More type infrastructure

From here on out, I can start playing with concrete code that attempts
to actually typecheck limited syntactic constructs, and see what I end
up with.

schala-lang/language/src/typechecking.rs

@@ -142,24 +142,35 @@ impl<'a> TypeContext<'a> {
   }
 
   pub fn typecheck(&mut self, ast: &AST) -> Result<String, String> {
+    let mut returned_type = Type::Const(TypeConst::Unit);
     for statement in ast.0.iter() {
-      self.typecheck_statement(statement.node()).map_err(|err| { err.msg })?
+      returned_type = self.typecheck_statement(statement.node()).map_err(|err| { err.msg })?
     }
-    Ok("unknown type".to_string())
+    Ok(format!("{:?}", returned_type))
   }
 
-  fn typecheck_statement(&mut self, statement: &Statement) -> InferResult<()> {
+  fn typecheck_statement(&mut self, statement: &Statement) -> InferResult<Type> {
     match statement {
       Statement::ExpressionStatement(e) => self.typecheck_expr(e.node()),
       Statement::Declaration(decl) => self.typecheck_decl(decl),
     }
   }
 
-  fn typecheck_decl(&mut self, decl: &Declaration) -> InferResult<()> {
-    Ok(())
+  fn typecheck_decl(&mut self, decl: &Declaration) -> InferResult<Type> {
+    Ok(Type::Const(TypeConst::Unit))
   }
 
-  fn typecheck_expr(&mut self, expr: &Expression) -> InferResult<()> {
-    Ok(())
+  fn typecheck_expr(&mut self, expr: &Expression) -> InferResult<Type> {
+    match expr {
+      Expression(expr_type, Some(_anno)) => {
+        //TODO here
+        self.typecheck_expr_type(expr_type)
+      },
+      Expression(expr_type, None) => self.typecheck_expr_type(expr_type)
+    }
+  }
+
+  fn typecheck_expr_type(&mut self, expr: &ExpressionType) -> InferResult<Type> {
+    Ok(Type::Const(TypeConst::Unit))
   }
 }