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Functional Reactive GUI for O'Caml
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Date: 2006-12-21 (03:36)
From: Chris King <colanderman@g...>
Subject: Re: [Caml-list] Functional Reactive GUI for O'Caml
On 12/20/06, Gabriel Kerneis <gabriel.kerneis@enst.fr> wrote:
> It might have been clearer if I had read a quick tutorial or how-to
> before. But your question targeted newbies, so let me give you a
> total newbie's answer : I just can't get it ;-)

You have a very good point :) nothing's intuitive unless it's
familiar, which I admit the syntax (especially lift) is not.  I intend
to post some examples as well as a tutorial in the next few weeks, but
for the time being here is a quick breakdown of the above example
(with a few math fixes :P):

> open Fr
> open FrGui
Fr is the library containing funcitons to create functional reactive
expressions; FrGui contains definitions of all the GUI widgets.

> let float_view f = text_view (lift (string_of_float @f))
This one-liner defines the float_view widget in terms of the text_view
widget.  Ignoring the definition (which should make more sense later),
it's simply a function which creates a widget which displays the
time-varying floating-point value f.

> let _ =
>   let temp_box, temp = spin_box 20. in
spin_box is a function which creates a widget (temp_box) which allows
the user to input a floating-point value (in this case, the
temperature we want to convert).  Here 20 is given as its initial
value.  The current value of temp_box is stored in temp.  This value
can change with time and is called a "behavior" (and has the type
float behavior).

>    let fahrenheit = lift (@temp *. 1.8 +. 32.)
Because temp is not a float but a float behavior, we must use a
special construct to access it.  lift (expr) allows us to access
values contained in behaviors and to create new ones.  Inside of expr
(which is otherwise a normal O'Caml expression), we can use the form
@expr to reference a behavior (think of this like dereferencing a
pointer in C). Here, fahrenheit is a behavior which is always equal to
the value of (temp *. 1.8 +. 32.).  Whenever temp changes, so does

>    and celsuis = lift ((@temp -. 32.) /. 1.8) in
Here, we similarly define celsius as a float behavior which is
dependent on the current value of temp.

>    let main_window = window
>        ~title: "Temperature converter"
>        (hbox [
>            hbox [label "Convert "; temp_box; label " to:"];
>            vbox [
>                hbox [label "degrees Fahrenheit: "; float_view fahrenheit];
>                hbox [label "degrees Celsius: "; float_view celsius]]]) in
Here we define the main window.  hbox, vbox, and label are all
functions which create widgets.  hbox and vbox pack lists of widgets
horizontally and vertically, and label creates a text label.  temp_box
is the widget instance we defined above, which allows the user to
enter the value of temp.  We also create two instances of the
float_view widget, to display the values of fahrenheit and celsius.

>    run_dialog (main_window, main_window#close)
This incantation displays main_window and enters the GUI's main loop
until the window's close button is clicked.

The end result is a window with a spin box into which the user can
input a temperature.  That temperature is instantly converted into
both Fahrenheit and Celsius and displayed in two other text boxes (the
float_views) in the window.

Hopefully this description helped more than it confused... if it did
more of the latter then I'll try to post a proper tutorial on the
O'Caml RT website as soon as possible, since one is in order anyway.
Thanks for you input :)

- Chris