IHaskell-Widgets

This package implements the ipython widgets 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.

Contributing examples

If you want to contribute with more Notebook examples, please do so on the Examples/ folder. Before commiting, please make sure they can be executed sequentialy and then remove the output from the Nootebooks with:

jupyter nbconvert *.ipynb --to notebook --inplace --clear-output

Things to do

• Automatic validation of the JSON implementation of widgets against the MsgSpec schema
• Create integration tests for the widgets
• Make the output widget capture output (problem: you have to get the message id of where the output is displayed)
• Make the layout widget values more 'Haskelian': Instead of checking if the string is valid at runtime, make some types so it's checked at compile-time
• Create a serializable color data type instead of using Maybe String
• Overload setField so it can be used with Maybes or other wrapper types without having to put Just every time.
• Add some "utils" work:
  • Create media widget from file
  • Get the selected label from a selection value

How to...

This is a mini-guide for developers that want to update to a more recent widgets specification, but without dwelling into the deeps of the project

Add a new attribute

If you want to add a new attribute you'll have to:

1. Create a new singleton in Singletons.hs inside the type data Field.
2. Write the serialization key of the field as specified in the model (see MsgSpec.md) inside the toKey function at Singletons.hs
3. Because we use the singletons-th library, you have to define an alias for the attribute at Common.hs to be able to use it at run-time more easily.
4. Now you have to specify the type of the field. Edit the type family Fieldtype at Types.hs

Add an attribute to a widget

First you have to check if the attribute is only for one widget, or is from a common class. You can check it at ipywidget's repo.

• If it's only for one widget:
  1. Edit the type instance WidgetFields <WidgetNameType> = ... at Types.hs, adding the new field to the field array.
  2. Modify the mk<WidgetName> at Module/WidgetName.hs, adding the default value of the attribute. If the widget doesn't have any attributes yet, you can check how to do it on other widgets.
• If it's for a common class:
  1. Edit the type <ClassName> = ... at Types.hs
  2. Edit the default<ClassName>Widget function from the same file, adding the default value for that attribute.

Some widgets receive messages from the frontend when a value is modified (such as sliders, text areas, buttons...). You'll have to modify the comm function instantiated from the class IHaskellWidget. You can find an example at IntSlider.hs

FAQ

When using widgets in ihaskell, you'll encounter a lot of compilation errors. If you are not very familiar with Haskell, they can be a bit hard to decipher, this is a mini guide that will (hopefully) appear when you paste the error in Google.

setField: No instance for...

You probably got this error when trying to use setField like this:

<interactive>:1:1: error:
    • No instance for (Data.Vinyl.Lens.RecElem
                         Data.Vinyl.Core.Rec
                         'ihaskell-widgets-0.3.0.0:IHaskell.Display.Widgets.Singletons.Index
                         'ihaskell-widgets-0.3.0.0:IHaskell.Display.Widgets.Singletons.Index
                         '[]
                         '[]
                         (Data.Vinyl.TypeLevel.RIndex 'ihaskell-widgets-0.3.0.0:IHaskell.Display.Widgets.Singletons.Index '[]))
        arising from a use of ‘setField’
    • In the expression: setField select Index 0
      In an equation for ‘it’: it = setField select Index 0

What this error means is that there is no field called Index for this particular widget. You can display on screen all the fields available for a widget using properties widget.

setField: Couldn't match expected type SField f with actual type

If you get an error like this, you probably forgot to put the field name in the second argument of setField.

<interactive>:1:25: error:
    • Couldn't match expected type ‘ihaskell-widgets-0.3.0.0:IHaskell.Display.Widgets.Singletons.SField f’ with actual type ‘[a0]’
    • In the second argument of ‘setField’, namely ‘["Apples", "Oranges", "Pears"]’
      In the expression: setField selectMultiple ["Apples", "Oranges", "Pears"]
      In an equation for ‘it’: it = setField selectMultiple ["Apples", "Oranges", "Pears"]
    • Relevant bindings include it :: ihaskell-widgets-0.3.0.0:IHaskell.Display.Widgets.Types.FieldType f -> IO () (bound at <interactive>:1:1)