Re: [Haskell-beginners] Beginners Digest, Vol 57, Issue 22

On 15/03/13 13:00, Răzvan Rotaru wrote:

Thanks for the reply.

On 15 March 2013 13:00, mailto:beginners-request@haskell.org> wrote:

Perhaps

data TextfieldProps = TextId | TextLabel | TextValue -- and similarly for ButtonProps?

I confess to not fully understanding the GUI modelling attempted here, which isn't to say that you aren't on the right track.

To help you and help us help you better, one approach known to be successful is to sketch out the code (better yet, the type signatures!) of what you'd like to write, e.g. how are TextfieldProps and ButtonProps used? What functions act on them?

-- Kim-Ee

I don't have any code to show you, since I just started trying to write the types. I'll try to explain what I'm trying to do.

So, I want model some GUI widgets. This means I should have type for each widget (or a type synomym), holding it's data, or state if you like. A textfield must have an id, a label and the entered text as value. A button must have an id, a label and a function to call when the button is clicked.

As previously said, records are not an acceptable solution because of the name clash of properties, and I don't want to prefix each property to avoid this. Next best thing is to use a map to hold the properties. I could use strings as keys, in which case my map would look like this (written as a list of tuples):

[ ("id", "name_textfield"), ("label", "Name:"), ("value", "Please enter name here ...")]

but then is no checking for the property names (e.g. if I mispelled "id", or I used "onclick" for textfields). I want to find a way to use the type system to check these properties. My idea was to use new types as keys for these maps. I would then have:

data TextfieldProperties = Id | Label | Value type Textfield = Map TextfieldProperties String

But then I get into the same name clash, because all widgets have Ids, most have labels, etc. And here I got stuck.

Then, how to create widgets? Record syntax would have been fine, but since I'm not using records I would have to write some make-... functions, which will receive a list of key value pairs and insert them into the map to create the widget. Also here I can put the default values of properties (for. example if id is not specified, one is generated, or use empty strings for unspecified textfield values).

How to use widgets? Well there will be functions to draw them on the screen. I could use a typeclass here. Then their properties must be accessed somehow, and this should be also polymorphic of course: I want one function to get the id of a widget, no matter of what type. Typeclasses can help here as well. Then there would be methods to search the GUI tree for certain widgets... you know, standard stuff you would want to do with GUI widgets.

Currently, my blocking point is how to define the types. And I don't want to flatten it all out, and use (Map String String) for any widget. I'm missing out on the type system if I do this.

Cheers, Răzvan

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How about a Widget typeclass that provides the ‘identifier’ function and all the other ones that all the widgets will have in common? You could then do something like

class (Widget a) => LabelWidget a where getLabel :: a -> String for all widgets that need to have the label functions defined for them. Granted, that's a lot of instances for a lot of GUI elements but that's what you get for developing such a large package.

-- Mateusz K.