From 318df648e88a492d0ddddd06e9b1424e31038f42 Mon Sep 17 00:00:00 2001
From: widmogrod <widmogrod@gmail.com>
Date: Wed, 20 Dec 2017 21:59:09 +0100
Subject: [PATCH] Introduce new functions tail, head, length

---
 src/Functional/listt.php | 133 ++++++++++++++++-------------------------------
 1 file changed, 44 insertions(+), 89 deletions(-)

diff --git a/src/Functional/listt.php b/src/Functional/listt.php
index f757a38..7f2504d 100644
--- a/src/Functional/listt.php
+++ b/src/Functional/listt.php
@@ -91,92 +91,47 @@ function append(Listt $a, Listt $b = null)
     })(...func_get_args());
 }
 
-///**
-// * head :: [a] -> a
-// *
-// * Extract the first element of a list, which must be non-empty.
-// *
-// * @param Listt $l
-// * @return mixed
-// */
-//function head(Listt $l)
-//{
-//    return $l->head();
-//}
-//
-///**
-// * tail :: [a] -> [a]
-// *
-// * Extract the elements after the head of a list, which must be non-empty.
-// *
-// * @param Listt $l
-// * @return Listt
-// */
-//function tail(Listt $l)
-//{
-//    return $l->tail();
-//}
-//
-///**
-// * last :: [a] -> a
-// *
-// * Extract the last element of a list, which must be finite and non-empty.
-// *
-// * @param Listt $l
-// */
-//function last(Listt $l)
-//{
-//    // TODO
-//}
-//
-///**
-// * init :: [a] -> [a]
-// *
-// * Return all the elements of a list except the last one. The list must be non-empty.
-// *
-// * @param Listt $l
-// */
-//function init(Listt $l)
-//{
-//    // TODO
-//}
-//
-//
-///**
-// * length :: Foldable t => t a -> Int
-// *
-// * Returns the size/length of a finite structure as an Int.
-// * The default implementation is optimized for structures that are similar to cons-lists,
-// * because there is no general way to do better.
-// *
-// * @param Listt $l
-// * @return int
-// */
-//function length(Listt $l): int
-//{
-//    // TODO
-//}
-//
-///**
-// * (!!) :: [a] -> Int -> a infixl 9
-// *
-// * List index (subscript) operator, starting from 0. It is an instance of the more general genericIndex, which takes an index of any integral type.
-// *
-// * @param Listt $L
-// */
-//function index(Listt $L, int $index)
-//{
-//    // TODO
-//}
-//
-///**
-// * reverse :: [a] -> [a]
-// *
-// * reverse xs returns the elements of xs in reverse order. xs must be finite.
-// *
-// * @param Listt $l
-// */
-//function reverse(Listt $l)
-//{
-//    // TODO
-//}
+/**
+ * head :: [a] -> a
+ *
+ * Extract the first element of a list, which must be non-empty.
+ *
+ * @param Listt $l
+ * @return mixed
+ * @throws \Widmogrod\Primitive\EmptyListError
+ */
+function head(Listt $l)
+{
+    return $l->head();
+}
+
+/**
+ * tail :: [a] -> [a]
+ *
+ * Extract the elements after the head of a list, which must be non-empty.
+ *
+ * @param Listt $l
+ * @return Listt
+ * @throws \Widmogrod\Primitive\EmptyListError
+ */
+function tail(Listt $l)
+{
+    return $l->tail();
+}
+
+/**
+ * length :: Foldable t => t a -> Int
+ *
+ * Returns the size/length of a finite structure as an Int.
+ * The default implementation is optimized for structures that are similar to cons-lists,
+ * because there is no general way to do better.
+ *
+ * @param Foldable $t
+ * @return int
+ */
+function length(Foldable $t): int
+{
+    return $t->reduce(function ($len) {
+        return $len + 1;
+    }, 0);
+}
-- 
2.11.0