make getCanvasElementById safe, fixes #2

Gruntfile.js

@@ -9,7 +9,7 @@ module.exports = function(grunt) {
             },
             main: {
                 files: {
-                    src: ["src/**/*.purs"]
+                    src: ["src/**/*.purs", "bower_components/**/src/**/*.purs"]
                 }
             },
         },

README.md

@@ -4,7 +4,7 @@
 
 ### Types
 
-    type Arc  = { end :: Number, start :: Number, r :: Number, y :: Number, x :: Number }
+    type Arc = { end :: Number, start :: Number, r :: Number, y :: Number, x :: Number }
 
     data Canvas :: !
 
@@ -27,7 +27,7 @@
 
     data Context2D :: *
 
-    type Dimensions  = { height :: Number, width :: Number }
+    type Dimensions = { height :: Number, width :: Number }
 
     data ImageData :: *
 
@@ -36,15 +36,15 @@
       Square :: LineCap
       Butt :: LineCap
 
-    type Rectangle  = { h :: Number, w :: Number, y :: Number, x :: Number }
+    type Rectangle = { h :: Number, w :: Number, y :: Number, x :: Number }
 
-    type ScaleTransform  = { scaleY :: Number, scaleX :: Number }
+    type ScaleTransform = { scaleY :: Number, scaleX :: Number }
 
-    type TextMetrics  = { width :: Number }
+    type TextMetrics = { width :: Number }
 
-    type Transform  = { m32 :: Number, m31 :: Number, m22 :: Number, m21 :: Number, m12 :: Number, m11 :: Number }
+    type Transform = { m32 :: Number, m31 :: Number, m22 :: Number, m21 :: Number, m12 :: Number, m11 :: Number }
 
-    type TranslateTransform  = { translateY :: Number, translateX :: Number }
+    type TranslateTransform = { translateY :: Number, translateX :: Number }
 
 
 ### Type Class Instances
@@ -82,7 +82,9 @@
 
     getCanvasDimensions :: forall eff. CanvasElement -> Eff (canvas :: Canvas | eff) Dimensions
 
-    getCanvasElementById :: forall eff. String -> Eff (canvas :: Canvas | eff) CanvasElement
+    getCanvasElementById :: forall eff. String -> Eff (canvas :: Canvas | eff) (Maybe CanvasElement)
+
+    getCanvasElementByIdImpl :: forall a eff. Fn3 String (a -> Maybe a) (Maybe a) (Eff (canvas :: Canvas | eff) (Maybe CanvasElement))
 
     getCanvasHeight :: forall eff. CanvasElement -> Eff (canvas :: Canvas | eff) Number
 

bower.json

@@ -14,5 +14,8 @@
     "bower_components",
     "test",
     "tests"
-  ]
+  ],
+  "dependencies": {
+    "purescript-maybe": "~0.2.1"
+  }
 }

src/Graphics/Canvas.purs

@@ -1,5 +1,7 @@
 module Graphics.Canvas where
 
+import Data.Function
+import Data.Maybe
 import Control.Monad.Eff
 
 foreign import data Canvas :: !
@@ -12,12 +14,27 @@ foreign import data ImageData :: *
 
 foreign import data CanvasPixelArray :: *
 
-foreign import getCanvasElementById
-  "function getCanvasElementById(id) {\
-  \  return function() {\
-  \    return document.getElementById(id);\
-  \  };\
-  \}" :: forall eff. String -> Eff (canvas :: Canvas | eff) CanvasElement 
+foreign import getCanvasElementByIdImpl
+  """function getCanvasElementByIdImpl(id, Just, Nothing) {
+    return function() {
+      var el = document.getElementById(id);
+      if (el && el instanceof HTMLCanvasElement) {
+        return Just(el);
+      } else {
+        return Nothing;
+      }
+    };
+  }""" :: forall a eff.
+  Fn3
+    String
+    (a -> Maybe a)
+    (Maybe a)
+    (Eff (canvas :: Canvas | eff) (Maybe CanvasElement))
+
+getCanvasElementById :: forall eff.
+  String -> Eff (canvas :: Canvas | eff) (Maybe CanvasElement)
+getCanvasElementById elId =
+  runFn3 getCanvasElementByIdImpl elId Just Nothing
 
 foreign import getContext2D 
   "function getContext2D(c) {\