Polish up the refactored code

- Rename IHaskell.Display.Widget.Types.Widget to IPythonWidget
- Add explanatory comments to the source files
- Add documentation for messaging protocol in MsgSpec.md
- Add README.md
- Remove unnecessary extensions and imports

ihaskell-display/ihaskell-widgets/MsgSpec.md

@@ -0,0 +1,102 @@
+# IPython widget messaging specification
+
+> Largely based on: https://github.com/ipython/ipython/wiki/IPEP-23:-Backbone.js-Widgets
+
+> The messaging specification as detailed is riddled with the assumptions IHaskell's widget
+> implementation makes. It works for us, so it should work for everyone.
+
+## Creating widgets
+
+Let's say the user types in some code, and the only effect of that code is the creation of a widget.
+The kernel will open a comm for the widget, and store a reference to that comm inside it. Then, to
+notify the frontend about the creation of a widget, an initial state update is sent on the widget's comm.
+
+> The comm should be opened with a `target_name` of `"ipython.widget"`.
+
+The initial state update message looks like this:
+
+```json
+{
+    "method": "update",
+    "state": { "<some/all widget properties>" }
+}
+```
+
+Any property initialized with the empty string is provided the default value by the frontend.
+
+The initial state update must *at least* have the following fields:
+
+  - `msg_throttle` (default 3): To prevent the kernel from flooding with messages, the messages from
+    the widget to the kernel are throttled. If `msg_throttle` messages were sent, and all are still
+    processing, the widget will not send anymore state messages.
+
+  - `_view_name` (depends on the widget): The frontend uses a generic model to represent
+    widgets. This field determines how a set of widget properties gets rendered into a
+    widget. Has the form `IPython.<widgetname>`, e.g `IPython.Button`.
+
+  - `_css` (default value = empty list): A list of 3-tuples, (selector, key, value).
+
+  - `visible` (default = True): Whether the widget is visible or not.
+
+  - Rest of the properties as required initially.
+
+This state update is also used with fragments of the overall state to sync changes between the
+frontend and the kernel.
+
+## Displaying widgets
+
+The creation of a widget does not display it. To display a widget, the kernel sends a display
+message to the frontend on the widget's comm.
+
+```json
+{
+    "method": "display"
+}
+```
+
+* Widgets can also send a custom message, having the form:
+
+```json
+{
+    "method": "custom",
+    "content": { "<message content>" }
+}
+```
+
+## Handling changes to widget in the frontend
+
+Changes to widgets in the frontend lead to messages being sent to the backend. These messages have
+two possible formats:
+
+1. Backbone.js initiated sync:
+
+```json
+{
+    "method": "backbone",
+    "sync_data": { "<changes to sync with the backend>" }
+}
+```
+
+These messages are sent by the Backbone.js library when some change is made to a widget. For
+example, whenever a change is made to the text inside a `TextWidget`, the complete contents are sent
+to the kernel so that the kernel stays up-to-date about the widget's contents.
+
+2. Custom message:
+
+```json
+{
+    "method": "custom",
+    "content": { "<custom message data>" }
+}
+```
+
+This form is generally used to notify the kernel about events. For example, the `TextWidget` sends a
+custom message when the text is submitted by hitting the 'Enter' key.
+
+---
+
+*NOTE*: It's important that the messages sent on the comm are in response to an execution message
+ from the front-end or another widget's comm message. This is required so the widget framework knows
+ what cell triggered the message and can display the widget in the correct location.
+
+---

ihaskell-display/ihaskell-widgets/README.md

@@ -0,0 +1,7 @@
+# IHaskell-Widgets
+
+This package implements the [ipython widgets](https://github.com/ipython/ipywidgets) in
+IHaskell. The frontend (javascript) is provided by the jupyter/ipython notebook environment, whereas
+the backend is implemented in haskell.
+
+To know more about the widget messaging protocol, see [MsgSpec.md](MsgSpec.md).

ihaskell-display/ihaskell-widgets/ihaskell-widgets.cabal

@@ -44,7 +44,7 @@ build-type:          Simple
 
 -- Extra files to be distributed with the package, such as examples or a 
 -- README.
--- extra-source-files:  
+extra-source-files:  README.md, MsgSpec.md
 
 -- Constraint on the version of Cabal needed to build this package.
 cabal-version:       >=1.10
@@ -91,6 +91,10 @@ library
   -- Base language which the package is written in.
   default-language:    Haskell2010
   
-  -- Deal with small -fcontext-stack on ghc-7.8
+  -- Deal with small -fcontext-stack on ghc-7.8.
+  -- Default values:
+  --   ghc-7.6.*  = 200
+  --   ghc-7.8.*  = 20   -- Too small for vinyl & singletons
+  --   ghc-7.10.* = 100
   if impl(ghc == 7.8.*)
     ghc-options:       -fcontext-stack=100

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Bool/CheckBox.hs

@@ -20,7 +20,7 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
@@ -28,7 +28,7 @@ import           IHaskell.Display.Widgets.Types
 import           IHaskell.Display.Widgets.Common
 
 -- | A 'CheckBoxWidget' represents a Checkbox widget from IPython.html.widgets.
-type CheckBoxWidget = Widget CheckBoxType
+type CheckBoxWidget = IPythonWidget CheckBoxType
 
 -- | Create a new output widget
 mkCheckBoxWidget :: IO CheckBoxWidget
@@ -40,7 +40,7 @@ mkCheckBoxWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.Checkbox"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Bool/ToggleButton.hs

@@ -20,7 +20,7 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
@@ -28,7 +28,7 @@ import           IHaskell.Display.Widgets.Types
 import           IHaskell.Display.Widgets.Common
 
 -- | A 'ToggleButton' represents a ToggleButton widget from IPython.html.widgets.
-type ToggleButton = Widget ToggleButtonType
+type ToggleButton = IPythonWidget ToggleButtonType
 
 -- | Create a new output widget
 mkToggleButton :: IO ToggleButton
@@ -45,7 +45,7 @@ mkToggleButton = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.ToggleButton"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Button.hs

@@ -22,7 +22,7 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
@@ -30,7 +30,7 @@ import           IHaskell.Display.Widgets.Types
 import           IHaskell.Display.Widgets.Common
 
 -- | A 'Button' represents a Button from IPython.html.widgets.
-type Button = Widget ButtonType
+type Button = IPythonWidget ButtonType
 
 -- | Create a new button
 mkButton :: IO Button
@@ -50,7 +50,7 @@ mkButton = do
 
   stateIO <- newIORef buttonState
 
-  let button = Widget uuid stateIO
+  let button = IPythonWidget uuid stateIO
 
   let initData = object
                    [ "model_name" .= str "WidgetModel"

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Common.hs

@@ -1,39 +1,18 @@
-{-# LANGUAGE FlexibleContexts #-}
-{-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE DataKinds #-}
-{-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeFamilies #-}
-{-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE QuasiQuotes #-}
-{-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE NoMonomorphismRestriction #-}
-{-# LANGUAGE TypeSynonymInstances #-}
 module IHaskell.Display.Widgets.Common where
 
-import Control.Monad (when)
-
 import Data.Aeson
-import Data.Aeson.Types (emptyObject, Pair)
 import Data.Text (pack, Text)
-import Data.IORef (IORef, readIORef, modifyIORef)
-import Data.Proxy
-
-import Data.Vinyl (Rec (..), (<+>), recordToList, reifyConstraint, rmap, Dict (..))
-import Data.Vinyl.Functor (Compose (..), Const (..))
-import Data.Vinyl.Lens (rget, rput, type (∈))
-import qualified Data.Vinyl.TypeLevel as TL
 
 import Data.Singletons.TH
 
-import IHaskell.Eval.Widgets (widgetSendUpdate)
-import IHaskell.Display (Base64, IHaskellWidget (..))
-import IHaskell.IPython.Message.UUID
-
 -- Widget properties
 singletons [d|
   data Field = ModelModule

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Image.hs

@@ -21,7 +21,7 @@ import           Data.Monoid (mempty)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
@@ -29,7 +29,7 @@ import           IHaskell.Display.Widgets.Types
 import           IHaskell.Display.Widgets.Common
 
 -- | An 'ImageWidget' represents a Image widget from IPython.html.widgets.
-type ImageWidget = Widget ImageType
+type ImageWidget = IPythonWidget ImageType
 
 -- | Create a new image widget
 mkImageWidget :: IO ImageWidget
@@ -45,7 +45,7 @@ mkImageWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
 
   let initData = object
                    [ "model_name" .= str "WidgetModel"

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Output.hs

@@ -25,14 +25,14 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
 import           IHaskell.Display.Widgets.Types
 
 -- | An 'OutputWidget' represents a Output widget from IPython.html.widgets.
-type OutputWidget = Widget OutputType
+type OutputWidget = IPythonWidget OutputType
 
 -- | Create a new output widget
 mkOutputWidget :: IO OutputWidget
@@ -44,7 +44,7 @@ mkOutputWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/String/HTML.hs

@@ -19,14 +19,14 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
 import           IHaskell.Display.Widgets.Types
 
 -- | A 'HTMLWidget' represents a HTML widget from IPython.html.widgets.
-type HTMLWidget = Widget HTMLType
+type HTMLWidget = IPythonWidget HTMLType
 
 -- | Create a new HTML widget
 mkHTMLWidget :: IO HTMLWidget
@@ -37,7 +37,7 @@ mkHTMLWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.HTML"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/String/Latex.hs

@@ -19,14 +19,14 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
 import           IHaskell.Display.Widgets.Types
 
 -- | A 'LatexWidget' represents a Latex widget from IPython.html.widgets.
-type LatexWidget = Widget LatexType
+type LatexWidget = IPythonWidget LatexType
 
 -- | Create a new Latex widget
 mkLatexWidget :: IO LatexWidget
@@ -37,7 +37,7 @@ mkLatexWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.Latex"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/String/Text.hs

@@ -22,7 +22,7 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
@@ -30,7 +30,7 @@ import           IHaskell.Display.Widgets.Types
 import           IHaskell.Display.Widgets.Common
 
 -- | A 'TextWidget' represents a Text widget from IPython.html.widgets.
-type TextWidget = Widget TextType
+type TextWidget = IPythonWidget TextType
 
 -- | Create a new Text widget
 mkTextWidget :: IO TextWidget
@@ -43,7 +43,7 @@ mkTextWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.Text"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/String/TextArea.hs

@@ -19,14 +19,14 @@ import           Data.IORef (newIORef)
 import           Data.Text (Text)
 import           Data.Vinyl (Rec (..), (<+>))
 
-import           IHaskell.Display hiding (Widget)
+import           IHaskell.Display
 import           IHaskell.Eval.Widgets
 import           IHaskell.IPython.Message.UUID as U
 
 import           IHaskell.Display.Widgets.Types
 
 -- | A 'TextAreaWidget' represents a Textarea widget from IPython.html.widgets.
-type TextAreaWidget = Widget TextAreaType
+type TextAreaWidget = IPythonWidget TextAreaType
 
 -- | Create a new TextArea widget
 mkTextAreaWidget :: IO TextAreaWidget
@@ -37,7 +37,7 @@ mkTextAreaWidget = do
 
   stateIO <- newIORef widgetState
 
-  let widget = Widget uuid stateIO
+  let widget = IPythonWidget uuid stateIO
       initData = object ["model_name" .= str "WidgetModel", "widget_class" .= str "IPython.Textarea"]
 
   -- Open a comm for this widget, and store it in the kernel state

ihaskell-display/ihaskell-widgets/src/IHaskell/Display/Widgets/Types.hs

@@ -6,17 +6,42 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE PolyKinds #-}
-{-# LANGUAGE GADTs #-}
 {-# LANGUAGE UndecidableInstances #-}
-{-# LANGUAGE TemplateHaskell #-}
-{-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE NoMonomorphismRestriction #-}
 {-# LANGUAGE TypeSynonymInstances #-}
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE ConstraintKinds #-}
 module IHaskell.Display.Widgets.Types where
 
+-- | This module houses all the type-trickery needed to make widgets happen.
+--
+-- All widgets have a corresponding 'WidgetType', and some fields/attributes/properties as defined by
+-- the 'WidgetFields' type-family.
+--
+-- Each widget field corresponds to a concrete haskell type, as given by the 'FieldType' type-family.
+--
+-- Vinyl records are used to wrap together widget fields into a single 'WidgetState'.
+--
+-- Singletons are used as a way to represent the promoted types of kind Field. For example:
+--
+-- @
+-- SViewName :: SField ViewName
+-- @
+--
+-- This allows the user to pass the type 'ViewName' without using Data.Proxy. In essence, a singleton
+-- is the only inhabitant (other than bottom) of a promoted type. Single element set/type == singleton.
+--
+-- It also allows the record to wrap values of properties with information about their Field type. A
+-- vinyl record is represented as @Rec f ts@, which means that a record is a list of @f x@, where @x@
+-- is a type present in the type-level list @ts@. Thus a 'WidgetState' is essentially a list of field
+-- properties wrapped together with the corresponding promoted Field type. See ('=::') for more.
+--
+-- The IPython widgets expect state updates of the form {"property": value}, where an empty string for
+-- value is ignored by the frontend and the default value is used instead.
+--
+-- To know more about the IPython messaging specification (as implemented in this package) take a look
+-- at the supplied MsgSpec.md.
+
 import Control.Monad (when)
 import Control.Applicative ((<$>))
 
@@ -24,7 +49,6 @@ import Data.Aeson
 import Data.Aeson.Types (emptyObject, Pair)
 import Data.Text (pack, Text)
 import Data.IORef (IORef, readIORef, modifyIORef)
-import Data.Proxy
 
 import Data.Vinyl (Rec (..), (<+>), recordToList, reifyConstraint, rmap, Dict (..))
 import Data.Vinyl.Functor (Compose (..), Const (..))
@@ -42,7 +66,7 @@ import IHaskell.IPython.Message.UUID
 
 import IHaskell.Display.Widgets.Common
 
--- Classes from IPython's widget hierarchy
+-- Classes from IPython's widget hierarchy. Defined as such to reduce code duplication.
 type WidgetClass = '[ModelModule, ModelName, ViewModule, ViewName, MsgThrottle, Version, OnDisplayed]
 type DOMWidgetClass = WidgetClass :++
    '[ Visible, CSS, DOMClasses, Width, Height, Padding, Margin, Color
@@ -52,7 +76,7 @@ type DOMWidgetClass = WidgetClass :++
 type StringClass = DOMWidgetClass :++ '[StringValue, Disabled, Description, Placeholder]
 type BoolClass = DOMWidgetClass :++ '[BoolValue, Disabled, Description]
 
--- Types associated with Fields
+-- Types associated with Fields.
 type family FieldType (f :: Field) :: * where
   FieldType ModelModule = Text
   FieldType ModelName = Text
@@ -91,6 +115,7 @@ type family FieldType (f :: Field) :: * where
   FieldType ImageFormat = ImageFormatValue
   FieldType BoolValue = Bool
 
+-- Different types of widgets. Every widget in IPython has a corresponding WidgetType
 data WidgetType = ButtonType
                 | ImageType
                 | OutputType
@@ -101,6 +126,7 @@ data WidgetType = ButtonType
                 | CheckBoxType
                 | ToggleButtonType
 
+-- Fields associated with a widget
 type family WidgetFields (w :: WidgetType) :: [Field] where
   WidgetFields ButtonType = DOMWidgetClass :++ '[Description, Tooltip, Disabled, Icon, ButtonStyle, ClickHandler]
   WidgetFields ImageType = DOMWidgetClass :++ '[ImageFormat, B64Value]
@@ -112,8 +138,10 @@ type family WidgetFields (w :: WidgetType) :: [Field] where
   WidgetFields CheckBoxType = BoolClass
   WidgetFields ToggleButtonType = BoolClass :++ '[Tooltip, Icon, ButtonStyle]
 
-newtype Attr f = Attr { _unAttr :: FieldType f }
+-- Wrapper around a field
+newtype Attr (f :: Field) = Attr { _unAttr :: FieldType f }
 
+-- Types that can be converted to Aeson Pairs.
 class ToPairs a where
   toPairs :: a -> [Pair]
 
@@ -155,9 +183,11 @@ instance ToPairs (Attr B64Value) where toPairs (Attr x) = ["_b64value" .= toJSON
 instance ToPairs (Attr ImageFormat) where toPairs (Attr x) = ["format" .= toJSON x]
 instance ToPairs (Attr BoolValue) where toPairs (Attr x) = ["value" .= toJSON x]
 
+-- | Store the value for a field, as an object parametrized by the Field
 (=::) :: sing f -> FieldType f -> Attr f
 _ =:: x = Attr x
 
+-- | A record representing an object of the Widget class from IPython
 defaultWidget :: FieldType ViewName -> Rec Attr WidgetClass
 defaultWidget viewName = (SModelModule =:: "")
                       :& (SModelName =:: "WidgetModel")
@@ -168,6 +198,7 @@ defaultWidget viewName = (SModelModule =:: "")
                       :& (SOnDisplayed =:: return ())
                       :& RNil
 
+-- | A record representing an object of the DOMWidget class from IPython
 defaultDOMWidget :: FieldType ViewName -> Rec Attr DOMWidgetClass
 defaultDOMWidget viewName = defaultWidget viewName <+> domAttrs
   where domAttrs = (SVisible =:: True)
@@ -189,6 +220,7 @@ defaultDOMWidget viewName = defaultWidget viewName <+> domAttrs
                 :& (SFontFamily =:: "")
                 :& RNil
 
+-- | A record representing a widget of the _String class from IPython
 defaultStringWidget :: FieldType ViewName -> Rec Attr StringClass
 defaultStringWidget viewName = defaultDOMWidget viewName <+> strAttrs
   where strAttrs = (SStringValue =:: "")
@@ -197,6 +229,7 @@ defaultStringWidget viewName = defaultDOMWidget viewName <+> strAttrs
                 :& (SPlaceholder =:: "")
                 :& RNil
 
+-- | A record representing a widget of the _Bool class from IPython
 defaultBoolWidget :: FieldType ViewName -> Rec Attr BoolClass
 defaultBoolWidget viewName = defaultDOMWidget viewName <+> boolAttrs
   where boolAttrs = (SBoolValue =:: False)
@@ -215,35 +248,28 @@ instance RecAll Attr (WidgetFields w) ToPairs => ToJSON (WidgetState w) where
     . rmap (\(Compose (Dict x)) -> Const $ toPairs x)
     $ reifyConstraint (Proxy :: Proxy ToPairs) $ _getState record
 
-data Widget (w :: WidgetType) = Widget { uuid :: UUID, state :: IORef (WidgetState w) }
-
--- | Reflect a (Proxy :: Proxy f) back to f
--- Copied from: http://stackoverflow.com/a/28033250/2388535
-reflect ::
-  forall (a :: k).
-  (SingI a, SingKind ('KProxy :: KProxy k)) =>
-  Proxy a -> Demote a
-reflect _ = fromSing (sing :: Sing a)
+data IPythonWidget (w :: WidgetType) = IPythonWidget { uuid :: UUID, state :: IORef (WidgetState w) }
 
 -- | Change the value for a field, and notify the frontend about it.
-setField :: (f ∈ WidgetFields w, IHaskellWidget (Widget w), SingI f, ToPairs (Attr f)) => Widget w -> SField f -> FieldType f -> IO ()
+setField :: (f ∈ WidgetFields w, IHaskellWidget (IPythonWidget w), ToPairs (Attr f)) => IPythonWidget w -> SField f -> FieldType f -> IO ()
 setField widget (sfield :: SField f) fval = do
   setField' widget sfield fval
   let pairs = toPairs (Attr fval :: Attr f)
   when (not . null $ pairs) $ widgetSendUpdate widget (object pairs)
 
 -- | Change the value of a field, without notifying the frontend. For internal use. Uses BangPattern.
-setField' :: (f ∈ WidgetFields w, IHaskellWidget (Widget w), SingI f) => Widget w -> SField f -> FieldType f -> IO ()
-setField' widget (sfield :: SField f) !fval = modifyIORef (state widget) (WidgetState . rput (sfield =:: fval) . _getState)
+setField' :: (f ∈ WidgetFields w, IHaskellWidget (IPythonWidget w)) => IPythonWidget w -> SField f -> FieldType f -> IO ()
+setField' widget sfield !fval = modifyIORef (state widget) (WidgetState . rput (sfield =:: fval) . _getState)
 
 -- | Get the value of a field.
-getField :: (f ∈ WidgetFields w) => Widget w -> SField f -> IO (FieldType f)
+getField :: (f ∈ WidgetFields w) => IPythonWidget w -> SField f -> IO (FieldType f)
 getField widget sfield = _unAttr <$> rget sfield <$> _getState <$> readIORef (state widget)
 
--- | Useful with toJSON, and OverloadedStrings
+-- | Useful with toJSON and OverloadedStrings
 str :: String -> String
 str = id
 
+-- | Send zero values as empty strings, which stands for default value in the frontend.
 instance ToJSON Natural where
   toJSON 0 = String ""
   toJSON n = String . pack $ show n