From b2039a7b673d9c7798f6d36bb0019bb439193ab6 Mon Sep 17 00:00:00 2001
From: greg <greg.shuflin@protonmail.com>
Date: Sat, 10 Nov 2018 16:33:42 -0800
Subject: [PATCH] Parameterize Type type over existential/universal

---
 schala-lang/language/src/typechecking.rs | 63 ++++++++++++++++++------
 1 file changed, 47 insertions(+), 16 deletions(-)

diff --git a/schala-lang/language/src/typechecking.rs b/schala-lang/language/src/typechecking.rs
index 4fa41a8..95ba5ec 100644
--- a/schala-lang/language/src/typechecking.rs
+++ b/schala-lang/language/src/typechecking.rs
@@ -6,7 +6,7 @@ use util::ScopeStack;
 pub type TypeName = Rc<String>;
 
 pub struct TypeContext<'a> {
-  variable_map: ScopeStack<'a, Rc<String>, Type<()>>,
+  variable_map: ScopeStack<'a, Rc<String>, Type<TVar>>,
   evar_count: u32
 }
 
@@ -28,16 +28,47 @@ enum Type<A> {
   Arrow(Box<Type<A>>, Box<Type<A>>),
 }
 
+#[derive(Debug, Clone)]
 enum TVar {
-  Univ(UniversalVar),
+  Univ(UVar),
   Exist(ExistentialVar)
 }
 
-struct UniversalVar(Rc<String>);
+#[derive(Debug, Clone)]
+struct UVar(Rc<String>);
+
+#[derive(Debug, Clone)]
 struct ExistentialVar(u32);
 
+impl Type<UVar> {
+  fn to_tvar(&self) -> Type<TVar> {
+    match self {
+      Type::Var(UVar(name)) => Type::Var(TVar::Univ(UVar(name.clone()))),
+      Type::Const(ref c) => Type::Const(c.clone()),
+      Type::Arrow(a, b) => Type::Arrow(
+        Box::new(a.to_tvar()),
+        Box::new(b.to_tvar())
+      )
+    }
+  }
+}
+
+impl Type<TVar> {
+  fn skolemize(&self) -> Type<UVar> {
+    match self {
+      Type::Var(TVar::Univ(uvar)) => Type::Var(uvar.clone()),
+      Type::Var(TVar::Exist(evar)) => unimplemented!(),
+      Type::Const(ref c) => Type::Const(c.clone()),
+      Type::Arrow(a, b) => Type::Arrow(
+        Box::new(a.skolemize()),
+        Box::new(b.skolemize())
+      )
+    }
+  }
+}
+
 impl TypeIdentifier {
-  fn to_monotype(&self) -> Type<()> {
+  fn to_monotype(&self) -> Type<UVar> {
     match self {
       TypeIdentifier::Tuple(items) => unimplemented!(),
       TypeIdentifier::Singleton(TypeSingletonName { name, .. }) => {
@@ -94,7 +125,7 @@ impl<'a> TypeContext<'a> {
 }
 
 impl<'a> TypeContext<'a> {
-  fn infer_ast(&mut self, ast: &AST) -> InferResult<Type<()>> {
+  fn infer_ast(&mut self, ast: &AST) -> InferResult<Type<UVar>> {
     let mut output = Type::Const(TConst::Unit);
     for statement in ast.0.iter() {
       output = self.infer_statement(statement)?;
@@ -102,29 +133,29 @@ impl<'a> TypeContext<'a> {
     Ok(output)
   }
 
-  fn infer_statement(&mut self, stmt: &Statement) -> InferResult<Type<()>> {
+  fn infer_statement(&mut self, stmt: &Statement) -> InferResult<Type<UVar>> {
     match stmt {
       Statement::ExpressionStatement(ref expr) => self.infer_expr(expr),
       Statement::Declaration(ref decl) => self.infer_decl(decl),
     }
   }
 
-  fn infer_expr(&mut self, expr: &Expression) -> InferResult<Type<()>> {
+  fn infer_expr(&mut self, expr: &Expression) -> InferResult<Type<UVar>> {
     match expr {
       Expression(expr_type, Some(type_anno)) => {
         let tx = self.infer_expr_type(expr_type)?;
         let ty = type_anno.to_monotype();
-        self.unify(&ty, &tx)
+        self.unify(&ty.to_tvar(), &tx.to_tvar()).map(|x| x.skolemize())
       },
       Expression(expr_type, None) => self.infer_expr_type(expr_type)
     }
   }
 
-  fn infer_decl(&mut self, expr: &Declaration) -> InferResult<Type<()>> {
+  fn infer_decl(&mut self, expr: &Declaration) -> InferResult<Type<UVar>> {
     Ok(Type::Const(TConst::user("unimplemented")))
   }
 
-  fn infer_expr_type(&mut self, expr_type: &ExpressionType) -> InferResult<Type<()>> {
+  fn infer_expr_type(&mut self, expr_type: &ExpressionType) -> InferResult<Type<UVar>> {
     use self::ExpressionType::*;
     Ok(match expr_type {
       NatLiteral(_) => Type::Const(TConst::Nat),
@@ -135,17 +166,17 @@ impl<'a> TypeContext<'a> {
         //TODO handle the distinction between 0-arg constructors and variables at some point
         // need symbol table for that
         match self.variable_map.lookup(name) {
-          Some(ty) => ty.clone(),
+          Some(ty) => ty.clone().skolemize(),
           None => return TypeError::new(&format!("Variable {} not found", name))
         }
       },
       IfExpression { discriminator, body } => self.infer_if_expr(discriminator, body)?,
       Call { f, arguments } => {
-        let tf: Type<()> = self.infer_expr(f)?; //has to be an Arrow Type
+        let tf = self.infer_expr(f)?; //has to be an Arrow Type
         let targ = self.infer_expr(&arguments[0])?; // TODO make this work with functions with more than one arg
         match tf {
           Type::Arrow(t1, t2) => {
-            self.unify(&t1, &targ)?;
+            self.unify(&t1.to_tvar(), &targ.to_tvar())?;
             *t2.clone()
           },
           _ => return TypeError::new("not a function")
@@ -163,7 +194,7 @@ impl<'a> TypeContext<'a> {
     })
   }
 
-  fn infer_if_expr(&mut self, discriminator: &Discriminator, body: &IfExpressionBody) -> InferResult<Type<()>> {
+  fn infer_if_expr(&mut self, discriminator: &Discriminator, body: &IfExpressionBody) -> InferResult<Type<UVar>> {
     let test = match discriminator {
       Discriminator::Simple(expr) => expr,
       _ => return TypeError::new("Dame desu")
@@ -177,7 +208,7 @@ impl<'a> TypeContext<'a> {
     unimplemented!()
   }
 
-  fn infer_block(&mut self, block: &Block) -> InferResult<Type<()>> {
+  fn infer_block(&mut self, block: &Block) -> InferResult<Type<UVar>> {
     let mut output = Type::Const(TConst::Unit);
     for statement in block.iter() {
       output = self.infer_statement(statement)?;
@@ -185,7 +216,7 @@ impl<'a> TypeContext<'a> {
     Ok(output)
   }
 
-  fn unify(&mut self, t1: &Type<()>, t2: &Type<()>) -> InferResult<Type<()>> {
+  fn unify(&mut self, t1: &Type<TVar>, t2: &Type<TVar>) -> InferResult<Type<TVar>> {
     unimplemented!()
   }