add pascals-triangle exercise

config.json

@@ -321,6 +321,17 @@
       "topics": [
 
       ]
+    },
+    {
+      "uuid": "a730ca85-057d-db80-e44e-25b8028acede6337cca",
+      "slug": "pascals-triangle",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 4,
+      "topics": [
+        "Lists",
+        "Control-flow"
+      ]
     }
   ],
   "foregone": [

exercises/pascals-triangle/README.md

@@ -0,0 +1,51 @@
+# Pascals Triangle
+
+Compute Pascal's triangle up to a given number of rows.
+
+In Pascal's Triangle each number is computed by adding the numbers to
+the right and left of the current position in the previous row.
+
+```text
+    1
+   1 1
+  1 2 1
+ 1 3 3 1
+1 4 6 4 1
+# ... etc
+```
+
+## Elm Installation
+
+Refer to the [Exercism help page](http://exercism.io/languages/elm) for Elm
+installation and learning resources.
+
+## Writing the Code
+
+The first time you start an exercise, you'll need to ensure you have the
+appropriate dependencies installed.
+
+```bash
+$ npm install
+```
+
+Execute the tests with:
+
+```bash
+$ npm test
+```
+
+Automatically run tests again when you save changes:
+
+```bash
+$ npm run watch
+```
+
+As you work your way through the test suite, be sure to remove the `skip <|`
+calls from each test until you get them all passing!
+
+## Source
+
+Pascal's Triangle at Wolfram Math World [http://mathworld.wolfram.com/PascalsTriangle.html](http://mathworld.wolfram.com/PascalsTriangle.html)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/pascals-triangle/Triangle.elm

@@ -0,0 +1 @@
+module Triangle exposing (..)

exercises/pascals-triangle/Triangle.example.elm

@@ -0,0 +1,23 @@
+module Triangle exposing (rows)
+
+import List
+
+
+nextRow : List Int -> List Int
+nextRow row =
+    List.map2 (+) (0 :: row) (row ++ [ 0 ])
+
+
+rows : Int -> List (List Int)
+rows n =
+    let
+        loop i row =
+            if i == n then
+                []
+            else
+                row :: loop (i + 1) (nextRow row)
+    in
+    if n < 0 then
+        []
+    else
+        loop 0 [ 1 ]

exercises/pascals-triangle/elm-package.json

@@ -0,0 +1,14 @@
+{
+    "version": "3.0.0",
+    "summary": "Exercism problems in Elm.",
+    "repository": "https://github.com/exercism/elm.git",
+    "license": "BSD3",
+    "source-directories": [
+        "."
+    ],
+    "exposed-modules": [],
+    "dependencies": {
+        "elm-lang/core": "5.0.0 <= v < 6.0.0"
+    },
+    "elm-version": "0.18.0 <= v < 0.19.0"
+}

exercises/pascals-triangle/package.json

@@ -0,0 +1,14 @@
+{
+    "description": "Exercism/Elm",
+    "repository": "https://github.com/exercism/elm.git",
+    "license": "MIT",
+    "scripts": {
+        "postinstall": "elm-package install -y",
+        "watch": "elm-test --watch",
+        "test": "elm-test"
+    },
+    "dependencies": {
+        "elm": "0.18.0",
+        "elm-test": "0.18.3"
+    }
+}

exercises/pascals-triangle/tests/Tests.elm

@@ -0,0 +1,34 @@
+module Tests exposing (..)
+
+import Expect
+import Test exposing (..)
+import Triangle exposing (rows)
+
+
+tests : Test
+tests =
+    describe "Triangle"
+        [ test "no rows" <|
+            \() ->
+                Expect.equal [] (rows 0)
+        , skip <|
+            test "single row" <|
+                \() ->
+                    Expect.equal [ [ 1 ] ] (rows 1)
+        , skip <|
+            test "two rows" <|
+                \() ->
+                    Expect.equal [ [ 1 ], [ 1, 1 ] ] (rows 2)
+        , skip <|
+            test "three rows" <|
+                \() ->
+                    Expect.equal [ [ 1 ], [ 1, 1 ], [ 1, 2, 1 ] ] (rows 3)
+        , skip <|
+            test "four rows" <|
+                \() ->
+                    Expect.equal [ [ 1 ], [ 1, 1 ], [ 1, 2, 1 ], [ 1, 3, 3, 1 ] ] (rows 4)
+        , skip <|
+            test "negative rows" <|
+                \() ->
+                    Expect.equal [] (rows -1)
+        ]

exercises/pascals-triangle/tests/elm-package.json

@@ -0,0 +1,16 @@
+{
+    "version": "3.0.0",
+    "summary": "Exercism problems in Elm.",
+    "repository": "https://github.com/exercism/elm.git",
+    "license": "BSD3",
+    "source-directories": [
+        ".",
+        ".."
+    ],
+    "exposed-modules": [],
+    "dependencies": {
+        "elm-lang/core": "5.0.0 <= v < 6.0.0",
+        "elm-community/elm-test": "4.0.0 <= v < 5.0.0"
+    },
+    "elm-version": "0.18.0 <= v < 0.19.0"
+}