From 7ae6955066801a4248bf6bcc516d2398ec212179 Mon Sep 17 00:00:00 2001
From: Jeremy Wall <jeremy@marzhillstudios.com>
Date: Mon, 25 Feb 2019 20:24:24 -0600
Subject: [PATCH] FEATURE: Add head and tail functions to our list library.

---
 docsite/site/content/stdlib/lists.md | 20 +++++++++++++++++++-
 std/lists.ucg                        | 18 ++++++++++++++++++
 std/tests/lists_test.ucg             | 10 ++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/docsite/site/content/stdlib/lists.md b/docsite/site/content/stdlib/lists.md
index ed02de0..f4d0c41 100644
--- a/docsite/site/content/stdlib/lists.md
+++ b/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.
diff --git a/std/lists.ucg b/std/lists.ucg
index 41342fe..a58cd7c 100644
--- a/std/lists.ucg
+++ b/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.
 //
diff --git a/std/tests/lists_test.ucg b/std/tests/lists_test.ucg
index ad6431c..2339524 100644
--- a/std/tests/lists_test.ucg
+++ b/std/tests/lists_test.ucg
@@ -42,4 +42,14 @@ assert asserts.equal{
 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],
 };
\ No newline at end of file