) = allocate_string_obj(content.len())?;
     slice.copy_from_slice(content.as_bytes());
     Ok(gc_handle)
 }
 
-pub unsafe fn concat_string(a: Object, b: Object) -> Result<GcHandle, LayoutError> {
+unsafe fn concat_string(a: ObjString, b: ObjString) -> Result<GcHandle, LayoutError> {
-    let a_head = a.ptr as *mut ObjStringHeader;
+    let (a_data,b_data) = (a.as_slice(), b.as_slice());
-    let b_head = b.ptr as *mut ObjStringHeader;
-    let a_data = ObjString::as_bytes(a_head);
-    let b_data = ObjString::as_bytes(b_head);
-    let new_len = a_data.len() + b_data.len();
 
+    let new_len = a_data.len() + b_data.len();
     let (gc_handle, slice) = allocate_string_obj(new_len)?;
 
     slice[..a_data.len()].copy_from_slice(a_data);
@@ -158,28 +262,12 @@ pub unsafe fn concat_string(a: Object, b: Object) -> Result<GcHandle, LayoutErro
 unsafe fn deallocate_object(object: Object) {
     match object.get_otype() {
         ObjectType::String => {
-            let header = object.ptr as *mut ObjStringHeader;
+            let header = object.ptr as *mut StringHeader;
             dealloc(
                 object.ptr as *mut u8,
-                ObjString::layout((*header).len).unwrap().0,
+                StringAlloc::layout((*header).len).unwrap().0,
             )
         }
     }
 }
 
-#[derive(Debug, Clone, PartialEq)]
-pub struct GcHandle {
-    object: Object,
-}
-
-impl Drop for GcHandle {
-    fn drop(&mut self) {
-        unsafe { deallocate_object(self.object) };
-    }
-}
-
-impl GcHandle {
-    pub fn get_object(&self) -> Object {
-        return self.object;
-    }
-}

rlox/src/lc.rs

@@ -1,10 +1,12 @@
-use std::fmt;
+#![allow(dead_code, unused)]
+
+use std::{collections::hash_map, fmt};
 use std::iter::Peekable;
 use std::str::CharIndices;
 use std::collections::HashMap;
 
-use crate::bc::{Chunk, Op};
+use crate::bc::Value;
-use crate::gc::allocate_string;
+use crate::{bc::{Chunk, Op}, gc::GC};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
 enum ScanErrorKind {
@@ -285,24 +287,22 @@ struct Parser<'src> {
 #[derive(Debug, PartialEq)]
 pub enum ParseErrorKind {
     InvalidNumber,
-    UnexpectedEOF,
     IncompleteExpression,
     NoSemicolonAfterValue,
     NoSemicolonAfterExpression,
+    NoVariableName,
+    NoSemicolonAfterVarDecl,
 }
 
 impl fmt::Display for ParseErrorKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             ParseErrorKind::InvalidNumber => todo!(),
-            ParseErrorKind::UnexpectedEOF => todo!(),
+            ParseErrorKind::IncompleteExpression => write!(f, "Expect expression."),
-            ParseErrorKind::IncompleteExpression => {
+            ParseErrorKind::NoSemicolonAfterValue => write!(f, "Expect ';' after value."),
-                write!(f, "Expect expression.")
+            ParseErrorKind::NoSemicolonAfterExpression => write!(f, "Expect ';' after expression."),
-            },
+            ParseErrorKind::NoVariableName => write!(f, "Expect variable name."),
-            ParseErrorKind::NoSemicolonAfterValue => todo!(),
+            ParseErrorKind::NoSemicolonAfterVarDecl => write!(f, "Expect ';' after variable declaration."),
-            ParseErrorKind::NoSemicolonAfterExpression => {
-                write!(f, "Expect ';' after expression.")
-            },
         }
     }
 }
@@ -336,7 +336,7 @@ enum Associativity {
     NonAssoc,
 }
 
-#[derive(Clone, Copy, PartialOrd, Ord, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialOrd, Ord, PartialEq, Eq)]
 enum Precedence {
     None,
     Assignment,
@@ -409,6 +409,21 @@ impl<'src> Parser<'src> {
         }
     }
 
+    fn add_string(&mut self, chunk: &mut Chunk, string: &'src str) -> u8 {
+        match self.intern_table.entry(string) {
+            hash_map::Entry::Occupied(entry) => {
+                entry.get().clone()
+            },
+            hash_map::Entry::Vacant(entry) => {
+                let handle = GC::new_string(string);
+                chunk.add_constant_value(Value::from(handle.get_object()));
+                chunk.allocations.push_front(handle);
+                let offset = chunk.constants.len() as u8 - 1;
+                entry.insert(offset).clone()
+            },
+        }
+    }
+
     fn _expression(&mut self, chunk: &mut Chunk, min_prec: Precedence) -> Result<'src, ()> {
         match self.scanner.next() {
             None => return Err(self.error_end(ParseErrorKind::IncompleteExpression)),
@@ -430,18 +445,13 @@ impl<'src> Parser<'src> {
                 }
                 TokenType::String => {
                     let without_quotes = &token.span[1..(token.span.len() - 1)];
-                    match self.intern_table.get(without_quotes) {
+                    let offset = self.add_string(chunk, without_quotes);
-                        Some(&index) => {
+                    chunk.add_op(
-                            chunk.add_op(Op::Constant { offset: index }, token.line);
+                        Op::Constant {
-                        }
+                            offset,
-                        None => {
+                        },
-                            let object = unsafe { allocate_string(without_quotes) }.unwrap();
+                        token.line
-                            chunk.add_constant(object.get_object().into(), token.line);
+                    );
-                            self.intern_table
-                                .insert(without_quotes, chunk.constants.len() as u8 - 1);
-                            chunk.allocations.push_front(object);
-                        }
-                    };
                 }
                 TokenType::LeftParen => {
                     self._expression(chunk, Precedence::None)?;
@@ -456,6 +466,10 @@ impl<'src> Parser<'src> {
                 TokenType::False => {
                     chunk.add_op(Op::False, token.line);
                 }
+                TokenType::Identifier => {
+                    let offset = self.add_string(chunk, token.span);
+                    chunk.add_op(Op::GetGlobal {offset}, token.line);
+                }
                 _ => {
                     return Err(self.error_at(token, ParseErrorKind::IncompleteExpression));
                 }
@@ -504,31 +518,31 @@ impl<'src> Parser<'src> {
         self._expression(chunk, Precedence::None)
     }
 
+    pub fn must_consume(&mut self, expected: TokenType, error_kind: ParseErrorKind) -> Result<'src, Token<'src>> {
+        match self.scanner.peek().cloned() {
+            Some(token) if token.ttype == expected => Ok(self.scanner.next().unwrap()),
+            Some(token) => Err(self.error_at(token.clone(), error_kind)),
+            _ => Err(self.error_end(error_kind)),
+        }
+    }
+
     pub fn print_statement(&mut self, print_token: Token<'src>, chunk: &mut Chunk) -> Result<'src, ()> {
         self.expression(chunk)?;
         chunk.add_op(Op::Print, print_token.line);
-        match self.scanner.next_if(|t| t.ttype == TokenType::Semicolon) {
+        self.must_consume(TokenType::Semicolon, ParseErrorKind::NoSemicolonAfterValue).map(|_| ())
-            Some(_) => Ok(()),
-            None => {
-                let location = self.scanner.peek().cloned();
-                Err(self.error_at_or_end(location, ParseErrorKind::NoSemicolonAfterValue))
-            },
-        }
     }
 
-    pub fn expr_statement(&mut self, chunk: &mut Chunk) -> Result<'src, ()> {
+    fn expr_statement(&mut self, chunk: &mut Chunk) -> Result<'src, ()> {
         self.expression(chunk)?;
-        chunk.add_op(Op::Pop, 0);
+        let pop_line =
-        match self.scanner.next_if(|t| t.ttype == TokenType::Semicolon) {
+            self.must_consume(TokenType::Semicolon, ParseErrorKind::NoSemicolonAfterExpression)
-            Some(_) => Ok(()),
+                .map(|tok| tok.line)?;
-            None => {
+        chunk.add_op(Op::Pop, pop_line);
-                let location = self.scanner.peek().cloned();
+
-                Err(self.error_at_or_end(location, ParseErrorKind::NoSemicolonAfterExpression))
+        Ok(())
-            },
-        }
     }
 
-    pub fn statement(&mut self, chunk: &mut Chunk) -> Result<'src, ()> {
+    fn statement(&mut self, chunk: &mut Chunk) -> Result<'src, ()> {
         match self.scanner.peek().unwrap().ttype {
             TokenType::Print => {
                 let print_token = self.scanner.next().unwrap();
@@ -538,7 +552,7 @@ impl<'src> Parser<'src> {
         }
     }
 
-    pub fn synchronize(&mut self) {
+    fn synchronize(&mut self) {
         use TokenType::*;
 
         while let Some(_token) = self.scanner.next_if(
@@ -546,7 +560,34 @@ impl<'src> Parser<'src> {
         ) {}
     }
 
+    fn var_declaration(&mut self, var_token: Token<'src>, chunk: &mut Chunk) ->  Result<'src, ()> {
+        let ident = self.must_consume(TokenType::Identifier, ParseErrorKind::NoVariableName)?;
+        let offset = self.add_string(chunk, ident.span);
+
+        match self.scanner.peek() {
+            Some(token) if token.ttype == TokenType::Equal => {
+                self.expression(chunk)?;
+            },
+            _ => {
+                chunk.add_op(Op::Nil, ident.line);
+            }
+        }
+
+        chunk.add_op(Op::DefineGlobal { offset }, ident.line);
+
+        self.must_consume(TokenType::Semicolon, ParseErrorKind::NoSemicolonAfterVarDecl)?;
+
+        Ok(())
+    }
+
     pub fn declaration(&mut self, chunk: &mut Chunk) {
+        let peeked = self.scanner.peek().unwrap().clone();
+        match peeked.ttype {
+            TokenType::Var => {
+                self.scanner.next();
+                self.var_declaration(peeked, chunk);
+            },
+            _ => {
                 self.statement(chunk).unwrap_or_else(
                     |err| {
                         self.errors.push(err);
@@ -554,6 +595,8 @@ impl<'src> Parser<'src> {
                     }
                 )
             }
+        }
+    }
 
     pub fn compile(&mut self, chunk: &mut Chunk) {
         while let Some(_) = self.scanner.peek() {
@@ -799,7 +842,7 @@ mod tests {
         use crate::bc::Op::*;
         let expected = Chunk::new_with(
             vec![Constant { offset: 0 }, Constant { offset: 1 }, Add, Print],
-            vec![],
+            vec![1, 1, 1, 1],
             vec![Value::from(1.0), Value::from(1.0)],
             LinkedList::new(),
         );
@@ -810,14 +853,14 @@ mod tests {
     #[test]
     fn basic_print_string_statement() {
         let source = "print \"string\";";
-        let allocation = unsafe { allocate_string("string").unwrap() };
+        let allocation = GC::new_string("string");
         let object = allocation.get_object();
         let mut allocations = LinkedList::new();
         allocations.push_front(allocation);
         use crate::bc::Op::*;
         let expected = Chunk::new_with(
             vec![Constant { offset: 0 }, Print],
-            vec![],
+            vec![1, 1],
             vec![Value::from(object)],
             allocations,
         );
@@ -831,11 +874,40 @@ mod tests {
         use crate::bc::Op::*;
         let expected = Chunk::new_with(
             vec![Constant { offset: 0 }, Constant { offset: 1 }, Divide, Pop],
-            vec![],
+            vec![1, 1, 1, 1],
             vec![Value::from(1.0), Value::from(1.0)],
             LinkedList::new(),
         );
 
         test_parse_program(source, &expected);
     }
+
+    #[test]
+    fn basic_var_decl() {
+        let source = "var x;";
+        use crate::bc::Op::*;
+        let x = GC::new_string("x");
+        let expected = Chunk::new_with(
+            vec![Nil, DefineGlobal { offset: 0 }],
+            vec![1, 1],
+            vec![x.get_object().into()],
+            LinkedList::new(),
+        );
+
+        test_parse_program(source, &expected);
+    }
+
+    fn basic_var_decl_with_initializer() {
+        let source = "var x = 1 + 1;";
+        use crate::bc::Op::*;
+        let x = GC::new_string("x");
+        let expected = Chunk::new_with(
+            vec![Constant {offset: 1}, Constant {offset: 2}, Add, DefineGlobal { offset: 0 }],
+            vec![],
+            vec![x.get_object().into(), Value::from(1.0), Value::from(1.0)],
+            LinkedList::new(),
+        );
+
+        test_parse_program(source, &expected);
+    }
 }

rlox/src/vm.rs

@@ -1,6 +1,6 @@
 use crate::bc::{Chunk, Op, TraceInfo, Value};
-use crate::gc::{concat_string, GcHandle, ObjectType};
+use crate::gc::{GcHandle, ObjString, ObjectType, GC};
-use std::collections::LinkedList;
+use std::collections::{HashMap, LinkedList};
 use std::io;
 use std::rc::Rc;
 
@@ -72,6 +72,7 @@ impl VM {
         output: &mut Output,
     ) -> Result<()> {
         let mut allocations: LinkedList<GcHandle> = LinkedList::new();
+        let mut globals: HashMap<ObjString, Value> = HashMap::new();
 
         while self.pc < chunk.code.len() {
             let instr = chunk.code[self.pc];
@@ -126,7 +127,8 @@ impl VM {
                                 match a {
                                     Value::Obj(a) => match a.get_otype() {
                                         ObjectType::String => {
-                                            let new_obj = unsafe { concat_string(a, b).unwrap() };
+                                            let (a, b) = (a.downcast().unwrap(), b.downcast().unwrap());
+                                            let new_obj = GC::new_concat_string(a, b);
                                             self.push(Value::from(new_obj.get_object()));
                                             allocations.push_front(new_obj);
                                             Ok(())
@@ -178,6 +180,34 @@ impl VM {
                 },
                 Op::Pop => {
                     self.pop()?;
+                },
+                Op::DefineGlobal { offset } => {
+                    let ident = chunk.constants[offset as usize].clone();
+                    if let Value::Obj(name) = ident {
+                        let name = name.downcast::<ObjString>().unwrap();
+                        globals.entry(name).or_insert(self.pop()?);
+                        Ok(())
+                    } else {
+                        unreachable!()
+                    }?
+                },
+                Op::GetGlobal { offset } => {
+                    let ident = match chunk.constants[offset as usize] {
+                        Value::Obj(object) => object.downcast::<ObjString>().unwrap(),
+                        _ => todo!(),
+                    };
+
+                    if let Some(value) = globals.get(&ident) {
+                        self.push(value.clone());
+
+                        Ok(())
+                    } else {
+                        Err(
+                        VMError::Runtime(
+                            format!("Undefined variable '{}'.", ident).into(),
+                            self.pc,
+                        ))
+                    }?
                 }
             }
         }
@@ -190,6 +220,8 @@ impl VM {
 mod tests {
     use std::collections::LinkedList;
 
+    use crate::gc::GC;
+
     use super::{Chunk, Op, VMError, Value, VM};
 
     #[test]
@@ -269,4 +301,29 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn read_write_globals() -> Result<(), VMError> {
+        let var = GC::new_string("global");
+        use Op::*;
+        let chunk = Chunk::new_with(
+            vec![
+                Constant { offset: 0 },
+                DefineGlobal { offset: 1 },
+                Constant { offset: 2 },
+                GetGlobal { offset: 1 },
+                Multiply,
+            ],
+            vec![],
+            vec![Value::from(5.0), Value::from(var.get_object()), Value::from(6.0)],
+            LinkedList::new()
+        );
+
+        let mut vm = VM::new();
+        vm.stdrun(&chunk)?;
+
+        assert_eq!(vm.stack, vec![Value::Number(30.0)]);
+
+        Ok(())
+    }
 }