Add nullary function shorthand

Fixes #70

src/ast/parse-form.lisp

@@ -35,9 +35,9 @@ This does not attempt to do any sort of analysis whatsoever. It is suitable for
 	(parse-seq expr subnodes sr package))
        ;; Application
        ((t &rest rands)
-        (when (= 0 (length rands))
-          (error-parsing expr "Unable to call nullary function."))
-        (parse-application expr (first expr) rands sr package))))
+        (if (= 0 (length rands))
+	    (parse-application expr (first expr) `(coalton-user::Unit) sr package)
+            (parse-application expr (first expr) rands sr package)))))
     (t (error-parsing expr "The expression is not a valid value expression."))))
 
 (defun invert-alist (alist)

src/typechecker/derive-type.lisp

@@ -73,6 +73,10 @@ Returns (VALUES type predicate-list typed-node subs)")
            (rands (node-application-rands value))
            (ret-ty (make-variable))
            (typed-rands nil))
+
+      (when (= 0 (length rands))
+	(coalton-impl::coalton-bug "Invalid application with 0 arguments ~A." rator))
+
       (multiple-value-bind (fun-ty fun-preds typed-rator substs)
           (derive-expression-type rator env substs)
         (unless (or (tvar-p fun-ty)