Add better reporting and new features

Units report expected results on end of input.
Opposite parser helps with the complexity explosion
on unit parsers.
Input and error printing has context.

1 files changed, 12 insertions, 3 deletions

diff --git a/core.lisp b/core.lisp
index 2cd0120..3b06936 100644
--- a/core.lisp
+++ b/core.lisp
@@ -1,5 +1,14 @@
 (in-package #:monparser)
 
+(defun opposite (p)
+  (lambda (input &key lazy)
+    (let ((result (funcall p input)))
+      (cond ((parsing-p result)
+             (make-failure :place input :message "Unexpected match."))
+            ((failure-p result)
+             (make-parsing :tree nil :left input))
+            (t (error "Unexpected result type."))))))
+
 (defun fail (message)
   (lambda (input &key lazy)
     (make-failure :place input :message message)))
@@ -28,7 +37,8 @@
            (make-parsing :tree it :left (advance input))
            (make-failure :place input
                          :message (format nil "Expected: ~a, Got: ~a" ',predicate it))))
-       (make-failure :place input :message "Reached end of input."))))
+       (make-failure :place input
+                     :message (format nil "Reached end of input. Expected: ~a" ',predicate)))))
 
 (defun lazily-select-parser (input parsers)
   (let ((intermediate-parsers '())
@@ -46,8 +56,7 @@
                             (input-cursor (parsing-left result))))
                  (setf result r)))
               ((failure-p r)
-               (when (or (failure-p result)
-                         (= (length parsers) 1))
+               (when (failure-p result)
                  (setf result r)))
               (t (error (format nil "Invalid return value: ~a" r))))))
     (if intermediate-parsers