diff --git a/src/02_syntax/expression.zig b/src/02_syntax/expression.zig
new file mode 100644
index 0000000..34a7cfa
--- /dev/null
+++ b/src/02_syntax/expression.zig
@@ -0,0 +1,47 @@
+const std = @import("std");
+const lexic = @import("lexic");
+const Token = lexic.Token;
+const TokenType = lexic.TokenType;
+const ParseError = @import("./types.zig").ParseError;
+
+const Expression = union(enum) {
+    number: *const Token,
+
+    /// Attempts to parse an expression from a token stream.
+    fn parse(tokens: *const std.ArrayList(Token), pos: usize) ParseError!@This() {
+        std.debug.assert(pos < tokens.items.len);
+
+        const t = tokens.items[pos];
+        if (t.token_type != TokenType.Int) {
+            return ParseError.Unmatched;
+        }
+
+        return .{
+            .number = &t,
+        };
+    }
+};
+
+test "should parse expression" {
+    const input = "322";
+    const tokens = try lexic.tokenize(input, std.testing.allocator);
+    defer tokens.deinit();
+
+    const expr = try Expression.parse(&tokens, 0);
+    try std.testing.expectEqualDeep("322", expr.number.value);
+    try std.testing.expectEqualDeep(TokenType.Int, expr.number.token_type);
+}
+
+test "should fail on non expression" {
+    const input = "identifier";
+    const tokens = try lexic.tokenize(input, std.testing.allocator);
+    defer tokens.deinit();
+
+    const expr = Expression.parse(&tokens, 0) catch |err| {
+        try std.testing.expectEqual(ParseError.Unmatched, err);
+        return;
+    };
+
+    std.debug.print("v: {s}", .{expr.number.value});
+    try std.testing.expect(false);
+}
diff --git a/src/02_syntax/root.zig b/src/02_syntax/root.zig
index deffc7e..67a2728 100644
--- a/src/02_syntax/root.zig
+++ b/src/02_syntax/root.zig
@@ -1,14 +1,9 @@
 const std = @import("std");
 const lexic = @import("lexic");
+const expression = @import("./expression.zig");
 const Token = lexic.Token;
 const TokenType = lexic.TokenType;
-
-const TokenStream = std.ArrayList(Token);
-
-const ParseError = error{
-    Unmatched,
-    Error,
-};
+const ParseError = @import("./types.zig").ParseError;
 
 const Statement = union(enum) {
     VariableBinding: u8,
@@ -18,39 +13,11 @@ const VariableBinding = struct {
     is_mutable: bool,
     datatype: ?*Token,
     identifier: *Token,
-    expression: Expression,
+    expression: expression.Expression,
 
     fn parse() !@This() {}
 };
 
-const Expression = union(enum) {
-    number: *const Token,
-
-    /// Attempts to parse an expression from a token stream.
-    fn parse(tokens: *const TokenStream, pos: usize) ParseError!@This() {
-        std.debug.assert(pos < tokens.items.len);
-
-        const t = tokens.items[pos];
-        if (t.token_type != TokenType.Int) {
-            return ParseError.Unmatched;
-        }
-
-        return .{
-            .number = &t,
-        };
-    }
-};
-
 test {
     std.testing.refAllDecls(@This());
 }
-
-test "should parse expression" {
-    const input = "322";
-    const tokens = try lexic.tokenize(input, std.testing.allocator);
-    defer tokens.deinit();
-
-    const expr = try Expression.parse(&tokens, 0);
-    try std.testing.expectEqualDeep("322", expr.number.value);
-    try std.testing.expectEqualDeep(TokenType.Int, expr.number.token_type);
-}
diff --git a/src/02_syntax/types.zig b/src/02_syntax/types.zig
new file mode 100644
index 0000000..d0e469f
--- /dev/null
+++ b/src/02_syntax/types.zig
@@ -0,0 +1,4 @@
+pub const ParseError = error{
+    Unmatched,
+    Error,
+};