Make typechecking slightly less buggy

- The statement ["x", "x"] no longer typechecks in the empty context
- The statement ["ident": {"x": "x"}] now correctly generalizes
- The statement ["compose", "compose", "compose"] no longer goes into an infinite loop

genir.py

@@ -146,13 +146,14 @@ def let_2_ir(
 ) -> Result[tuple[Expression, MonoType, Sequence[Substitution]], SemanticError]:
 	# Parse the rhs
 	standin_ty = context.new_type_var()
-	match json_to_ir(rhs, context.with_mono(lhs, standin_ty)):
+	context_for_rhs = context.with_mono(lhs, standin_ty) if isinstance(rhs, Mapping) else context
+	match json_to_ir(rhs, context_for_rhs):
 		case Ok(( rhs_expr, rhs_ty, rhs_subst )):
 
 			# Unify the rhs type with the generated type of the rhs from earlier
 			match subst_all_monotype(standin_ty, rhs_subst).unify(rhs_ty):
 				case Ok(recursion_substs):
-					updated_ctx = context.subst_all(rhs_subst).with_mono(lhs, rhs_ty).subst_all(recursion_substs)
+					updated_ctx = context.subst_all(rhs_subst).with_(lhs, context.generalize(rhs_ty)).subst_all(recursion_substs)
 
 					# Parse the body
 					match json_to_ir(body, updated_ctx):

types_.py

@@ -188,7 +188,8 @@ class Context:
 		return FSet(fv for ty in self.variable_types.values() for fv in ty.free_vars())
 
 	def generalize(self, mt: MonoType) -> PolyType:
-		return PolyType(mt.free_vars(), mt)
+		variable_mappings = [(VarTy(fv.id, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'[i]), fv) for (i, fv) in enumerate(mt.free_vars())]
+		return PolyType([new_var for (new_var, _) in variable_mappings], subst_all_monotype(mt, variable_mappings))
 
 	def __contains__(self, name: str) -> bool:
 		return name in self.variable_types