An Up/Down Counter

This example shows how to handle input from more than one source.

The two buttons affect the same counter.

Here is the source code:

import Fudgets

main = fudlogue (shellF "Up/Down Counter" counterF)

counterF = intDispF >==< mapstateF count 0 >==<
           (buttonF "Up" >+< buttonF "Down")

count n (Left Click) = (n+1,[n+1])
count n (Right Click) = (n-1,[n-1])

Things to note

• The Up/Down Counter is a small extension of the Up Counter. We have added a button by replacing

  buttonF "Up"

  with

  (buttonF "Up" >+< buttonF "Down")

  using the operator >+< for parallel composition

• The output from a parallel composition is the merged output from the two components. Output from the left component is tagged Left and output from the right component is tagged Right.

• The count function will now receive Left Click or Right Click, depending on which button was pressed. It has been adjusted accordingly.

• Everything else is as in the previous example.