FEATURE: Add head and tail functions to our list library.

docsite/site/content/stdlib/lists.md

@@ -36,7 +36,7 @@ l.str_join{
 
 ## len
 
-The len module returns the length of a list. It has a single required parameter.
+The len function returns the length of a list. It has a single required parameter.
 
 * `list` The list to reverse.
 
@@ -45,6 +45,24 @@ let l = import "std/lists.ucg";
 l.len{list=[0, 1, 2, 3]} == 4;
 ```
 
+## head and tail
+
+The `tail` function returns the tail of a list minus it's head.
+
+```
+let l = import "std/lists.ucg";
+let tail = l.tail([0,1,2,3]);
+tail == [1,2,3];
+```
+
+The `head` function returns the head of the list as a list of one item.
+
+```
+let l = import "std/lists.ucg";
+let hd = l.head([0,1,2,3]);
+tail == [0];
+```
+
 ## enumerate
 
 The enumerate module enumerates the elements of a list. It has three parameters.

std/lists.ucg

@@ -29,6 +29,24 @@ let len = func(list) => reduce(func(acc, item) => acc + 1, 0, list);
 // Reverses the provided list.
 let reverse = func(list) => reduce(func (acc, item) =>  [item] + acc, [], list);
 
+// head returns the first item in a list as a list of one item.
+// This function is safe for empty list inputs but is not safe
+// for NULL inputs.
+let head = func(list) => select len(list) > 0, [], {
+    true = [list.0],
+};
+
+// tail returns the tail of a list without the head.
+// This function is safe for empty lists but is not safe
+// for NULL inputs.
+let tail = func(list) => reduce(
+    func (acc, item) => select acc.count > 0, acc{count=1, tail=[]}, {
+        true = acc{count = acc.count + 1, tail = acc.tail + [item]},
+    },
+    {count=0},
+    list,
+).tail;
+
 // Enumerates the provided list with optional start and step parameters for the
 // enumeration. Prodices a list of pairs with the numeration and the list item.
 //

std/tests/lists_test.ucg

@@ -43,3 +43,13 @@ assert asserts.equal{
     left=list.zip{list1=[0, 1], list2=[3, 4]},
     right=[[0, 3], [1, 4]],
 };
+
+assert asserts.equal{
+    left=list.tail([0,1,2,3,4]),
+    right=[1,2,3,4],
+};
+
+assert asserts.equal{
+    left=list.head([0,1,2,3,4]),
+    right=[0],
+};