From 5fbe12129d3df1671b98ab2b6e914f3ff8741374 Mon Sep 17 00:00:00 2001
From: Emmanuel Delaborde <emmanuel.delaborde@modicagroup.com>
Date: Fri, 4 Oct 2019 13:03:21 +1300
Subject: [PATCH 1/3] implement binary-search-tree exercise

---
 config.json                                   | 12 ++++
 exercises/binary-search-tree/Makefile         |  9 +++
 exercises/binary-search-tree/README.md        | 60 +++++++++++++++++++
 .../binary-search-tree/binary_search_tree.ml  | 16 +++++
 .../binary-search-tree/binary_search_tree.mli | 17 ++++++
 exercises/binary-search-tree/dune             | 16 +++++
 exercises/binary-search-tree/dune-project     |  2 +
 exercises/binary-search-tree/example.ml       | 26 ++++++++
 exercises/binary-search-tree/test.ml          | 58 ++++++++++++++++++
 9 files changed, 216 insertions(+)
 create mode 100644 exercises/binary-search-tree/Makefile
 create mode 100644 exercises/binary-search-tree/README.md
 create mode 100644 exercises/binary-search-tree/binary_search_tree.ml
 create mode 100644 exercises/binary-search-tree/binary_search_tree.mli
 create mode 100644 exercises/binary-search-tree/dune
 create mode 100644 exercises/binary-search-tree/dune-project
 create mode 100644 exercises/binary-search-tree/example.ml
 create mode 100644 exercises/binary-search-tree/test.ml

diff --git a/config.json b/config.json
index 7ff2143f6f..292ca14978 100644
--- a/config.json
+++ b/config.json
@@ -447,6 +447,18 @@
       "difficulty": 0,
       "topics": null,
       "deprecated": true
+    },
+    {
+      "slug": "binary-search-tree",
+      "uuid": "ce05137f-d4e5-40f2-ae6b-6c43e00d458f",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 3,
+      "topics": [
+        "recursion",
+        "searching",
+        "trees"
+      ]
     }
   ]
 }
diff --git a/exercises/binary-search-tree/Makefile b/exercises/binary-search-tree/Makefile
new file mode 100644
index 0000000000..b71d6af2de
--- /dev/null
+++ b/exercises/binary-search-tree/Makefile
@@ -0,0 +1,9 @@
+default: clean test
+
+test: 
+	dune runtest
+
+clean:
+	dune clean
+
+.PHONY: clean
diff --git a/exercises/binary-search-tree/README.md b/exercises/binary-search-tree/README.md
new file mode 100644
index 0000000000..01d4572a16
--- /dev/null
+++ b/exercises/binary-search-tree/README.md
@@ -0,0 +1,60 @@
+# Binary Search Tree
+
+Insert and search for numbers in a binary tree.
+
+When we need to represent sorted data, an array does not make a good
+data structure.
+
+Say we have the array `[1, 3, 4, 5]`, and we add 2 to it so it becomes
+`[1, 3, 4, 5, 2]` now we must sort the entire array again! We can
+improve on this by realizing that we only need to make space for the new
+item `[1, nil, 3, 4, 5]`, and then adding the item in the space we
+added. But this still requires us to shift many elements down by one.
+
+Binary Search Trees, however, can operate on sorted data much more
+efficiently.
+
+A binary search tree consists of a series of connected nodes. Each node
+contains a piece of data (e.g. the number 3), a variable named `left`,
+and a variable named `right`. The `left` and `right` variables point at
+`nil`, or other nodes. Since these other nodes in turn have other nodes
+beneath them, we say that the left and right variables are pointing at
+subtrees. All data in the left subtree is less than or equal to the
+current node's data, and all data in the right subtree is greater than
+the current node's data.
+
+For example, if we had a node containing the data 4, and we added the
+data 2, our tree would look like this:
+
+      4
+     /
+    2
+
+If we then added 6, it would look like this:
+
+      4
+     / \
+    2   6
+
+If we then added 3, it would look like this
+
+       4
+     /   \
+    2     6
+     \
+      3
+
+And if we then added 1, 5, and 7, it would look like this
+
+          4
+        /   \
+       /     \
+      2       6
+     / \     / \
+    1   3   5   7
+## Source
+
+Josh Cheek [https://twitter.com/josh_cheek](https://twitter.com/josh_cheek)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.
\ No newline at end of file
diff --git a/exercises/binary-search-tree/binary_search_tree.ml b/exercises/binary-search-tree/binary_search_tree.ml
new file mode 100644
index 0000000000..ce27643a7a
--- /dev/null
+++ b/exercises/binary-search-tree/binary_search_tree.ml
@@ -0,0 +1,16 @@
+open Base
+
+type bst
+
+let empty = failwith "'empty' is missing"
+
+let value _ = failwith "'empty' is missing"
+
+let left _ = failwith "'empty' is missing"
+
+let right _ = failwith "'empty' is missing"
+
+let insert _ _ = failwith "'empty' is missing"
+
+let to_list _ = failwith "'empty' is missing"
+
diff --git a/exercises/binary-search-tree/binary_search_tree.mli b/exercises/binary-search-tree/binary_search_tree.mli
new file mode 100644
index 0000000000..b83b66560e
--- /dev/null
+++ b/exercises/binary-search-tree/binary_search_tree.mli
@@ -0,0 +1,17 @@
+open Base
+
+type bst
+
+val empty : bst
+
+val value : bst -> (int, string) Result.t
+
+val left : bst -> (bst, string) Result.t
+
+val right : bst -> (bst, string) Result.t
+
+val insert : int -> bst -> bst
+
+val to_list : bst -> int list
+
+
diff --git a/exercises/binary-search-tree/dune b/exercises/binary-search-tree/dune
new file mode 100644
index 0000000000..3d0b594af7
--- /dev/null
+++ b/exercises/binary-search-tree/dune
@@ -0,0 +1,16 @@
+(executable
+ (name test)
+ (libraries base oUnit))
+
+(alias
+  (name    runtest)
+  (deps    (:x test.exe))
+  (action  (run %{x})))
+
+(alias
+  (name    buildtest)
+  (deps    (:x test.exe)))
+
+(env
+  (dev
+    (flags (:standard -warn-error -A))))
\ No newline at end of file
diff --git a/exercises/binary-search-tree/dune-project b/exercises/binary-search-tree/dune-project
new file mode 100644
index 0000000000..f4f8feafff
--- /dev/null
+++ b/exercises/binary-search-tree/dune-project
@@ -0,0 +1,2 @@
+(lang dune 1.1)
+(version 1.3.0)
diff --git a/exercises/binary-search-tree/example.ml b/exercises/binary-search-tree/example.ml
new file mode 100644
index 0000000000..3f784f8584
--- /dev/null
+++ b/exercises/binary-search-tree/example.ml
@@ -0,0 +1,26 @@
+open Base
+
+type 'a bst = Leaf | Node of 'a bst * 'a * 'a bst
+
+let empty = Leaf
+
+let value = function
+  | Leaf -> Error "empty tree"
+  | Node(_, v, _) -> v
+
+let left = function
+  | Leaf -> Error "empty tree"
+  | Node(l, _, _) -> l
+
+let right = function
+  | Leaf -> Error "empty tree"
+  | Node(_, _, r) -> r
+
+let insert v = function
+  | Leaf -> Node(Leaf, v, Leaf)
+  | Node(l, v', r) when v <= v' -> Node(insert v l, v', r)
+  | Node(l, v', r) -> Node(l, v', insert v r)
+
+let to_list = function
+| Leaf -> []
+| Node(l, v, r) -> to_list(l) @ [v] @ to_list(r)
\ No newline at end of file
diff --git a/exercises/binary-search-tree/test.ml b/exercises/binary-search-tree/test.ml
new file mode 100644
index 0000000000..d43b1e947b
--- /dev/null
+++ b/exercises/binary-search-tree/test.ml
@@ -0,0 +1,58 @@
+open Base
+open OUnit2
+open Binary_search_tree
+
+let result_to_string f = function
+  | Error m -> Printf.sprintf "Error \"%s\"" m
+  | Ok x -> f x |> Printf.sprintf "Some %s"
+
+let ae exp got _test_ctxt =
+  assert_equal ~printer:(result_to_string Int.to_string) exp got
+
+let intlist_to_string l = 
+  List.map l ~f:Int.to_string
+  |> List.intersperse ~sep:"; "
+  |> List.fold ~init:"" ~f:(^) 
+  |> fun s -> "[" ^ s ^ "]"
+
+let ael exp got _test_ctxt =
+  assert_equal ~printer:intlist_to_string exp got
+
+let tests = 
+  let t4 = empty |> insert 4 in 
+  let t42 = t4 |> insert 2 in  
+  let l2 =  t42 |> left in  
+  let t44 = t4 |> insert 4 in 
+  let l4 =  t44 |> left in 
+  let t45 = t4 |> insert 5 in
+  let r5 = t45 |> right in  
+  let t4261357 = t42 |> insert 6 |> insert 1 |> insert 3 |> insert 5 |> insert 7 in
+  let t2 = empty |> insert 2 in
+  let t21 = t2 |> insert 1 in
+  let t22 = t2 |> insert 2 in
+  let t23 = t2 |> insert 3 in
+  let t213675 = t21 |> insert 3 |> insert 6 |> insert 7 |> insert 5 in
+  [
+    "data is retained" >:: ae (Ok 4) (value t4);
+    "smaller number at left node 1" >:: ae (Ok 4) (value t42);
+    "smaller number at left node 2" >:: ae (Ok 2) (Result.bind l2 ~f:value);
+    "same number at left node 1" >:: ae (Ok 4) (value t44);
+    "same number at left node 2" >:: ae (Ok 4) (Result.bind l4 ~f:value);
+    "greater number at right node 1" >:: ae (Ok 4) (value t45);
+    "greater number at right node 2" >:: ae (Ok 5) (Result.bind r5 ~f:value);
+    "can create complex tree 1" >:: ae (Ok 4) (value t4261357);
+    "can create complex tree 2" >:: ae (Ok 2) (Result.bind (t4261357 |> left) ~f:value);
+    "can create complex tree 3" >:: ae (Ok 1) (Result.bind (Result.bind (t4261357 |> left) ~f:left) ~f:value);
+    "can create complex tree 4" >:: ae (Ok 3) (Result.bind (Result.bind (t4261357 |> left) ~f:right) ~f:value);
+    "can create complex tree 5" >:: ae (Ok 6) (Result.bind (t4261357 |> right) ~f:value);
+    "can create complex tree 6" >:: ae (Ok 5) (Result.bind (Result.bind (t4261357 |> right) ~f:left) ~f:value);
+    "can create complex tree 7" >:: ae (Ok 7) (Result.bind (Result.bind (t4261357 |> right) ~f:right) ~f:value);
+    "can sort single number" >:: ael [2] (to_list t2);
+    "can sort if second number is smaller than first" >:: ael [1;2] (to_list t21);
+    "can sort if second number is same as first" >:: ael [2;2] (to_list t22);
+    "can sort if second number is greater than first" >:: ael [2;3] (to_list t23);
+    "can sort complex tree" >:: ael [1; 2; 3; 5; 6; 7] (to_list t213675);
+  ]
+
+let () =
+  run_test_tt_main ("binary-search-tree tests" >::: tests)

From d50f903c98939ce70c9755d00efc17df89e87f5f Mon Sep 17 00:00:00 2001
From: Emmanuel Delaborde <emmanuel.delaborde@modicagroup.com>
Date: Sat, 5 Oct 2019 20:26:55 +1300
Subject: [PATCH 2/3] implement linked-list exercise

---
 config.json                                   | 16 +--
 exercises/binary-search-tree/README.md        | 60 ------------
 .../binary-search-tree/binary_search_tree.ml  | 16 ---
 .../binary-search-tree/binary_search_tree.mli | 17 ----
 exercises/binary-search-tree/example.ml       | 26 -----
 exercises/binary-search-tree/test.ml          | 58 -----------
 .../Makefile                                  |  0
 exercises/linked-list/README.md               | 29 ++++++
 .../{binary-search-tree => linked-list}/dune  |  0
 .../dune-project                              |  0
 exercises/linked-list/example.ml              | 48 +++++++++
 exercises/linked-list/linked_list.ml          | 15 +++
 exercises/linked-list/linked_list.mli         | 15 +++
 exercises/linked-list/test.ml                 | 97 +++++++++++++++++++
 14 files changed, 212 insertions(+), 185 deletions(-)
 delete mode 100644 exercises/binary-search-tree/README.md
 delete mode 100644 exercises/binary-search-tree/binary_search_tree.ml
 delete mode 100644 exercises/binary-search-tree/binary_search_tree.mli
 delete mode 100644 exercises/binary-search-tree/example.ml
 delete mode 100644 exercises/binary-search-tree/test.ml
 rename exercises/{binary-search-tree => linked-list}/Makefile (100%)
 create mode 100644 exercises/linked-list/README.md
 rename exercises/{binary-search-tree => linked-list}/dune (100%)
 rename exercises/{binary-search-tree => linked-list}/dune-project (100%)
 create mode 100644 exercises/linked-list/example.ml
 create mode 100644 exercises/linked-list/linked_list.ml
 create mode 100644 exercises/linked-list/linked_list.mli
 create mode 100644 exercises/linked-list/test.ml

diff --git a/config.json b/config.json
index 292ca14978..3ce9dae714 100644
--- a/config.json
+++ b/config.json
@@ -449,16 +449,16 @@
       "deprecated": true
     },
     {
-      "slug": "binary-search-tree",
-      "uuid": "ce05137f-d4e5-40f2-ae6b-6c43e00d458f",
+      "slug": "linked-list",
+      "uuid": "840f4011-029a-4f5c-970f-f8e585cd4fdf",
       "core": false,
-      "unlocked_by": null,
-      "difficulty": 3,
+      "unlocked_by": "null",
+      "difficulty": 8,
       "topics": [
-        "recursion",
-        "searching",
-        "trees"
+        "data_structures",
+        "lists",
+        "recursion"
       ]
     }
   ]
-}
+}
\ No newline at end of file
diff --git a/exercises/binary-search-tree/README.md b/exercises/binary-search-tree/README.md
deleted file mode 100644
index 01d4572a16..0000000000
--- a/exercises/binary-search-tree/README.md
+++ /dev/null
@@ -1,60 +0,0 @@
-# Binary Search Tree
-
-Insert and search for numbers in a binary tree.
-
-When we need to represent sorted data, an array does not make a good
-data structure.
-
-Say we have the array `[1, 3, 4, 5]`, and we add 2 to it so it becomes
-`[1, 3, 4, 5, 2]` now we must sort the entire array again! We can
-improve on this by realizing that we only need to make space for the new
-item `[1, nil, 3, 4, 5]`, and then adding the item in the space we
-added. But this still requires us to shift many elements down by one.
-
-Binary Search Trees, however, can operate on sorted data much more
-efficiently.
-
-A binary search tree consists of a series of connected nodes. Each node
-contains a piece of data (e.g. the number 3), a variable named `left`,
-and a variable named `right`. The `left` and `right` variables point at
-`nil`, or other nodes. Since these other nodes in turn have other nodes
-beneath them, we say that the left and right variables are pointing at
-subtrees. All data in the left subtree is less than or equal to the
-current node's data, and all data in the right subtree is greater than
-the current node's data.
-
-For example, if we had a node containing the data 4, and we added the
-data 2, our tree would look like this:
-
-      4
-     /
-    2
-
-If we then added 6, it would look like this:
-
-      4
-     / \
-    2   6
-
-If we then added 3, it would look like this
-
-       4
-     /   \
-    2     6
-     \
-      3
-
-And if we then added 1, 5, and 7, it would look like this
-
-          4
-        /   \
-       /     \
-      2       6
-     / \     / \
-    1   3   5   7
-## Source
-
-Josh Cheek [https://twitter.com/josh_cheek](https://twitter.com/josh_cheek)
-
-## Submitting Incomplete Solutions
-It's possible to submit an incomplete solution so you can see how others have completed the exercise.
\ No newline at end of file
diff --git a/exercises/binary-search-tree/binary_search_tree.ml b/exercises/binary-search-tree/binary_search_tree.ml
deleted file mode 100644
index ce27643a7a..0000000000
--- a/exercises/binary-search-tree/binary_search_tree.ml
+++ /dev/null
@@ -1,16 +0,0 @@
-open Base
-
-type bst
-
-let empty = failwith "'empty' is missing"
-
-let value _ = failwith "'empty' is missing"
-
-let left _ = failwith "'empty' is missing"
-
-let right _ = failwith "'empty' is missing"
-
-let insert _ _ = failwith "'empty' is missing"
-
-let to_list _ = failwith "'empty' is missing"
-
diff --git a/exercises/binary-search-tree/binary_search_tree.mli b/exercises/binary-search-tree/binary_search_tree.mli
deleted file mode 100644
index b83b66560e..0000000000
--- a/exercises/binary-search-tree/binary_search_tree.mli
+++ /dev/null
@@ -1,17 +0,0 @@
-open Base
-
-type bst
-
-val empty : bst
-
-val value : bst -> (int, string) Result.t
-
-val left : bst -> (bst, string) Result.t
-
-val right : bst -> (bst, string) Result.t
-
-val insert : int -> bst -> bst
-
-val to_list : bst -> int list
-
-
diff --git a/exercises/binary-search-tree/example.ml b/exercises/binary-search-tree/example.ml
deleted file mode 100644
index 3f784f8584..0000000000
--- a/exercises/binary-search-tree/example.ml
+++ /dev/null
@@ -1,26 +0,0 @@
-open Base
-
-type 'a bst = Leaf | Node of 'a bst * 'a * 'a bst
-
-let empty = Leaf
-
-let value = function
-  | Leaf -> Error "empty tree"
-  | Node(_, v, _) -> v
-
-let left = function
-  | Leaf -> Error "empty tree"
-  | Node(l, _, _) -> l
-
-let right = function
-  | Leaf -> Error "empty tree"
-  | Node(_, _, r) -> r
-
-let insert v = function
-  | Leaf -> Node(Leaf, v, Leaf)
-  | Node(l, v', r) when v <= v' -> Node(insert v l, v', r)
-  | Node(l, v', r) -> Node(l, v', insert v r)
-
-let to_list = function
-| Leaf -> []
-| Node(l, v, r) -> to_list(l) @ [v] @ to_list(r)
\ No newline at end of file
diff --git a/exercises/binary-search-tree/test.ml b/exercises/binary-search-tree/test.ml
deleted file mode 100644
index d43b1e947b..0000000000
--- a/exercises/binary-search-tree/test.ml
+++ /dev/null
@@ -1,58 +0,0 @@
-open Base
-open OUnit2
-open Binary_search_tree
-
-let result_to_string f = function
-  | Error m -> Printf.sprintf "Error \"%s\"" m
-  | Ok x -> f x |> Printf.sprintf "Some %s"
-
-let ae exp got _test_ctxt =
-  assert_equal ~printer:(result_to_string Int.to_string) exp got
-
-let intlist_to_string l = 
-  List.map l ~f:Int.to_string
-  |> List.intersperse ~sep:"; "
-  |> List.fold ~init:"" ~f:(^) 
-  |> fun s -> "[" ^ s ^ "]"
-
-let ael exp got _test_ctxt =
-  assert_equal ~printer:intlist_to_string exp got
-
-let tests = 
-  let t4 = empty |> insert 4 in 
-  let t42 = t4 |> insert 2 in  
-  let l2 =  t42 |> left in  
-  let t44 = t4 |> insert 4 in 
-  let l4 =  t44 |> left in 
-  let t45 = t4 |> insert 5 in
-  let r5 = t45 |> right in  
-  let t4261357 = t42 |> insert 6 |> insert 1 |> insert 3 |> insert 5 |> insert 7 in
-  let t2 = empty |> insert 2 in
-  let t21 = t2 |> insert 1 in
-  let t22 = t2 |> insert 2 in
-  let t23 = t2 |> insert 3 in
-  let t213675 = t21 |> insert 3 |> insert 6 |> insert 7 |> insert 5 in
-  [
-    "data is retained" >:: ae (Ok 4) (value t4);
-    "smaller number at left node 1" >:: ae (Ok 4) (value t42);
-    "smaller number at left node 2" >:: ae (Ok 2) (Result.bind l2 ~f:value);
-    "same number at left node 1" >:: ae (Ok 4) (value t44);
-    "same number at left node 2" >:: ae (Ok 4) (Result.bind l4 ~f:value);
-    "greater number at right node 1" >:: ae (Ok 4) (value t45);
-    "greater number at right node 2" >:: ae (Ok 5) (Result.bind r5 ~f:value);
-    "can create complex tree 1" >:: ae (Ok 4) (value t4261357);
-    "can create complex tree 2" >:: ae (Ok 2) (Result.bind (t4261357 |> left) ~f:value);
-    "can create complex tree 3" >:: ae (Ok 1) (Result.bind (Result.bind (t4261357 |> left) ~f:left) ~f:value);
-    "can create complex tree 4" >:: ae (Ok 3) (Result.bind (Result.bind (t4261357 |> left) ~f:right) ~f:value);
-    "can create complex tree 5" >:: ae (Ok 6) (Result.bind (t4261357 |> right) ~f:value);
-    "can create complex tree 6" >:: ae (Ok 5) (Result.bind (Result.bind (t4261357 |> right) ~f:left) ~f:value);
-    "can create complex tree 7" >:: ae (Ok 7) (Result.bind (Result.bind (t4261357 |> right) ~f:right) ~f:value);
-    "can sort single number" >:: ael [2] (to_list t2);
-    "can sort if second number is smaller than first" >:: ael [1;2] (to_list t21);
-    "can sort if second number is same as first" >:: ael [2;2] (to_list t22);
-    "can sort if second number is greater than first" >:: ael [2;3] (to_list t23);
-    "can sort complex tree" >:: ael [1; 2; 3; 5; 6; 7] (to_list t213675);
-  ]
-
-let () =
-  run_test_tt_main ("binary-search-tree tests" >::: tests)
diff --git a/exercises/binary-search-tree/Makefile b/exercises/linked-list/Makefile
similarity index 100%
rename from exercises/binary-search-tree/Makefile
rename to exercises/linked-list/Makefile
diff --git a/exercises/linked-list/README.md b/exercises/linked-list/README.md
new file mode 100644
index 0000000000..137ef7aa39
--- /dev/null
+++ b/exercises/linked-list/README.md
@@ -0,0 +1,29 @@
+# Linked List
+
+Implement a doubly linked list.
+
+Like an array, a linked list is a simple linear data structure. Several
+common data types can be implemented using linked lists, like queues,
+stacks, and associative arrays.
+
+A linked list is a collection of data elements called *nodes*. In a
+*singly linked list* each node holds a value and a link to the next node.
+In a *doubly linked list* each node also holds a link to the previous
+node.
+
+You will write an implementation of a doubly linked list. Implement a
+Node to hold a value and pointers to the next and previous nodes. Then
+implement a List which holds references to the first and last node and
+offers an array-like interface for adding and removing items:
+
+* `push` (*insert value at back*);
+* `pop` (*remove value at back*);
+* `shift` (*remove value at front*).
+* `unshift` (*insert value at front*);
+* `count` (*count elements*);
+
+To keep your implementation simple, the tests will not cover error
+conditions. Specifically: `pop` or `shift` will never be called on an
+empty list.
+
+If you want to know more about linked lists, check [Wikipedia](https://en.wikipedia.org/wiki/Linked_list).
\ No newline at end of file
diff --git a/exercises/binary-search-tree/dune b/exercises/linked-list/dune
similarity index 100%
rename from exercises/binary-search-tree/dune
rename to exercises/linked-list/dune
diff --git a/exercises/binary-search-tree/dune-project b/exercises/linked-list/dune-project
similarity index 100%
rename from exercises/binary-search-tree/dune-project
rename to exercises/linked-list/dune-project
diff --git a/exercises/linked-list/example.ml b/exercises/linked-list/example.ml
new file mode 100644
index 0000000000..67b734add4
--- /dev/null
+++ b/exercises/linked-list/example.ml
@@ -0,0 +1,48 @@
+open Base
+
+type 'a node = 
+  | Empty
+  | Node of { value: 'a; mutable prev: 'a node; mutable next: 'a node}
+and 'a linked_list = { mutable first: 'a node; mutable last:'a node}
+
+let empty () = {first = Empty; last = Empty}
+
+let push v l = 
+    let elt = Node { value = v; prev = l.last; next = Empty } in ( 
+      match l.last with
+      | Empty -> l.first <- elt
+      | Node n -> n.next <- elt
+    ); 
+    l.last <- elt
+
+let pop = function
+  | {last = Empty; _} -> failwith "empty list!"
+  | {last = Node {value; prev; _}; _} as l -> (
+      match prev with 
+      | Empty -> (l.last <- Empty; l.first <- Empty)
+      | Node n -> (l.last <- prev; n.next <- Empty)
+    );
+    value
+
+let shift = function
+  | {first = Empty; _} -> failwith "empty list!"
+  | {first = Node {value; next; _}; _} as l -> (
+    match next with 
+    | Empty -> (l.first <- Empty; l.last <- Empty)
+    | Node n -> (l.first <- next; n.prev <- Empty)
+  );
+  value
+  
+let unshift v l =
+  let elt = Node { value = v; prev = Empty; next = l.first } in ( 
+    match l.first with
+    | Empty -> l.last <- elt
+    | Node n -> n.prev <- elt
+  ); 
+  l.first <- elt
+
+let count l = 
+  let rec count_node acc = function
+  | Empty -> acc
+  | Node {next = n; _} -> count_node (1 + acc) n
+  in count_node 0 l.first
\ No newline at end of file
diff --git a/exercises/linked-list/linked_list.ml b/exercises/linked-list/linked_list.ml
new file mode 100644
index 0000000000..e40fd6f129
--- /dev/null
+++ b/exercises/linked-list/linked_list.ml
@@ -0,0 +1,15 @@
+open Base
+
+type 'a linked_list
+
+let empty _ = failwith "'empty' is missing"
+
+let push _ _ = failwith "'push' is missing"
+
+let pop _ = failwith "'pop' is missing"
+
+let shift _ = failwith "'shift' is missing"
+
+let unshift _ = failwith "'unshift' is missing"
+
+let count _ = failwith "'count' is missing"
diff --git a/exercises/linked-list/linked_list.mli b/exercises/linked-list/linked_list.mli
new file mode 100644
index 0000000000..976f151c6c
--- /dev/null
+++ b/exercises/linked-list/linked_list.mli
@@ -0,0 +1,15 @@
+open Base
+
+type 'a linked_list
+
+val empty : unit -> 'a linked_list
+
+val push : 'a -> 'a linked_list -> unit
+
+val pop : 'a linked_list -> 'a
+
+val shift : 'a linked_list -> 'a
+
+val unshift : 'a -> 'a linked_list -> unit
+
+val count : 'a linked_list -> int
\ No newline at end of file
diff --git a/exercises/linked-list/test.ml b/exercises/linked-list/test.ml
new file mode 100644
index 0000000000..0b8cd6f30b
--- /dev/null
+++ b/exercises/linked-list/test.ml
@@ -0,0 +1,97 @@
+open Base
+open OUnit2
+open Linked_list
+
+let ae exp got _test_ctxt =
+  assert_equal ~printer:Int.to_string exp got
+
+let test1 = 
+  let l = empty () in
+  push 10 l;
+  push 20 l;
+  let v1 = pop l in
+  let v2 = pop l in 
+  ["add/extract elements to the end of the list with push/pop" >:: ae 20 v1;
+  "add/extract elements to the end of the list with push/pop" >:: ae 10 v2]
+
+let test2 = 
+  let l = empty () in
+  push 10 l;
+  push 20 l;
+  let v1 = shift l in
+  let v2 = shift l in 
+  ["extract elements from the beginning of the list with shift" >:: ae 10 v1;
+  "extract elements from the beginning of the list with shift" >:: ae 20 v2]
+
+let test3 = 
+  let l = empty () in
+  unshift 10 l;
+  unshift 20 l;
+  let v1 = shift l in
+  let v2 = shift l in 
+  ["add/extract elements from the beginning of the list with unshift/shift" >:: ae 20 v1;
+  "add/extract elements from the beginning of the list with unshift/shift" >:: ae 10 v2]
+
+let test4 = 
+  let l = empty () in
+  unshift 10 l;
+  unshift 20 l;
+  let v1 = pop l in
+  let v2 = pop l in 
+  ["add/extract elements from the beginning of the list with unshift/shift" >:: ae 10 v1;
+  "add/extract elements from the beginning of the list with unshift/shift" >:: ae 20 v2]
+
+let test5 = 
+  let l = empty () in
+  push 10 l;
+  push 20 l;
+  let v1 = pop l in
+  push 30 l;
+  let v2 = shift l in 
+  unshift 40 l;
+  push 50 l;
+  let v3 = shift l in
+  let v4 = pop l in
+  let v5 = shift l in
+  ["example" >:: ae 20 v1;
+  "example" >:: ae 10 v2;
+  "example" >:: ae 40 v3;
+  "example" >:: ae 50 v4;
+  "example" >:: ae 30 v5]
+
+let test6 =
+  let l = empty () in
+  let v1 = count l in
+  push 10 l;
+  let v2 = count l in
+  push 20 l;
+  let v3 = count l in
+  ["can count its elements" >:: ae 0 v1;
+  "can count its elements" >:: ae 1 v2;
+  "can count its elements" >:: ae 2 v3]
+
+let test7 = 
+  let l = empty () in
+  push 10 l;
+  let _ = pop l in
+  unshift 20 l;
+  let v1 = count l in
+  let v2 = pop l in
+  ["sets head/tail after popping last element" >:: ae 1 v1;
+  "sets head/tail after popping last element" >:: ae 20 v2]
+
+let test8 = 
+  let l = empty () in
+  unshift 10 l;
+  let _ = shift l in
+  push 20 l;
+  let v1 = count l in
+  let v2 = shift l in
+  ["sets head/tail after shifting last element" >:: ae 1 v1;
+  "sets head/tail after shifting last element" >:: ae 20 v2]
+
+let tests = 
+  test1 @ test2 @ test3 @ test4 @ test5 @ test6 @ test7 @ test8 
+
+let () =
+  run_test_tt_main ("linked-list tests" >::: tests)

From 6cc34fcc023b1baecf8b84e966ccb12f39a1c0dc Mon Sep 17 00:00:00 2001
From: voila <th3rac25@gmail.com>
Date: Fri, 18 Oct 2019 21:53:32 +1300
Subject: [PATCH 3/3] Update exercises/linked-list/dune-project

Co-Authored-By: Simon Shine <shreddedglory@gmail.com>
---
 exercises/linked-list/dune-project | 1 -
 1 file changed, 1 deletion(-)

diff --git a/exercises/linked-list/dune-project b/exercises/linked-list/dune-project
index f4f8feafff..7655de0773 100644
--- a/exercises/linked-list/dune-project
+++ b/exercises/linked-list/dune-project
@@ -1,2 +1 @@
 (lang dune 1.1)
-(version 1.3.0)