--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/arith_test.py	Sun May 19 14:15:45 2019 +0200
@@ -0,0 +1,98 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Let's test the combinators in a real world application!
+
+@author: lbo
+"""
+
+from combinators import *
+
+class Parens(Parser):
+    def parse(self, st):
+        initial = st.index()
+        p1, st = Operator('(').parse(st)
+        print('pr', p1, st)
+        if p1 is None:
+            st.reset(initial)
+            return None, st
+        term, st = Term().parse(st)
+        print('pr', term, st)
+        if term is None:
+            st.reset(initial)
+            return None, st
+        p2, st = Operator(')').parse(st)
+        print('pr', p2, st)
+        if p2 is None:
+            print('No closing paren!')
+            st.reset(initial)
+            return None, st
+        return term, st
+
+def Atom():
+    """An atom is a variable or a float."""
+    return (Float() | Parens() | Regex('\w+'))
+
+def Operator(set):
+    return Last(Skip(Whitespace()) + CharSet(set))
+
+class Product(Parser):
+
+    def parse(self, st):
+        initial = st.index()
+
+        left, st = Atom().parse(st)
+        print('p', left, st)
+        if left is None:
+            st.reset(initial)
+            return None, st
+
+        op, st = Operator('*/').parse(st)
+        print('p', op, st)
+        if op is None:
+            return left, st
+
+        right, st = Product().parse(st)
+        print('p', right, st)
+        if right is None:
+            st.reset(initial)
+            return None, st
+        return ((left, op, right), st)
+
+class Term(Parser):
+
+    def parse(self, st):
+        initial = st.index()
+
+        left, st = Product().parse(st)
+        print('t', left, st)
+        if left is None:
+            st.reset(initial)
+            return None, st
+
+        op, st = Operator('+-').parse(st)
+        print('t', op, st)
+        if op is None:
+            return left, st
+
+        right, st = Term().parse(st)
+        print(right, st)
+        if right is None:
+            st.reset(initial)
+            return None, st
+
+        return (left, op, right), st
+
+def pretty_print(tpl):
+    # tpl is a (left, op, right) tuple or a scalar.
+    if not isinstance(tpl, tuple):
+        return str(tpl)
+    assert len(tpl) == 3
+    return '({} {} {})'.format(pretty_print(tpl[0]), tpl[1], pretty_print(tpl[2]))
+
+def parse_and_print(expr):
+    parsed, st = Term().parse(ps(expr))
+    if parsed is None:
+        print('Parse error :(', st)
+        return
+    print(pretty_print(parsed))
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