Difference between revisions of "Haskell in web browser/Calc example"

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===Conclusion: all together===
 
 
In conclusion, let's finally develop a program that does something useful. As another "classical" example, this will be a [http://en.wikipedia.org/wiki/Reverse_Polish_notation RPN] calculator.
 
 
<div style="border: 1px solid gray">
 
<haskell>
 
-- Begin Pasteable Code --
 
module TutEx6 where
 
 
import CPS
 
import Data.Char
 
import Graphics.UI.HsWTK
 
import DOM.Level2.Event
 
import DOM.Level2.Events
 
import DOM.Level2.KeyEvent
 
import DOM.Level2.HTMLDivElement
 
import DOM.Level2.HTMLSpanElement
 
import Control.Concurrent.JSThreads
 
 
main = docBodyC mainW
 
 
mainW = msgBox $ \calcmb ->
 
msgBox $ \dispmb ->
 
mkDiv `withStyle` ["border" := "1px solid black"
 
,"width" := "200px"
 
,"height" := "150px"
 
,"text-align" := "center"]
 
|<< (mkDiv `withStyle` ["height" := "20%"
 
,"text-align" := "right"
 
,"vertical-align" := "middle"
 
,"overflow" := "hidden"]
 
|<< (textP "\160" +++ textP "\160"
 
+++ active (updMapA "0" id dispmb))
 
+++ foldr (+++) nowidget (map (cbtn calcmb) btxt)
 
+++ active (passMapStateA ci calc (show . acc) calcmb dispmb)
 
+++ active (evtBCastA "keydown" clkey [calcmb])
 
+++ active (tabIndexA 0))
 
 
clkey :: TKeyEvent -> CPS Bool Char
 
 
clkey e k = preventDefault e $ \_ -> get'keyCode e $ \c -> k $ case c of
 
38 -> '^'
 
107 -> '+'
 
61 -> '+'
 
109 -> '-'
 
106 -> '*'
 
191 -> '/'
 
111 -> '/'
 
59 -> '/'
 
_ -> chr c
 
 
data CalcST = CalcST {acc :: Int, stack :: [Int], rst :: Int}
 
 
ci = CalcST {acc = 0, stack = [], rst = 0}
 
 
calc :: CalcST -> Char -> CalcST
 
 
calc st c = case c of
 
'C' -> ci
 
d | isDigit d -> let acc' = rst st * acc st * 10 + ord d - ord '0'
 
in st {acc = acc', rst = 1}
 
'^' -> st {rst = 0, stack = (acc st) : (stack st)}
 
o | o `elem` ['+', '-', '*', '/'] ->
 
let op '+' = (+)
 
op '-' = (-)
 
op '*' = (*)
 
op '/' = div
 
in if null (stack st)
 
then st
 
else let acc' = op o (head $ stack st) (acc st)
 
in st {rst = 0,
 
stack = acc' : tail (stack st),
 
acc = acc'}
 
_ -> st
 
 
btxt = ['1', '2', '3', '+',
 
'4', '5', '6', '-',
 
'7', '8', '9', '*',
 
'C', '0', '^', '/']
 
 
cbtn mb txt = mkDiv `withStyle` ["float" := "left"
 
,"width" := "25%"
 
,"height" := "20%"
 
,"margin" := "0px"
 
,"vertical-align" := "middle"
 
,"text-align" := "center"]
 
|<< buttonI `withStyle` ["width" := "80%"
 
,"height" := "80%"
 
,"font-size" := "80%"]
 
|<< (textP [txt]
 
+++ active (evtBCastA "click" (evt2ConstU txt) [mb]))
 
 
-- End Pasteable Code --
 
</haskell>
 
</div>
 
 
The main ''Widget'', <hask>mainW</hask> consists of the following pieces:
 
 
* Two ''Message Boxes''. One (<code>calcmb</code>) will be used to transmit keystrokes and mouse clicks received from the browser's input facilities. Another (<code>dispmb</code>) will be used to update the calculator's display element.
 
 
* A &lt;DIV&gt; element with visible border: it encloses all other elements of the calculator. It is also responsible for receiving keyboard input.
 
 
The following elements are nested within the above mentioned &lt;DIV&gt;:
 
 
* A &lt;DIV&gt; element that contains the calculator's display.
 
 
* 16 buttons for mouse input. Note the use of <hask>foldr</hask> and <hask>map</hask>. All buttons use the same ''Message Box'' <code>calcmb</code>, so it is passed to each button's creation function. Each button uses its own character taken from the <code>btxt</code> list: button creation function is mapped over this list. Finally, the resulting list is folded with <code>+++</code> to sequence all buttons properly.
 
 
* ''Activator'' which implements the internal (stateful) logic of the calculator. It is based on <code>[http://www.golubovsky.org:5984/_utils/yhcws/Graphics-UI-HsWTK.html#v%3ApassMapStateA passMapStateA]</code> which receives messages from its input ''Message Box'', maintains internal state transitions based on what is received, and maps internal state to messages sent to the output ''Message Box''. That's how the two parts (user input and result display) are wired together.
 
 
* ''Activator'' based on <code>evtBCastA</code> to catch keystrokes and route them to the same ''Message Box'' where mouse clicks go.
 
 
* ''Activator'' based on <code>[http://www.golubovsky.org:5984/_utils/yhcws/Graphics-UI-HsWTK.html#v%3AtabIndexA tabIndexA]</code>. Although DOM does not define the ''tabIndex'' property for most HTML elements, according to [http://codingforums.com/archive/index.php?t-86479.html this (last comment)], it is necessary to initialize this property in order to be able to receive keystrokes on &lt;DIV&gt; elements.
 
 
The <code>clkey</code> function performs proper mapping of keystrokes to simulate mouse clicks on buttons. It extracts the value of ''keyCode'' property from the event received, and remaps certain codes, finally passing a character to the continuation.
 
 
The <code>CalcST</code> defines the internal state of the calculator. It includes the accumulator (where the user input goes), the stack (a list of numbers), and the reset flag which helps update the display properly when entry of a new number begins.
 
 
Internal logic of the calculator (<code>calc</code>) is pretty straighforward: numbers typed in are pushed down the stack when "^" button is clicked, or the "up arrow" key is pressed. Operation buttons/keys perform binary operations between the accumulator and the topmost stack element. Finally copy of the result is pushed back onto the stack, so next number typed in may be used for the next computation.
 
 
The <code>btxt</code> list of characters defines what shows on buttons.
 
 
The button creation function (<code>cbtn</code>) defines a &lt;DIV&gt; element nesting a &lt;BUTTON&gt; element. Each button has an ''Activator'' based on <code>evtBCastA</code> that maps each click to the character showing on the button. Note the ''float: left'' style and percentage width defined for &lt;DIV&gt;s: no matrix layout logic is necessary; buttons fill the &lt;DIV&gt; with borders with correct placement.
 

Revision as of 14:17, 6 February 2021