Add grains exercise

config.json

@@ -153,6 +153,11 @@
             "slug": "gigasecond",
             "difficulty": 1,
             "topics": []
+        },
+        {
+            "slug": "grains",
+            "difficulty": 1,
+            "topics": []
         }
     ],
     "deprecated": [],

exercises/grains/Grains.elm

@@ -0,0 +1,2 @@
+module Grains exposing (..)
+

exercises/grains/Grains.example.elm

@@ -0,0 +1,9 @@
+module Grains exposing (..)
+
+
+square : Int -> Maybe Int
+square n =
+    if n < 1 then
+        Nothing
+    else
+        Just <| 2 ^ (n - 1)

exercises/grains/Tests.elm

@@ -0,0 +1,39 @@
+port module Main exposing (..)
+
+import Test.Runner.Node exposing (run, TestProgram)
+import Json.Encode exposing (Value)
+import Test exposing (..)
+import Expect
+import Grains exposing (square)
+
+
+tests : Test
+tests =
+    describe "Grains"
+        [ describe "square"
+            [ test "of 1" <|
+                \() -> Expect.equal (Just 1) (square 1)
+            , test "of 2" <|
+                \() -> Expect.equal (Just 2) (square 2)
+            , test "of 3" <|
+                \() -> Expect.equal (Just 4) (square 3)
+            , test "of 4" <|
+                \() -> Expect.equal (Just 8) (square 4)
+            , test "of 16" <|
+                \() -> Expect.equal (Just 32768) (square 16)
+            , test "of 32" <|
+                \() -> Expect.equal (Just 2147483648) (square 32)
+            , test "square 0 raises an exception" <|
+                \() -> Expect.equal Nothing (square 0)
+            , test "negative square raises an exception" <|
+                \() -> Expect.equal Nothing (square -1)
+            ]
+        ]
+
+
+main : TestProgram
+main =
+    run emit tests
+
+
+port emit : ( String, Value ) -> Cmd msg

exercises/grains/elm-package.json

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

exercises/grains/package.json

@@ -0,0 +1,13 @@
+{
+    "description": "Exercism/Elm",
+    "repository": "https://github.com/exercism/xelm.git",
+    "license": "MIT",
+    "scripts": {
+        "postinstall": "elm-package install -y",
+        "test": "elm-test Tests.elm"
+    },
+    "dependencies": {
+        "elm": "^0.18.0",
+        "elm-test": "^0.18.0"
+    }
+}