diff --git a/code/compiler/12/ast.hpp b/code/compiler/12/ast.hpp
index 68082ab..d3887bd 100644
--- a/code/compiler/12/ast.hpp
+++ b/code/compiler/12/ast.hpp
@@ -125,8 +125,8 @@ struct ast_let : public ast {
     definition_group definitions;
     ast_ptr in;
 
-    ast_let(ast_ptr i, definition_group g)
-        : in(std::move(i)), definitions(std::move(g)) {}
+    ast_let(definition_group g, ast_ptr i)
+        : definitions(std::move(g)), in(std::move(i)) {}
 
     void print(int indent, std::ostream& to) const;
     void find_free(type_mgr& mgr, type_env_ptr& env, std::set<std::string>& into);
diff --git a/code/compiler/12/parser.y b/code/compiler/12/parser.y
index 5256fae..bae7ad0 100644
--- a/code/compiler/12/parser.y
+++ b/code/compiler/12/parser.y
@@ -22,6 +22,8 @@ extern yy::parser::symbol_type yylex();
 %token DATA
 %token CASE
 %token OF
+%token LET
+%token IN
 %token OCURLY
 %token CCURLY
 %token OPAREN
@@ -40,8 +42,9 @@ extern yy::parser::symbol_type yylex();
 %type <std::vector<branch_ptr>> branches
 %type <std::vector<constructor_ptr>> constructors
 %type <std::vector<parsed_type_ptr>> typeList
+%type <definition_group> definitions
 %type <parsed_type_ptr> type nonArrowType typeListElement
-%type <ast_ptr> aAdd aMul case app appBase
+%type <ast_ptr> aAdd aMul case let app appBase
 %type <definition_data_ptr> data 
 %type <definition_defn_ptr> defn
 %type <branch_ptr> branch
@@ -53,17 +56,13 @@ extern yy::parser::symbol_type yylex();
 %%
 
 program
-    : definitions { }
+    : definitions { global_defs = std::move($1); }
     ;
 
 definitions
-    : definitions definition { }
-    | definition { }
-    ;
-
-definition
-    : defn { auto name = $1->name; global_defs.defs_defn[name] = std::move($1); }
-    | data { auto name = $1->name; global_defs.defs_data[name] = std::move($1); }
+    : definitions defn { $$ = std::move($1); auto name = $2->name; $$.defs_defn[name] = std::move($2); }
+    | definitions data { $$ = std::move($1); auto name = $2->name; $$.defs_data[name] = std::move($2); }
+    | %empty { $$ = definition_group(); }
     ;
 
 defn
@@ -100,6 +99,12 @@ appBase
     | UID { $$ = ast_ptr(new ast_uid(std::move($1))); }
     | OPAREN aAdd CPAREN { $$ = std::move($2); }
     | case { $$ = std::move($1); }
+    | let { $$ = std::move($1); }
+    ;
+
+let
+    : LET OCURLY definitions CCURLY IN OCURLY aAdd CCURLY
+        { $$ = ast_ptr(new ast_let(std::move($3), std::move($7))); }
     ;
 
 case
diff --git a/code/compiler/12/scanner.l b/code/compiler/12/scanner.l
index c8a4429..c417de1 100644
--- a/code/compiler/12/scanner.l
+++ b/code/compiler/12/scanner.l
@@ -22,6 +22,8 @@ defn { return yy::parser::make_DEFN(); }
 data { return yy::parser::make_DATA(); }
 case { return yy::parser::make_CASE(); }
 of { return yy::parser::make_OF(); }
+let { return yy::parser::make_LET(); }
+in { return yy::parser::make_IN(); }
 \{ { return yy::parser::make_OCURLY(); }
 \} { return yy::parser::make_CCURLY(); }
 \( { return yy::parser::make_OPAREN(); }