From 80f06d5293797f2b1a4161d11c08df3d19958cfa Mon Sep 17 00:00:00 2001
From: Kylee Tilley <kyleetilley@gmail.com>
Date: Sat, 30 Dec 2023 23:46:59 -0600
Subject: [PATCH 1/2] Rename StmtAssign to StmtLet to better reflect what it's
 doing

---
 res/grammar.lalrpop | 12 ++++++------
 src/syntax/ast.rs   |  9 ++++-----
 src/vm/compiler.rs  |  2 +-
 3 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/res/grammar.lalrpop b/res/grammar.lalrpop
index 3b7346e..3cd5ac7 100644
--- a/res/grammar.lalrpop
+++ b/res/grammar.lalrpop
@@ -16,7 +16,7 @@ pub Program: ast::Program = <stmts:Spanned<Decl>*> => ast::Program { <> };
 Decl = {
     DeclClass,
     DeclFn,
-    DeclLetVar,
+    DeclLet,
     Stmt,
 }
 
@@ -24,7 +24,7 @@ DeclClass: ast::Stmt = <class:StmtClass> => ast::Stmt::Class(<>);
 
 StmtClass: ast::StmtClass =
     "class" <name:identifier> <super_:("extends" <Spanned<ExprIdentifier>>)?> "{"
-        <fields:(<Spanned<StmtAssign>>)*>
+        <fields:(<Spanned<StmtLet>>)*>
         <methods:(<Spanned<StmtFn>>)*>
     "}" =>
         ast::StmtClass { <> };
@@ -67,10 +67,10 @@ StmtFn: ast::StmtFn = {
     }
 }
 
-DeclLetVar: ast::Stmt = <assign:StmtAssign> =>
-    ast::Stmt::Assign(<>);
+DeclLet: ast::Stmt = <assign:StmtLet> =>
+    ast::Stmt::Let(<>);
 
-StmtAssign: ast::StmtAssign = "let" <name:identifier> <value:("=" <ExprS>)?> ";" => ast::StmtAssign {
+StmtLet: ast::StmtLet = "let" <name:identifier> <value:("=" <ExprS>)?> ";" => ast::StmtLet {
     identifier: ast::Identifier {
         name,
         depth: None
@@ -111,7 +111,7 @@ StmtClosed: ast::Stmt = {
 }
 
 ForInit: Option<ast::StmtS> = {
-    <Spanned<DeclLetVar>> => Some(<>),
+    <Spanned<DeclLet>> => Some(<>),
     <Spanned<StmtExpr>> => Some(<>),
     ";" => None,
 }
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index ecd41be..44fbd27 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -20,7 +20,7 @@ pub enum Stmt {
     If(Box<StmtIf>),
     Print(StmtPrint),
     Return(StmtReturn),
-    Assign(StmtAssign),
+    Let(StmtLet),
     While(Box<StmtWhile>),
     Error,
 }
@@ -36,7 +36,7 @@ impl Debug for Stmt {
             Self::If(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Print(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Return(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
-            Self::Assign(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
+            Self::Let(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::While(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Error => write!(f, "Error"),
         }
@@ -53,7 +53,7 @@ pub struct StmtClass {
     pub name: String,
     pub super_: Option<ExprS>,
     pub methods: Vec<Spanned<StmtFn>>,
-    pub fields: Vec<Spanned<StmtAssign>>,
+    pub fields: Vec<Spanned<StmtLet>>,
 }
 
 /// An expression statement evaluates an expression and discards the result.
@@ -94,9 +94,8 @@ pub struct StmtReturn {
     pub value: Option<ExprS>,
 }
 
-/// Statement that sets `var.name` to `value`
 #[derive(Clone, Debug, PartialEq)]
-pub struct StmtAssign {
+pub struct StmtLet {
     pub identifier: Identifier,
     pub value: Option<ExprS>,
 }
diff --git a/src/vm/compiler.rs b/src/vm/compiler.rs
index f06b9cc..56d95cb 100644
--- a/src/vm/compiler.rs
+++ b/src/vm/compiler.rs
@@ -276,7 +276,7 @@ impl Compiler {
                 }
                 self.emit_u8(op::RETURN, span);
             }
-            Stmt::Assign(assign) => {
+            Stmt::Let(assign) => {
                 let name = &assign.identifier.name;
                 if self.is_global() {
                     let name = gc.alloc(name);

From fc4fd2ee142e919300c26c0c72dc3a9d0c7e2868 Mon Sep 17 00:00:00 2001
From: Kylee Tilley <kyleetilley@gmail.com>
Date: Sun, 31 Dec 2023 14:16:40 -0600
Subject: [PATCH 2/2] Add StmtConst

---
 .../const/collide_with_parameter.locks        |  3 +++
 res/examples/const/duplicate_local.locks      |  4 ++++
 res/examples/const/in_middle_of_block.locks   | 10 ++++++++++
 res/examples/const/in_nested_block.locks      |  6 ++++++
 res/examples/const/local_from_method.locks    |  9 +++++++++
 res/examples/const/redeclare_global.locks     |  2 ++
 res/examples/const/redefine_global.locks      |  2 ++
 .../scope_reuse_in_different_blocks.locks     |  9 +++++++++
 res/examples/const/shadow_and_local.locks     |  7 +++++++
 res/examples/const/shadow_global.locks        |  6 ++++++
 res/examples/const/shadow_local.locks         |  8 ++++++++
 res/examples/const/uninitialized.locks        |  2 ++
 res/examples/const/use_clock_as_var.locks     |  1 +
 res/examples/const/use_false_as_var.locks     |  2 ++
 .../const/use_global_in_initializer.locks     |  3 +++
 .../const/use_local_in_initializer.locks      |  4 ++++
 res/examples/const/use_nil_as_var.locks       |  2 ++
 res/examples/const/use_this_as_var.locks      |  2 ++
 .../collide_with_parameter.locks              |  0
 .../{variable => let}/duplicate_local.locks   |  0
 .../duplicate_parameter.locks                 |  0
 .../{variable => let}/early_bound.locks       |  0
 .../in_middle_of_block.locks                  |  0
 .../{variable => let}/in_nested_block.locks   |  0
 .../{variable => let}/local_from_method.locks |  0
 .../{variable => let}/redeclare_global.locks  |  0
 .../{variable => let}/redefine_global.locks   |  0
 .../scope_reuse_in_different_blocks.locks     |  0
 .../{variable => let}/shadow_and_local.locks  |  0
 .../{variable => let}/shadow_global.locks     |  0
 .../{variable => let}/shadow_local.locks      |  0
 .../{variable => let}/undefined_global.locks  |  0
 .../{variable => let}/undefined_local.locks   |  0
 .../{variable => let}/uninitialized.locks     |  0
 .../unreached_undefined.locks                 |  0
 .../{variable => let}/use_clock_as_var.locks  |  0
 .../{variable => let}/use_false_as_var.locks  |  0
 .../use_global_in_initializer.locks           |  0
 .../use_local_in_initializer.locks            |  0
 .../{variable => let}/use_nil_as_var.locks    |  0
 .../{variable => let}/use_this_as_var.locks   |  0
 res/grammar.lalrpop                           | 13 +++++++++++++
 src/syntax/ast.rs                             |  8 ++++++++
 src/syntax/lexer.rs                           |  3 ++-
 src/syntax/mod.rs                             |  8 ++++++--
 src/vm/compiler.rs                            | 19 +++++++++++++++++++
 46 files changed, 130 insertions(+), 3 deletions(-)
 create mode 100644 res/examples/const/collide_with_parameter.locks
 create mode 100644 res/examples/const/duplicate_local.locks
 create mode 100644 res/examples/const/in_middle_of_block.locks
 create mode 100644 res/examples/const/in_nested_block.locks
 create mode 100644 res/examples/const/local_from_method.locks
 create mode 100644 res/examples/const/redeclare_global.locks
 create mode 100644 res/examples/const/redefine_global.locks
 create mode 100644 res/examples/const/scope_reuse_in_different_blocks.locks
 create mode 100644 res/examples/const/shadow_and_local.locks
 create mode 100644 res/examples/const/shadow_global.locks
 create mode 100644 res/examples/const/shadow_local.locks
 create mode 100644 res/examples/const/uninitialized.locks
 create mode 100644 res/examples/const/use_clock_as_var.locks
 create mode 100644 res/examples/const/use_false_as_var.locks
 create mode 100644 res/examples/const/use_global_in_initializer.locks
 create mode 100644 res/examples/const/use_local_in_initializer.locks
 create mode 100644 res/examples/const/use_nil_as_var.locks
 create mode 100644 res/examples/const/use_this_as_var.locks
 rename res/examples/{variable => let}/collide_with_parameter.locks (100%)
 rename res/examples/{variable => let}/duplicate_local.locks (100%)
 rename res/examples/{variable => let}/duplicate_parameter.locks (100%)
 rename res/examples/{variable => let}/early_bound.locks (100%)
 rename res/examples/{variable => let}/in_middle_of_block.locks (100%)
 rename res/examples/{variable => let}/in_nested_block.locks (100%)
 rename res/examples/{variable => let}/local_from_method.locks (100%)
 rename res/examples/{variable => let}/redeclare_global.locks (100%)
 rename res/examples/{variable => let}/redefine_global.locks (100%)
 rename res/examples/{variable => let}/scope_reuse_in_different_blocks.locks (100%)
 rename res/examples/{variable => let}/shadow_and_local.locks (100%)
 rename res/examples/{variable => let}/shadow_global.locks (100%)
 rename res/examples/{variable => let}/shadow_local.locks (100%)
 rename res/examples/{variable => let}/undefined_global.locks (100%)
 rename res/examples/{variable => let}/undefined_local.locks (100%)
 rename res/examples/{variable => let}/uninitialized.locks (100%)
 rename res/examples/{variable => let}/unreached_undefined.locks (100%)
 rename res/examples/{variable => let}/use_clock_as_var.locks (100%)
 rename res/examples/{variable => let}/use_false_as_var.locks (100%)
 rename res/examples/{variable => let}/use_global_in_initializer.locks (100%)
 rename res/examples/{variable => let}/use_local_in_initializer.locks (100%)
 rename res/examples/{variable => let}/use_nil_as_var.locks (100%)
 rename res/examples/{variable => let}/use_this_as_var.locks (100%)

diff --git a/res/examples/const/collide_with_parameter.locks b/res/examples/const/collide_with_parameter.locks
new file mode 100644
index 0000000..4a5800a
--- /dev/null
+++ b/res/examples/const/collide_with_parameter.locks
@@ -0,0 +1,3 @@
+fn foo(a) {
+  const a = 123; // out: NameError: name "a" is already defined
+}
diff --git a/res/examples/const/duplicate_local.locks b/res/examples/const/duplicate_local.locks
new file mode 100644
index 0000000..b88bc55
--- /dev/null
+++ b/res/examples/const/duplicate_local.locks
@@ -0,0 +1,4 @@
+{
+  const a = "value";
+  const a = "other"; // out: NameError: name "a" is already defined
+}
diff --git a/res/examples/const/in_middle_of_block.locks b/res/examples/const/in_middle_of_block.locks
new file mode 100644
index 0000000..0e5f33d
--- /dev/null
+++ b/res/examples/const/in_middle_of_block.locks
@@ -0,0 +1,10 @@
+{
+  const a = "a";
+  print a; // out: a
+  const b = a + " b";
+  print b; // out: a b
+  const c = a + " c";
+  print c; // out: a c
+  const d = b + " d";
+  print d; // out: a b d
+}
diff --git a/res/examples/const/in_nested_block.locks b/res/examples/const/in_nested_block.locks
new file mode 100644
index 0000000..8808d2c
--- /dev/null
+++ b/res/examples/const/in_nested_block.locks
@@ -0,0 +1,6 @@
+{
+  const a = "outer";
+  {
+    print a; // out: outer
+  }
+}
diff --git a/res/examples/const/local_from_method.locks b/res/examples/const/local_from_method.locks
new file mode 100644
index 0000000..d12d1ec
--- /dev/null
+++ b/res/examples/const/local_from_method.locks
@@ -0,0 +1,9 @@
+const foo = "variable";
+
+class Foo {
+  fn method() {
+    print foo;
+  }
+}
+
+Foo().method(); // out: variable
diff --git a/res/examples/const/redeclare_global.locks b/res/examples/const/redeclare_global.locks
new file mode 100644
index 0000000..b967044
--- /dev/null
+++ b/res/examples/const/redeclare_global.locks
@@ -0,0 +1,2 @@
+const a = "1";
+const a; // out: NameError: name "a" is already defined
diff --git a/res/examples/const/redefine_global.locks b/res/examples/const/redefine_global.locks
new file mode 100644
index 0000000..e2929a4
--- /dev/null
+++ b/res/examples/const/redefine_global.locks
@@ -0,0 +1,2 @@
+const a = "1";
+const a = "2";  // out: NameError: name "a" is already defined
diff --git a/res/examples/const/scope_reuse_in_different_blocks.locks b/res/examples/const/scope_reuse_in_different_blocks.locks
new file mode 100644
index 0000000..f583b23
--- /dev/null
+++ b/res/examples/const/scope_reuse_in_different_blocks.locks
@@ -0,0 +1,9 @@
+{
+  const a = "first";
+  print a; // out: first
+}
+
+{
+  const a = "second";
+  print a; // out: second
+}
diff --git a/res/examples/const/shadow_and_local.locks b/res/examples/const/shadow_and_local.locks
new file mode 100644
index 0000000..29915ef
--- /dev/null
+++ b/res/examples/const/shadow_and_local.locks
@@ -0,0 +1,7 @@
+{
+  const a = "outer";
+  {
+    print a; // out: outer
+    let a = "inner"; // out: NameError: name "a" is already defined
+  }
+}
diff --git a/res/examples/const/shadow_global.locks b/res/examples/const/shadow_global.locks
new file mode 100644
index 0000000..a7f22cc
--- /dev/null
+++ b/res/examples/const/shadow_global.locks
@@ -0,0 +1,6 @@
+const a = "global";
+{
+  let a = "shadow"; // out: NameError: name "a" is already defined
+  print a;
+}
+print a;
diff --git a/res/examples/const/shadow_local.locks b/res/examples/const/shadow_local.locks
new file mode 100644
index 0000000..ee6f280
--- /dev/null
+++ b/res/examples/const/shadow_local.locks
@@ -0,0 +1,8 @@
+{
+  const a = "local";
+  {
+    let a = "shadow"; // out: NameError: name "a" is already defined
+    print a;
+  }
+  print a;
+}
diff --git a/res/examples/const/uninitialized.locks b/res/examples/const/uninitialized.locks
new file mode 100644
index 0000000..1390f78
--- /dev/null
+++ b/res/examples/const/uninitialized.locks
@@ -0,0 +1,2 @@
+const a; // out: Error: const must be initialized with a value
+print a;
diff --git a/res/examples/const/use_clock_as_var.locks b/res/examples/const/use_clock_as_var.locks
new file mode 100644
index 0000000..90ed20c
--- /dev/null
+++ b/res/examples/const/use_clock_as_var.locks
@@ -0,0 +1 @@
+const clock = "new clock"; // out: NameError: name "clock" is already defined
diff --git a/res/examples/const/use_false_as_var.locks b/res/examples/const/use_false_as_var.locks
new file mode 100644
index 0000000..bab0a63
--- /dev/null
+++ b/res/examples/const/use_false_as_var.locks
@@ -0,0 +1,2 @@
+// out: SyntaxError: unexpected "false"
+const false = "value";
diff --git a/res/examples/const/use_global_in_initializer.locks b/res/examples/const/use_global_in_initializer.locks
new file mode 100644
index 0000000..4002e40
--- /dev/null
+++ b/res/examples/const/use_global_in_initializer.locks
@@ -0,0 +1,3 @@
+const a = "value";
+const a = a; // out: NameError: name "a" is already defined
+print a;
diff --git a/res/examples/const/use_local_in_initializer.locks b/res/examples/const/use_local_in_initializer.locks
new file mode 100644
index 0000000..da3dc9b
--- /dev/null
+++ b/res/examples/const/use_local_in_initializer.locks
@@ -0,0 +1,4 @@
+{
+  // out: NameError: cannot access variable "a" in its own initializer
+  const a = a;
+}
diff --git a/res/examples/const/use_nil_as_var.locks b/res/examples/const/use_nil_as_var.locks
new file mode 100644
index 0000000..c5ec8d2
--- /dev/null
+++ b/res/examples/const/use_nil_as_var.locks
@@ -0,0 +1,2 @@
+// out: SyntaxError: unexpected "nil"
+const nil = "value";
diff --git a/res/examples/const/use_this_as_var.locks b/res/examples/const/use_this_as_var.locks
new file mode 100644
index 0000000..65e73ef
--- /dev/null
+++ b/res/examples/const/use_this_as_var.locks
@@ -0,0 +1,2 @@
+// out: SyntaxError: unexpected "this"
+const this = "value";
diff --git a/res/examples/variable/collide_with_parameter.locks b/res/examples/let/collide_with_parameter.locks
similarity index 100%
rename from res/examples/variable/collide_with_parameter.locks
rename to res/examples/let/collide_with_parameter.locks
diff --git a/res/examples/variable/duplicate_local.locks b/res/examples/let/duplicate_local.locks
similarity index 100%
rename from res/examples/variable/duplicate_local.locks
rename to res/examples/let/duplicate_local.locks
diff --git a/res/examples/variable/duplicate_parameter.locks b/res/examples/let/duplicate_parameter.locks
similarity index 100%
rename from res/examples/variable/duplicate_parameter.locks
rename to res/examples/let/duplicate_parameter.locks
diff --git a/res/examples/variable/early_bound.locks b/res/examples/let/early_bound.locks
similarity index 100%
rename from res/examples/variable/early_bound.locks
rename to res/examples/let/early_bound.locks
diff --git a/res/examples/variable/in_middle_of_block.locks b/res/examples/let/in_middle_of_block.locks
similarity index 100%
rename from res/examples/variable/in_middle_of_block.locks
rename to res/examples/let/in_middle_of_block.locks
diff --git a/res/examples/variable/in_nested_block.locks b/res/examples/let/in_nested_block.locks
similarity index 100%
rename from res/examples/variable/in_nested_block.locks
rename to res/examples/let/in_nested_block.locks
diff --git a/res/examples/variable/local_from_method.locks b/res/examples/let/local_from_method.locks
similarity index 100%
rename from res/examples/variable/local_from_method.locks
rename to res/examples/let/local_from_method.locks
diff --git a/res/examples/variable/redeclare_global.locks b/res/examples/let/redeclare_global.locks
similarity index 100%
rename from res/examples/variable/redeclare_global.locks
rename to res/examples/let/redeclare_global.locks
diff --git a/res/examples/variable/redefine_global.locks b/res/examples/let/redefine_global.locks
similarity index 100%
rename from res/examples/variable/redefine_global.locks
rename to res/examples/let/redefine_global.locks
diff --git a/res/examples/variable/scope_reuse_in_different_blocks.locks b/res/examples/let/scope_reuse_in_different_blocks.locks
similarity index 100%
rename from res/examples/variable/scope_reuse_in_different_blocks.locks
rename to res/examples/let/scope_reuse_in_different_blocks.locks
diff --git a/res/examples/variable/shadow_and_local.locks b/res/examples/let/shadow_and_local.locks
similarity index 100%
rename from res/examples/variable/shadow_and_local.locks
rename to res/examples/let/shadow_and_local.locks
diff --git a/res/examples/variable/shadow_global.locks b/res/examples/let/shadow_global.locks
similarity index 100%
rename from res/examples/variable/shadow_global.locks
rename to res/examples/let/shadow_global.locks
diff --git a/res/examples/variable/shadow_local.locks b/res/examples/let/shadow_local.locks
similarity index 100%
rename from res/examples/variable/shadow_local.locks
rename to res/examples/let/shadow_local.locks
diff --git a/res/examples/variable/undefined_global.locks b/res/examples/let/undefined_global.locks
similarity index 100%
rename from res/examples/variable/undefined_global.locks
rename to res/examples/let/undefined_global.locks
diff --git a/res/examples/variable/undefined_local.locks b/res/examples/let/undefined_local.locks
similarity index 100%
rename from res/examples/variable/undefined_local.locks
rename to res/examples/let/undefined_local.locks
diff --git a/res/examples/variable/uninitialized.locks b/res/examples/let/uninitialized.locks
similarity index 100%
rename from res/examples/variable/uninitialized.locks
rename to res/examples/let/uninitialized.locks
diff --git a/res/examples/variable/unreached_undefined.locks b/res/examples/let/unreached_undefined.locks
similarity index 100%
rename from res/examples/variable/unreached_undefined.locks
rename to res/examples/let/unreached_undefined.locks
diff --git a/res/examples/variable/use_clock_as_var.locks b/res/examples/let/use_clock_as_var.locks
similarity index 100%
rename from res/examples/variable/use_clock_as_var.locks
rename to res/examples/let/use_clock_as_var.locks
diff --git a/res/examples/variable/use_false_as_var.locks b/res/examples/let/use_false_as_var.locks
similarity index 100%
rename from res/examples/variable/use_false_as_var.locks
rename to res/examples/let/use_false_as_var.locks
diff --git a/res/examples/variable/use_global_in_initializer.locks b/res/examples/let/use_global_in_initializer.locks
similarity index 100%
rename from res/examples/variable/use_global_in_initializer.locks
rename to res/examples/let/use_global_in_initializer.locks
diff --git a/res/examples/variable/use_local_in_initializer.locks b/res/examples/let/use_local_in_initializer.locks
similarity index 100%
rename from res/examples/variable/use_local_in_initializer.locks
rename to res/examples/let/use_local_in_initializer.locks
diff --git a/res/examples/variable/use_nil_as_var.locks b/res/examples/let/use_nil_as_var.locks
similarity index 100%
rename from res/examples/variable/use_nil_as_var.locks
rename to res/examples/let/use_nil_as_var.locks
diff --git a/res/examples/variable/use_this_as_var.locks b/res/examples/let/use_this_as_var.locks
similarity index 100%
rename from res/examples/variable/use_this_as_var.locks
rename to res/examples/let/use_this_as_var.locks
diff --git a/res/grammar.lalrpop b/res/grammar.lalrpop
index 3cd5ac7..c33302b 100644
--- a/res/grammar.lalrpop
+++ b/res/grammar.lalrpop
@@ -17,6 +17,7 @@ Decl = {
     DeclClass,
     DeclFn,
     DeclLet,
+    DeclConst,
     Stmt,
 }
 
@@ -67,6 +68,17 @@ StmtFn: ast::StmtFn = {
     }
 }
 
+DeclConst: ast::Stmt = <assign:StmtConst> =>
+    ast::Stmt::Const(<>);
+
+StmtConst: ast::StmtConst = "const" <name:identifier> <value:("=" <ExprS>)?> ";" => ast::StmtConst {
+    identifier: ast::Identifier {
+        name,
+        depth: None
+    },
+    value,
+};
+
 DeclLet: ast::Stmt = <assign:StmtLet> =>
     ast::Stmt::Let(<>);
 
@@ -365,6 +377,7 @@ extern {
         "this" => lexer::Token::This,
         "true" => lexer::Token::True,
         "let" => lexer::Token::Let,
+        "const" => lexer::Token::Const,
         "while" => lexer::Token::While,
         "extends" => lexer::Token::Extends,
     }
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index 44fbd27..ac2f7d8 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -21,6 +21,7 @@ pub enum Stmt {
     Print(StmtPrint),
     Return(StmtReturn),
     Let(StmtLet),
+    Const(StmtConst),
     While(Box<StmtWhile>),
     Error,
 }
@@ -37,6 +38,7 @@ impl Debug for Stmt {
             Self::Print(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Return(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Let(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
+            Self::Const(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::While(arg0) => f.write_fmt(format_args!("{:#?}", arg0)),
             Self::Error => write!(f, "Error"),
         }
@@ -100,6 +102,12 @@ pub struct StmtLet {
     pub value: Option<ExprS>,
 }
 
+#[derive(Clone, Debug, PartialEq)]
+pub struct StmtConst {
+    pub identifier: Identifier,
+    pub value: Option<ExprS>,
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub struct StmtWhile {
     pub cond: ExprS,
diff --git a/src/syntax/lexer.rs b/src/syntax/lexer.rs
index e26137c..852b791 100644
--- a/src/syntax/lexer.rs
+++ b/src/syntax/lexer.rs
@@ -146,8 +146,9 @@ pub enum Token {
     This,
     #[token("true")]
     True,
-    #[token("let")]
     Let,
+    #[token("const")]
+    Const,
     #[token("while")]
     While,
     #[token("extends")]
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 9aa3120..eefdb3f 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -49,7 +49,7 @@ pub fn parse(source: &str, offset: usize) -> Result<Program, Vec<ErrorS>> {
         }
         ParseError::UnrecognizedToken { token: (start, _, end), expected } => (
             Error::SyntaxError(SyntaxError::UnrecognizedToken {
-                token: source[start..end].to_string(),
+                token: source[start - offset..end - offset].to_string(),
                 expected,
             }),
             start..end,
@@ -57,5 +57,9 @@ pub fn parse(source: &str, offset: usize) -> Result<Program, Vec<ErrorS>> {
         ParseError::User { error } => error,
     }));
 
-    if errors.is_empty() { Ok(program) } else { Err(errors) }
+    if errors.is_empty() {
+        Ok(program)
+    } else {
+        Err(errors)
+    }
 }
diff --git a/src/vm/compiler.rs b/src/vm/compiler.rs
index 56d95cb..b685ff5 100644
--- a/src/vm/compiler.rs
+++ b/src/vm/compiler.rs
@@ -295,6 +295,25 @@ impl Compiler {
                     self.define_local();
                 }
             }
+            Stmt::Const(assign) => {
+                let name = &assign.identifier.name;
+                if self.is_global() {
+                    let name = gc.alloc(name);
+                    match &assign.value {
+                        Some(value) => self.compile_expr(value, gc)?,
+                        None => self.emit_u8(op::NIL, span),
+                    }
+                    self.emit_u8(op::DEFINE_GLOBAL, span);
+                    self.emit_constant(name.into(), span)?;
+                } else {
+                    self.declare_local(name, span)?;
+                    match &assign.value {
+                        Some(value) => self.compile_expr(value, gc)?,
+                        None => self.emit_u8(op::NIL, span),
+                    }
+                    self.define_local();
+                }
+            }
             Stmt::While(while_) => {
                 // START:
                 let loop_start = self.start_loop();