From f63d892662d02c92591119e0800847276d93634f Mon Sep 17 00:00:00 2001
From: Sam Vervaeck <samvv@pm.me>
Date: Sat, 3 Jun 2023 14:35:02 +0200
Subject: [PATCH] Re-introduce let declarations

Closes #42
---
 include/bolt/CST.hpp        | 121 ++++++++++++++-----------------
 include/bolt/CSTVisitor.hpp |  43 ++++-------
 include/bolt/Checker.hpp    |   4 +-
 include/bolt/Parser.hpp     |   4 +-
 include/bolt/Type.hpp       |   1 -
 src/CST.cc                  |  47 ++++--------
 src/Checker.cc              | 108 +++++++++++++++-------------
 src/Diagnostics.cc          |  10 ++-
 src/Parser.cc               | 138 +++++++-----------------------------
 src/Scanner.cc              |   6 +-
 10 files changed, 173 insertions(+), 309 deletions(-)

diff --git a/include/bolt/CST.hpp b/include/bolt/CST.hpp
index 8f3582a79..ef94e9179 100644
--- a/include/bolt/CST.hpp
+++ b/include/bolt/CST.hpp
@@ -104,9 +104,9 @@ namespace bolt {
     RArrow,
     RArrowAlt,
     LetKeyword,
-    FnKeyword,
     MutKeyword,
     PubKeyword,
+    ForeignKeyword,
     TypeKeyword,
     ReturnKeyword,
     ModKeyword,
@@ -164,8 +164,7 @@ namespace bolt {
     Parameter,
     LetBlockBody,
     LetExprBody,
-    FunctionDeclaration,
-    VariableDeclaration,
+    LetDeclaration,
     RecordDeclarationField,
     RecordDeclaration,
     VariantDeclaration,
@@ -549,20 +548,6 @@ namespace bolt {
 
   };
 
-  class FnKeyword : public Token {
-  public:
-
-    inline FnKeyword(TextLoc StartLoc):
-      Token(NodeKind::FnKeyword, StartLoc) {}
-
-    std::string getText() const override;
-
-    static bool classof(const Node* N) {
-      return N->getKind() == NodeKind::FnKeyword;
-    }
-
-  };
-
   class MutKeyword : public Token {
   public:
 
@@ -591,6 +576,20 @@ namespace bolt {
 
   };
 
+  class ForeignKeyword : public Token {
+  public:
+
+    inline ForeignKeyword(TextLoc StartLoc):
+      Token(NodeKind::ForeignKeyword, StartLoc) {}
+
+    std::string getText() const override;
+
+    static bool classof(const Node* N) {
+      return N->getKind() == NodeKind::ForeignKeyword;
+    }
+
+  };
+
   class TypeKeyword : public Token {
   public:
 
@@ -1718,7 +1717,7 @@ namespace bolt {
 
   };
 
-  class FunctionDeclaration : public Node {
+  class LetDeclaration : public TypedNode {
 
     Scope* TheScope = nullptr;
 
@@ -1726,26 +1725,31 @@ namespace bolt {
 
     bool IsCycleActive = false;
     InferContext* Ctx;
-    class Type* Ty;
 
     class PubKeyword* PubKeyword;
-    class FnKeyword* FnKeyword;
-    class Identifier* Name;
+    class ForeignKeyword* ForeignKeyword;
+    class LetKeyword* LetKeyword;
+    class MutKeyword* MutKeyword;
+    class Pattern* Pattern;
     std::vector<Parameter*> Params;
     class TypeAssert* TypeAssert;
     LetBody* Body;
 
-    FunctionDeclaration(
+    LetDeclaration(
       class PubKeyword* PubKeyword,
-      class FnKeyword* FnKeyword,
-      class Identifier* Name,
+      class ForeignKeyword* ForeignKeyword,
+      class LetKeyword* LetKeyword,
+      class MutKeyword* MutKeyword,
+      class Pattern* Pattern,
       std::vector<Parameter*> Params,
       class TypeAssert* TypeAssert,
       LetBody* Body
-    ): Node(NodeKind::FunctionDeclaration),
+    ): TypedNode(NodeKind::LetDeclaration),
        PubKeyword(PubKeyword),
-       FnKeyword(FnKeyword),
-       Name(Name),
+       ForeignKeyword(ForeignKeyword),
+       LetKeyword(LetKeyword),
+       MutKeyword(MutKeyword),
+       Pattern(Pattern),
        Params(Params),
        TypeAssert(TypeAssert),
        Body(Body) {}
@@ -1766,54 +1770,36 @@ namespace bolt {
     }
 
     bool isSignature() const noexcept {
-      return !Body;
+      return ForeignKeyword;
+    }
+
+    bool isVariable() const noexcept {
+      // Variables in classes and instances are never possible, so we reflect this by excluding them here.
+      return !isSignature() && !isClass() && !isInstance() && (Pattern->getKind() != NodeKind::BindPattern || !Body);
+    }
+
+    bool isFunction() const noexcept {
+      return !isSignature() && !isVariable();
+    }
+
+    Identifier* getName() const noexcept {
+      ZEN_ASSERT(Pattern->getKind() == NodeKind::BindPattern);
+      return static_cast<BindPattern*>(Pattern)->Name;
+    }
+
+    ByteString getNameAsString() const noexcept {
+      return getName()->getCanonicalText();
     }
 
     Token* getFirstToken() const override;
     Token* getLastToken() const override;
 
     static bool classof(const Node* N) {
-      return N->getKind() == NodeKind::FunctionDeclaration;
+      return N->getKind() == NodeKind::LetDeclaration;
     }
 
   };
 
-  class VariableDeclaration : public TypedNode {
-
-    Scope* TheScope = nullptr;
-
-  public:
-
-    bool IsCycleActive = false;
-
-    class PubKeyword* PubKeyword;
-    class LetKeyword* LetKeyword;
-    class MutKeyword* MutKeyword;
-    class Pattern* Pattern;
-    std::vector<Parameter*> Params;
-    class TypeAssert* TypeAssert;
-    LetBody* Body;
-
-    VariableDeclaration(
-      class PubKeyword* PubKeyword,
-      class LetKeyword* LetKeyword,
-      class MutKeyword* MutKeyword,
-      class Pattern* Pattern,
-      class TypeAssert* TypeAssert,
-      LetBody* Body
-    ): TypedNode(NodeKind::VariableDeclaration),
-       PubKeyword(PubKeyword),
-       LetKeyword(LetKeyword),
-       MutKeyword(MutKeyword),
-       Pattern(Pattern),
-       TypeAssert(TypeAssert),
-       Body(Body) {}
-
-    Token* getFirstToken() const override;
-    Token* getLastToken() const override;
-
-  };
-
   class InstanceDeclaration : public Node {
   public:
 
@@ -2063,7 +2049,7 @@ namespace bolt {
   template<> inline NodeKind getNodeType<RArrow>() { return NodeKind::RArrow; }
   template<> inline NodeKind getNodeType<RArrowAlt>() { return NodeKind::RArrowAlt; }
   template<> inline NodeKind getNodeType<LetKeyword>() { return NodeKind::LetKeyword; }
-  template<> inline NodeKind getNodeType<FnKeyword>() { return NodeKind::FnKeyword; }
+  template<> inline NodeKind getNodeType<ForeignKeyword>() { return NodeKind::ForeignKeyword; }
   template<> inline NodeKind getNodeType<MutKeyword>() { return NodeKind::MutKeyword; }
   template<> inline NodeKind getNodeType<PubKeyword>() { return NodeKind::PubKeyword; }
   template<> inline NodeKind getNodeType<TypeKeyword>() { return NodeKind::TypeKeyword; }
@@ -2106,8 +2092,7 @@ namespace bolt {
   template<> inline NodeKind getNodeType<Parameter>() { return NodeKind::Parameter; }
   template<> inline NodeKind getNodeType<LetBlockBody>() { return NodeKind::LetBlockBody; }
   template<> inline NodeKind getNodeType<LetExprBody>() { return NodeKind::LetExprBody; }
-  template<> inline NodeKind getNodeType<FunctionDeclaration>() { return NodeKind::FunctionDeclaration; }
-  template<> inline NodeKind getNodeType<VariableDeclaration>() { return NodeKind::VariableDeclaration; }
+  template<> inline NodeKind getNodeType<LetDeclaration>() { return NodeKind::LetDeclaration; }
   template<> inline NodeKind getNodeType<RecordDeclarationField>() { return NodeKind::RecordDeclarationField; }
   template<> inline NodeKind getNodeType<RecordDeclaration>() { return NodeKind::RecordDeclaration; }
   template<> inline NodeKind getNodeType<ClassDeclaration>() { return NodeKind::ClassDeclaration; }
diff --git a/include/bolt/CSTVisitor.hpp b/include/bolt/CSTVisitor.hpp
index c45015643..feee600d0 100644
--- a/include/bolt/CSTVisitor.hpp
+++ b/include/bolt/CSTVisitor.hpp
@@ -33,7 +33,7 @@ namespace bolt {
         BOLT_GEN_CASE(RArrow)
         BOLT_GEN_CASE(RArrowAlt)
         BOLT_GEN_CASE(LetKeyword)
-        BOLT_GEN_CASE(FnKeyword)
+        BOLT_GEN_CASE(ForeignKeyword)
         BOLT_GEN_CASE(MutKeyword)
         BOLT_GEN_CASE(PubKeyword)
         BOLT_GEN_CASE(TypeKeyword)
@@ -93,8 +93,7 @@ namespace bolt {
         BOLT_GEN_CASE(Parameter)
         BOLT_GEN_CASE(LetBlockBody)
         BOLT_GEN_CASE(LetExprBody)
-        BOLT_GEN_CASE(FunctionDeclaration)
-        BOLT_GEN_CASE(VariableDeclaration)
+        BOLT_GEN_CASE(LetDeclaration)
         BOLT_GEN_CASE(RecordDeclaration)
         BOLT_GEN_CASE(RecordDeclarationField)
         BOLT_GEN_CASE(VariantDeclaration)
@@ -176,7 +175,7 @@ namespace bolt {
       visitToken(N);
     }
 
-    void visitFnKeyword(FnKeyword* N) {
+    void visitForeignKeyword(ForeignKeyword* N) {
       visitToken(N);
     }
 
@@ -440,11 +439,7 @@ namespace bolt {
       visitLetBody(N);
     }
 
-    void visitFunctionDeclaration(FunctionDeclaration* N) {
-      visitNode(N);
-    }
-
-    void visitVariableDeclaration(VariableDeclaration* N) {
+    void visitLetDeclaration(LetDeclaration* N) {
       visitNode(N);
     }
 
@@ -509,7 +504,7 @@ namespace bolt {
         BOLT_GEN_CHILD_CASE(RArrow)
         BOLT_GEN_CHILD_CASE(RArrowAlt)
         BOLT_GEN_CHILD_CASE(LetKeyword)
-        BOLT_GEN_CHILD_CASE(FnKeyword)
+        BOLT_GEN_CHILD_CASE(ForeignKeyword)
         BOLT_GEN_CHILD_CASE(MutKeyword)
         BOLT_GEN_CHILD_CASE(PubKeyword)
         BOLT_GEN_CHILD_CASE(TypeKeyword)
@@ -569,8 +564,7 @@ namespace bolt {
         BOLT_GEN_CHILD_CASE(Parameter)
         BOLT_GEN_CHILD_CASE(LetBlockBody)
         BOLT_GEN_CHILD_CASE(LetExprBody)
-        BOLT_GEN_CHILD_CASE(FunctionDeclaration)
-        BOLT_GEN_CHILD_CASE(VariableDeclaration)
+        BOLT_GEN_CHILD_CASE(LetDeclaration)
         BOLT_GEN_CHILD_CASE(RecordDeclaration)
         BOLT_GEN_CHILD_CASE(RecordDeclarationField)
         BOLT_GEN_CHILD_CASE(VariantDeclaration)
@@ -629,7 +623,7 @@ namespace bolt {
     void visitEachChild(LetKeyword* N) {
     }
 
-    void visitEachChild(FnKeyword* N) {
+    void visitEachChild(ForeignKeyword* N) {
     }
 
     void visitEachChild(MutKeyword* N) {
@@ -951,29 +945,18 @@ namespace bolt {
       BOLT_VISIT(N->Expression);
     }
 
-    void visitEachChild(FunctionDeclaration* N) {
+    void visitEachChild(LetDeclaration* N) {
       if (N->PubKeyword) {
         BOLT_VISIT(N->PubKeyword);
       }
-      BOLT_VISIT(N->FnKeyword);
-      BOLT_VISIT(N->Name);
-      for (auto Param: N->Params) {
-        BOLT_VISIT(Param);
-      }
-      if (N->TypeAssert) {
-        BOLT_VISIT(N->TypeAssert);
-      }
-      if (N->Body) {
-        BOLT_VISIT(N->Body);
-      }
-    }
-
-    void visitEachChild(VariableDeclaration* N) {
-      if (N->PubKeyword) {
-        BOLT_VISIT(N->PubKeyword);
+      if (N->ForeignKeyword) {
+        BOLT_VISIT(N->ForeignKeyword);
       }
       BOLT_VISIT(N->LetKeyword);
       BOLT_VISIT(N->Pattern);
+      for (auto Param: N->Params) {
+        BOLT_VISIT(Param);
+      }
       if (N->TypeAssert) {
         BOLT_VISIT(N->TypeAssert);
       }
diff --git a/include/bolt/Checker.hpp b/include/bolt/Checker.hpp
index f310b7ed9..4eec0104f 100644
--- a/include/bolt/Checker.hpp
+++ b/include/bolt/Checker.hpp
@@ -192,7 +192,7 @@ namespace bolt {
     /// Type inference
 
     void forwardDeclare(Node* Node);
-    void forwardDeclareFunctionDeclaration(FunctionDeclaration* N, TVSet* TVs, ConstraintSet* Constraints);
+    void forwardDeclareFunctionDeclaration(LetDeclaration* N, TVSet* TVs, ConstraintSet* Constraints);
 
     Type* inferExpression(Expression* Expression);
     Type* inferTypeExpression(TypeExpression* TE, bool IsPoly = true);
@@ -200,7 +200,7 @@ namespace bolt {
     Type* inferPattern(Pattern* Pattern, ConstraintSet* Constraints = new ConstraintSet, TVSet* TVs = new TVSet);
 
     void infer(Node* node);
-    void inferFunctionDeclaration(FunctionDeclaration* N);
+    void inferFunctionDeclaration(LetDeclaration* N);
     void inferConstraintExpression(ConstraintExpression* C);
 
     /// Factory methods 
diff --git a/include/bolt/Parser.hpp b/include/bolt/Parser.hpp
index 86b26555f..e28f2e5a5 100644
--- a/include/bolt/Parser.hpp
+++ b/include/bolt/Parser.hpp
@@ -126,9 +126,7 @@ namespace bolt {
 
     Node* parseLetBodyElement();
 
-    FunctionDeclaration* parseFunctionDeclaration();
-
-    VariableDeclaration* parseVariableDeclaration();
+    LetDeclaration* parseLetDeclaration();
 
     Node* parseClassElement();
 
diff --git a/include/bolt/Type.hpp b/include/bolt/Type.hpp
index e15a873b6..ab5ae8b71 100644
--- a/include/bolt/Type.hpp
+++ b/include/bolt/Type.hpp
@@ -321,7 +321,6 @@ namespace bolt {
       return Curr;
     }
 
-
     static bool classof(const Type* Ty) {
       return Ty->getKind() == TypeKind::Arrow;
     }
diff --git a/src/CST.cc b/src/CST.cc
index 2d223ed31..dc5a6705e 100644
--- a/src/CST.cc
+++ b/src/CST.cc
@@ -78,9 +78,10 @@ namespace bolt {
         }
         break;
       }
-      case NodeKind::FunctionDeclaration:
+      case NodeKind::LetDeclaration:
       {
-        auto Decl = static_cast<FunctionDeclaration*>(X);
+        auto Decl = static_cast<LetDeclaration*>(X);
+        ZEN_ASSERT(Decl->isFunction());
         for (auto Param: Decl->Params) {
           visitPattern(Param->Pattern, Param);
         }
@@ -121,18 +122,14 @@ namespace bolt {
         }
         break;
       }
-      case NodeKind::VariableDeclaration:
+      case NodeKind::LetDeclaration:
       {
-        auto Decl = static_cast<VariableDeclaration*>(X);
+        auto Decl = static_cast<LetDeclaration*>(X);
+        // No matter if it  is a function or a variable, by visiting the pattern
+        // we add all relevant bindings to the current scope.
         visitPattern(Decl->Pattern, Decl);
         break;
       }
-      case NodeKind::FunctionDeclaration:
-      {
-        auto Decl = static_cast<FunctionDeclaration*>(X);
-        addSymbol(Decl->Name->getCanonicalText(), Decl, SymbolKind::Var);
-        break;
-      }
       case NodeKind::RecordDeclaration:
       {
         auto Decl = static_cast<RecordDeclaration*>(X);
@@ -651,14 +648,17 @@ namespace bolt {
     return Expression->getLastToken();
   }
 
-  Token* FunctionDeclaration::getFirstToken() const {
+  Token* LetDeclaration::getFirstToken() const {
     if (PubKeyword) {
       return PubKeyword;
     }
-    return FnKeyword;
+    if (ForeignKeyword) {
+      return ForeignKeyword;
+    }
+    return LetKeyword;
   }
 
-  Token* FunctionDeclaration::getLastToken() const {
+  Token* LetDeclaration::getLastToken() const {
     if (Body) {
       return Body->getLastToken();
     }
@@ -668,23 +668,6 @@ namespace bolt {
     if (Params.size()) {
       return Params.back()->getLastToken();
     }
-    return Name;
-  }
-
-  Token* VariableDeclaration::getFirstToken() const {
-    if (PubKeyword) {
-      return PubKeyword;
-    }
-    return LetKeyword;
-  }
-
-  Token* VariableDeclaration::getLastToken() const {
-    if (Body) {
-      return Body->getLastToken();
-    }
-    if (TypeAssert) {
-      return TypeAssert->getLastToken();
-    }
     return Pattern->getLastToken();
   }
 
@@ -837,8 +820,8 @@ namespace bolt {
     return "let";
   }
 
-  std::string FnKeyword::getText() const {
-    return "fn";
+  std::string ForeignKeyword::getText() const {
+    return "foreign";
   }
 
   std::string MutKeyword::getText() const {
diff --git a/src/Checker.cc b/src/Checker.cc
index 1b0652c03..4fbce7c40 100644
--- a/src/Checker.cc
+++ b/src/Checker.cc
@@ -274,13 +274,22 @@ namespace bolt {
         break;
       }
 
-      case NodeKind::FunctionDeclaration:
-        // These declarations will be handled separately in check()
-        break;
-
-      case NodeKind::VariableDeclaration:
-        // All of this node's semantics will be handled in infer()
+      case NodeKind::LetDeclaration:
+      {
+        // Function declarations are handled separately in forwardDeclareLetDeclaration()
+        auto Decl = static_cast<LetDeclaration*>(X);
+        if (!Decl->isVariable()) {
+          break;
+        }
+        Type* Ty;
+        if (Decl->TypeAssert) {
+          Ty = inferTypeExpression(Decl->TypeAssert->TypeExpression, false);
+        } else {
+          Ty = createTypeVar();
+        }
+        Decl->setType(Ty);
         break;
+      }
 
       case NodeKind::VariantDeclaration:
       {
@@ -405,11 +414,13 @@ namespace bolt {
         Contexts.pop();
       }
 
-      void visitFunctionDeclaration(FunctionDeclaration* Let) {
-        Let->Ctx = createDerivedContext();
-        Contexts.push(Let->Ctx);
-        visitEachChild(Let);
-        Contexts.pop();
+      void visitLetDeclaration(LetDeclaration* Let) {
+        if (Let->isFunction()) {
+          Let->Ctx = createDerivedContext();
+          Contexts.push(Let->Ctx);
+          visitEachChild(Let);
+          Contexts.pop();
+        }
       }
 
       // void visitVariableDeclaration(VariableDeclaration* Var) {
@@ -424,7 +435,11 @@ namespace bolt {
 
   }
 
-  void Checker::forwardDeclareFunctionDeclaration(FunctionDeclaration* Let, TVSet* TVs, ConstraintSet* Constraints) {
+  void Checker::forwardDeclareFunctionDeclaration(LetDeclaration* Let, TVSet* TVs, ConstraintSet* Constraints) {
+
+    if (!Let->isFunction()) {
+      return;
+    }
 
     setContext(Let->Ctx);
 
@@ -460,7 +475,7 @@ namespace bolt {
     } else {
       Ty = createTypeVar();
     }
-    Let->Ty = Ty;
+    Let->setType(Ty);
 
     // If declaring a let-declaration inside a type instance declaration,
     // we need to perform some work to make sure the type asserts of the
@@ -470,7 +485,7 @@ namespace bolt {
 
       auto Instance = static_cast<InstanceDeclaration*>(Let->Parent);
       auto Class = llvm::cast<ClassDeclaration>(Instance->getScope()->lookup({ {}, Instance->Name->getCanonicalText() }, SymbolKind::Class));
-      auto SigLet = llvm::cast<FunctionDeclaration>(Class->getScope()->lookupDirect({ {}, Let->Name->getCanonicalText() }, SymbolKind::Var));
+      auto SigLet = llvm::cast<LetDeclaration>(Class->getScope()->lookupDirect({ {}, Let->getNameAsString() }, SymbolKind::Var));
 
       auto Params = addClassVars(Class, false);
 
@@ -526,14 +541,14 @@ namespace bolt {
     }
 
     if (!Let->isInstance()) {
-      Let->Ctx->Parent->Env.emplace(Let->Name->getCanonicalText(), new Forall(Let->Ctx->TVs, Let->Ctx->Constraints, Ty));
+      Let->Ctx->Parent->Env.emplace(Let->getNameAsString(), new Forall(Let->Ctx->TVs, Let->Ctx->Constraints, Ty));
     }
 
   }
 
-  void Checker::inferFunctionDeclaration(FunctionDeclaration* Decl) {
+  void Checker::inferFunctionDeclaration(LetDeclaration* Decl) {
 
-    if (Decl->isSignature()) {
+    if (!Decl->isFunction()) {
       return;
     }
 
@@ -570,7 +585,7 @@ namespace bolt {
       RetType = createTypeVar();
     }
 
-    makeEqual(Decl->Ty, TArrow::build(ParamTypes, RetType), Decl);
+    makeEqual(Decl->getType(), TArrow::build(ParamTypes, RetType), Decl);
 
   }
 
@@ -624,9 +639,6 @@ namespace bolt {
         break;
       }
 
-      case NodeKind::FunctionDeclaration:
-        break;
-
       case NodeKind::ReturnStatement:
       {
         auto RetStmt = static_cast<ReturnStatement*>(N);
@@ -640,30 +652,22 @@ namespace bolt {
         break;
       }
 
-      case NodeKind::VariableDeclaration:
+      case NodeKind::LetDeclaration:
       {
-        auto Decl = static_cast<VariableDeclaration*>(N);
-        Type* Ty = nullptr;
-        if (Decl->TypeAssert) {
-          Ty = inferTypeExpression(Decl->TypeAssert->TypeExpression, false);
+        // Function declarations are handled separately in inferLetDeclaration()
+        auto Decl = static_cast<LetDeclaration*>(N);
+        if (!Decl->isVariable()) {
+          break;
         }
+        auto Ty = Decl->getType();
         if (Decl->Body) {
           ZEN_ASSERT(Decl->Body->getKind() == NodeKind::LetExprBody);
           auto E = static_cast<LetExprBody*>(Decl->Body);
           auto Ty2 = inferExpression(E->Expression);
-          if (Ty) {
-            makeEqual(Ty, Ty2, Decl);
-          } else {
-            Ty = Ty2;
-          }
+          makeEqual(Ty, Ty2, Decl);
         }
         auto Ty3 = inferPattern(Decl->Pattern);
-        if (Ty) {
-          makeEqual(Ty, Ty3, Decl);
-        } else {
-          Ty = Ty3;
-        }
-        Decl->setType(Ty);
+        makeEqual(Ty, Ty3, Decl);
         break;
       }
 
@@ -759,7 +763,9 @@ namespace bolt {
         auto D = static_cast<TypeclassConstraintExpression*>(C);
         std::vector<Type*> Types;
         for (auto TE: D->TEs) {
-          auto TV = static_cast<TVarRigid*>(inferTypeExpression(TE));
+          auto Ty = inferTypeExpression(TE);
+          ZEN_ASSERT(Ty->getKind() == TypeKind::Var && static_cast<TVar*>(Ty)->isRigid());
+          auto TV = static_cast<TVarRigid*>(Ty);
           TV->Provided.emplace(D->Name->getCanonicalText());
           Types.push_back(TV);
         }
@@ -938,10 +944,10 @@ namespace bolt {
         auto Ref = static_cast<ReferenceExpression*>(X);
         ZEN_ASSERT(Ref->ModulePath.empty());
         auto Target = Ref->getScope()->lookup(Ref->getSymbolPath());
-        if (Target && llvm::isa<FunctionDeclaration>(Target)) {
-          auto Let = static_cast<FunctionDeclaration*>(Target);
+        if (Target && llvm::isa<LetDeclaration>(Target)) {
+          auto Let = static_cast<LetDeclaration*>(Target);
           if (Let->IsCycleActive) {
-            return Let->Ty;
+            return Let->getType();
           }
         }
         auto Scm = lookup(Ref->Name->getCanonicalText());
@@ -1133,7 +1139,7 @@ namespace bolt {
 
       std::stack<Node*> Stack;
 
-      void visitFunctionDeclaration(FunctionDeclaration* N) {
+      void visitLetDeclaration(LetDeclaration* N) {
         RefGraph.addVertex(N);
         Stack.push(N);
         visitEachChild(N);
@@ -1154,7 +1160,7 @@ namespace bolt {
           RefGraph.addEdge(Stack.top(), Def->Parent);
           return;
         }
-        ZEN_ASSERT(Def->getKind() == NodeKind::FunctionDeclaration || Def->getKind() == NodeKind::VariableDeclaration);
+        ZEN_ASSERT(Def->getKind() == NodeKind::LetDeclaration);
         if (!Stack.empty()) {
           RefGraph.addEdge(Def, Stack.top());
         }
@@ -1193,33 +1199,33 @@ namespace bolt {
       auto TVs = new TVSet;
       auto Constraints = new ConstraintSet;
       for (auto N: Nodes) {
-        if (N->getKind() != NodeKind::FunctionDeclaration) {
+        if (N->getKind() != NodeKind::LetDeclaration) {
           continue;
         }
-        auto Decl = static_cast<FunctionDeclaration*>(N);
+        auto Decl = static_cast<LetDeclaration*>(N);
         forwardDeclareFunctionDeclaration(Decl, TVs, Constraints);
       }
     }
     for (auto Nodes: SCCs) {
       for (auto N: Nodes) {
-        if (N->getKind() != NodeKind::FunctionDeclaration) {
+        if (N->getKind() != NodeKind::LetDeclaration) {
           continue;
         }
-        auto Decl = static_cast<FunctionDeclaration*>(N);
+        auto Decl = static_cast<LetDeclaration*>(N);
         Decl->IsCycleActive = true;
       }
       for (auto N: Nodes) {
-        if (N->getKind() != NodeKind::FunctionDeclaration) {
+        if (N->getKind() != NodeKind::LetDeclaration) {
           continue;
         }
-        auto Decl = static_cast<FunctionDeclaration*>(N);
+        auto Decl = static_cast<LetDeclaration*>(N);
         inferFunctionDeclaration(Decl);
       }
       for (auto N: Nodes) {
-        if (N->getKind() != NodeKind::FunctionDeclaration) {
+        if (N->getKind() != NodeKind::LetDeclaration) {
           continue;
         }
-        auto Decl = static_cast<FunctionDeclaration*>(N);
+        auto Decl = static_cast<LetDeclaration*>(N);
         Decl->IsCycleActive = false;
       }
     }
diff --git a/src/Diagnostics.cc b/src/Diagnostics.cc
index 30569f8a4..6740f1f6e 100644
--- a/src/Diagnostics.cc
+++ b/src/Diagnostics.cc
@@ -100,8 +100,8 @@ namespace bolt {
         return "'pub'";
       case NodeKind::LetKeyword:
         return "'let'";
-      case NodeKind::FnKeyword:
-        return "'fn'";
+      case NodeKind::ForeignKeyword:
+        return "'foreign'";
       case NodeKind::MutKeyword:
         return "'mut'";
       case NodeKind::MatchKeyword:
@@ -112,10 +112,8 @@ namespace bolt {
         return "'type'";
       case NodeKind::ReferenceTypeExpression:
         return "a type reference";
-      case NodeKind::FunctionDeclaration:
-        return "a function declaration";
-      case NodeKind::VariableDeclaration:
-        return "a variable declaration";
+      case NodeKind::LetDeclaration:
+        return "a let-declaration";
       case NodeKind::CallExpression:
         return "a call-expression";
       case NodeKind::InfixExpression:
diff --git a/src/Parser.cc b/src/Parser.cc
index 2e57f8b16..56e2d08b5 100644
--- a/src/Parser.cc
+++ b/src/Parser.cc
@@ -919,9 +919,10 @@ finish:
     return new IfStatement(Parts);
   }
 
-VariableDeclaration* Parser::parseVariableDeclaration() {
+  LetDeclaration* Parser::parseLetDeclaration() {
 
     PubKeyword* Pub = nullptr;
+    ForeignKeyword* Foreign = nullptr;
     LetKeyword* Let;
     MutKeyword* Mut = nullptr;
     TypeAssert* TA = nullptr;
@@ -932,26 +933,36 @@ VariableDeclaration* Parser::parseVariableDeclaration() {
       Pub = static_cast<PubKeyword*>(T0);
       T0 = Tokens.get();
     }
+    if (T0->getKind() == NodeKind::ForeignKeyword) {
+      Foreign = static_cast<ForeignKeyword*>(T0);
+      T0 = Tokens.get();
+    }
     if (T0->getKind() != NodeKind::LetKeyword) {
       DE.add<UnexpectedTokenDiagnostic>(File, T0, std::vector { NodeKind::LetKeyword });
       if (Pub) {
         Pub->unref();
       }
+      if (Foreign) {
+        Foreign->unref();
+      }
       skipToLineFoldEnd();
       return nullptr;
     }
     Let = static_cast<LetKeyword*>(T0);
     auto T1 = Tokens.peek();
     if (T1->getKind() == NodeKind::MutKeyword) {
-      Mut = static_cast<MutKeyword*>(T1);
       Tokens.get();
+      Mut = static_cast<MutKeyword*>(T1);
     }
 
-    auto P = parseWidePattern();
-    if (!P) {
+    auto Pattern = parseNarrowPattern();
+    if (!Pattern) {
       if (Pub) {
         Pub->unref();
       }
+      if (Foreign) {
+        Foreign->unref();
+      }
       Let->unref();
       if (Mut) {
         Mut->unref();
@@ -960,103 +971,6 @@ VariableDeclaration* Parser::parseVariableDeclaration() {
       return nullptr;
     }
 
-    auto T2 = Tokens.peek();
-    if (T2->getKind() == NodeKind::Colon) {
-      Tokens.get();
-      auto TE = parseTypeExpression();
-      if (TE) {
-        TA = new TypeAssert(static_cast<Colon*>(T2), TE);
-      } else {
-        skipToLineFoldEnd();
-        goto finish;
-      }
-      T2 = Tokens.peek();
-    }
-
-    switch (T2->getKind()) {
-      case NodeKind::BlockStart:
-      {
-        Tokens.get();
-        std::vector<Node*> Elements;
-        for (;;) {
-          auto T3 = Tokens.peek();
-          if (T3->getKind() == NodeKind::BlockEnd) {
-            break;
-          }
-          auto Element = parseLetBodyElement();
-          if (Element) {
-            Elements.push_back(Element);
-          }
-        }
-        Tokens.get()->unref(); // Always a BlockEnd
-        Body = new LetBlockBody(static_cast<BlockStart*>(T2), Elements);
-        break;
-      }
-      case NodeKind::Equals:
-      {
-        Tokens.get();
-        auto E = parseExpression();
-        if (E == nullptr) {
-          skipToLineFoldEnd();
-          goto finish;
-        }
-        if (E) {
-          Body = new LetExprBody(static_cast<Equals*>(T2), E);
-        }
-        break;
-      }
-      case NodeKind::LineFoldEnd:
-        break;
-      default:
-        std::vector<NodeKind> Expected { NodeKind::BlockStart, NodeKind::LineFoldEnd, NodeKind::Equals };
-        if (TA == nullptr) {
-          // First tokens of TypeAssert
-          Expected.push_back(NodeKind::Colon);
-          // First tokens of Pattern
-          Expected.push_back(NodeKind::Identifier);
-        }
-        DE.add<UnexpectedTokenDiagnostic>(File, T2, Expected);
-    }
-
-    checkLineFoldEnd();
-
-finish:
-
-    return new VariableDeclaration { Pub, Let, Mut, P, TA, Body };
-  }
-
-  FunctionDeclaration* Parser::parseFunctionDeclaration() {
-
-    PubKeyword* Pub = nullptr;
-    FnKeyword* Fn;
-    TypeAssert* TA = nullptr;
-    LetBody* Body = nullptr;
-
-    auto T0 = Tokens.get();
-    if (T0->getKind() == NodeKind::PubKeyword) {
-      Pub = static_cast<PubKeyword*>(T0);
-      T0 = Tokens.get();
-    }
-    if (T0->getKind() != NodeKind::FnKeyword) {
-      DE.add<UnexpectedTokenDiagnostic>(File, T0, std::vector { NodeKind::FnKeyword });
-      if (Pub) {
-        Pub->unref();
-      }
-      skipToLineFoldEnd();
-      return nullptr;
-    }
-    Fn = static_cast<FnKeyword*>(T0);
-
-    auto Name = expectToken<Identifier>();
-    if (!Name) {
-      if (Pub) {
-        Pub->unref();
-      }
-      Fn->unref();
-      skipToLineFoldEnd();
-      return nullptr;
-    }
-
     std::vector<Parameter*> Params;
     Token* T2;
     for (;;) {
@@ -1138,10 +1052,12 @@ after_params:
 
 finish:
 
-    return new FunctionDeclaration(
+    return new LetDeclaration(
       Pub,
-      Fn,
-      Name,
+      Foreign,
+      Let,
+      Mut,
+      Pattern,
       Params,
       TA,
       Body
@@ -1152,9 +1068,7 @@ finish:
     auto T0 = peekFirstTokenAfterModifiers();
     switch (T0->getKind()) {
       case NodeKind::LetKeyword:
-        return parseVariableDeclaration();
-      case NodeKind::FnKeyword:
-        return parseFunctionDeclaration();
+        return parseLetDeclaration();
       case NodeKind::ReturnKeyword:
         return parseReturnStatement();
       case NodeKind::IfKeyword:
@@ -1558,12 +1472,12 @@ next_member:
   Node* Parser::parseClassElement() {
     auto T0 = Tokens.peek();
     switch (T0->getKind()) {
-      case NodeKind::FnKeyword:
-        return parseFunctionDeclaration();
+      case NodeKind::LetKeyword:
+        return parseLetDeclaration();
       case NodeKind::TypeKeyword:
         // TODO
       default:
-        DE.add<UnexpectedTokenDiagnostic>(File, T0, std::vector<NodeKind> { NodeKind::FnKeyword, NodeKind::TypeKeyword });
+        DE.add<UnexpectedTokenDiagnostic>(File, T0, std::vector<NodeKind> { NodeKind::LetKeyword, NodeKind::TypeKeyword });
         skipToLineFoldEnd();
         return nullptr;
     }
@@ -1573,9 +1487,7 @@ next_member:
     auto T0 = peekFirstTokenAfterModifiers();
     switch (T0->getKind()) {
       case NodeKind::LetKeyword:
-        return parseVariableDeclaration();
-      case NodeKind::FnKeyword:
-        return parseFunctionDeclaration();
+        return parseLetDeclaration();
       case NodeKind::IfKeyword:
         return parseIfStatement();
       case NodeKind::ClassKeyword:
diff --git a/src/Scanner.cc b/src/Scanner.cc
index 505fe8364..f6f3ab461 100644
--- a/src/Scanner.cc
+++ b/src/Scanner.cc
@@ -62,7 +62,7 @@ namespace bolt {
   std::unordered_map<ByteString, NodeKind> Keywords = {
     { "pub", NodeKind::PubKeyword },
     { "let", NodeKind::LetKeyword },
-    { "fn", NodeKind::FnKeyword },
+    { "foreign", NodeKind::ForeignKeyword },
     { "mut", NodeKind::MutKeyword },
     { "return", NodeKind::ReturnKeyword },
     { "type", NodeKind::TypeKeyword },
@@ -227,8 +227,8 @@ digit_finish:
               return new PubKeyword(StartLoc);
             case NodeKind::LetKeyword:
               return new LetKeyword(StartLoc);
-            case NodeKind::FnKeyword:
-              return new FnKeyword(StartLoc);
+            case NodeKind::ForeignKeyword:
+              return new ForeignKeyword(StartLoc);
             case NodeKind::MutKeyword:
               return new MutKeyword(StartLoc);
             case NodeKind::TypeKeyword: