Note: those "transactions" must be syncing to the OS buffers and not the disk.]]> Mon, 31 Mar 2008 01:49:57 PDT First, let us add a Image method to help:

  Image grabScreen := method(
     data := Sequence clone 
     width  := glutGet(GLUT_WINDOW_WIDTH)
     height := glutGet(GLUT_WINDOW_HEIGHT)
     glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data)	
     self setDataWidthHeightComponentCount(data, width, height, 4)
     self flipY
  )

Image clone grabScreen save("screenshot.jpg")

Constant := Object clone do (
  with := method(msg, 
    self msg := msg
    self activate := method(call message setName(msg asString); msg)
  )
)

// Lets try it out: Make PI a regular number
PI := 3.1415926

// Make b a method that uses it
b := method(PI+PI)
writeln(b) // -> 6.283185

// Inspect bs "source" code
writeln(getSlot("b") code) // -> block(PI +(PI))

// Make PI a constant
PI := Constant with(3.1415926)
writeln(b) // -> 6.283185
writeln(getSlot("b") code) // -> block(3.141593 +(3.141593))

// Still works but when we look at the code of b we see that
// the PIs have been replaced with PIs value!!!

A typical multiple-dispatch example is a game with Asteriods, Bullets and Ships that have collision methods for each combination of interactions. In this case (and perhaps almost all cases), we only care about the first argument. And the problem is having to write switch statements like this:

Ship collideWith := method(other,
   if(other isKindOf(Ship), ....; return)
   if(other isKindOf(Asteriod), ....; return)
   if(other isKindOf(Bullet), ....; return)
)

Here is a simple solution provided by Rich Collins:

Ship collideWith := method(other, other collideWithShip(self))
Ship collideWithShip := method(aShip, ....)
Ship collideWithAsteriod := method(anAsteriod, ....)
Ship collideWithBullet := method(aBullet, ....)

// with similar implementations for other objects