Some code cleanup, removed unused functions, changed == empty-list to isNil to avoid unnecessary construction of empty lists, replaced some casts by generic declarations. Added simple maybe, failable and state monad classes to help return optional objects or failures and carry state around. Added utility functions to zip and unzip list.

git-svn-id: http://svn.us.apache.org/repos/asf/tuscany@823981 13f79535-47bb-0310-9956-ffa450edef68

1 files changed, 36 insertions, 8 deletions

diff --git a/cpp/sca/kernel/list.hpp b/cpp/sca/kernel/list.hpp
index 4047e91c1b..189eace9b2 100644
--- a/cpp/sca/kernel/list.hpp
+++ b/cpp/sca/kernel/list.hpp
@@ -123,7 +123,7 @@ template<typename T> const bool isNil(const list<T>& p) {
  * Write a list to an output stream.
  */
 template<typename X> std::ostream& operator<<(std::ostream& out, const list<X>& l) {
-    if(l == list<X> ())
+    if(isNil(l))
         return out << "()";
     return out << "(" << car(l) << ", " << cdr(l) << ")";
 }
@@ -139,14 +139,14 @@ template<typename T> const list<T> cons(const T& car, const lambda<list<T> ()>&
  * Construct a list from a value and a cdr list.
  */
 template<typename T> const list<T> cons(const T& car, const list<T>& cdr) {
-    return list<T> (car, unit(cdr));
+    return list<T> (car, result(cdr));
 }
 
 /**
  * Construct a list of one value.
  */
 template<typename T> const list<T> makeList(const T& car) {
-    return list<T> (car, unit(list<T> ()));
+    return list<T> (car, result(list<T> ()));
 }
 
 /**
@@ -245,14 +245,14 @@ template<typename T> const list<T> append(const list<T>&a, const lambda<list<T>
     if(isNil(a))
         return fb();
 
-    return cons(car(a), lambda<list<T> ()> (appendCdr<T> (cdr(a), fb)));
+    return cons<T>(car(a), appendCdr<T> (cdr(a), fb));
 }
 
 /**
  * Appends two lists.
  */
 template<typename T> const list<T> append(const list<T>&a, const list<T>& b) {
-    return append(a, unit(b));
+    return append(a, result(b));
 }
 
 /**
@@ -340,12 +340,12 @@ template<typename T> const list<T> seq(const T& start, const T& end) {
     if(start == end)
         return makeList(start);
     if(start < end)
-        return cons(start, lambda<list<T> ()> (seqGenerate<T> (start + 1, end)));
-    return cons(start, lambda<list<T> ()> (seqGenerate<T> (start - 1, end)));
+        return cons<T>(start, seqGenerate<T> (start + 1, end));
+    return cons<T>(start, seqGenerate<T> (start - 1, end));
 }
 
 /**
- * Equivalent of the list assoc function.
+ * Returns the first pair matching a key from a list of key value pairs.
  */
 template<typename T> const list<T> assoc(const T& k, const list<list<T> >& p) {
     if(isNil(p))
@@ -356,6 +356,34 @@ template<typename T> const list<T> assoc(const T& k, const list<list<T> >& p) {
 }
 
 /**
+ * Returns a list of lists containing elements from two input lists.
+ */
+template<typename T> const list<list<T> > zip(const list<T>& a, const list<T>& b) {
+    if (isNil(a) || isNil(b))
+        return list<list<T> >();
+    return cons<list<T> >(makeList<T>(car(a), car(b)), zip(cdr(a), cdr(b)));
+}
+
+/**
+ * Converts a list of key value pairs to a list containing the list of keys and the list of values.
+ */
+template<typename T> const list<T> unzipKeys(const list<list<T> >& l) {
+    if (isNil(l))
+        return list<T>();
+    return cons(car(car(l)), unzipKeys(cdr(l)));
+}
+
+template<typename T> const list<T> unzipValues(const list<list<T> >& l) {
+    if (isNil(l))
+        return list<T>();
+    return cons(cadr(car(l)), unzipValues(cdr(l)));
+}
+
+template<typename T> const list<list<T> > unzip(const list<list<T> >& l) {
+    return makeList<list<T> >(unzipKeys(l), unzipValues(l));
+}
+
+/**
  * Pretty print a list.
  */
 template<typename T> std::ostream& print(const list<T>& l, std::ostream& os) {