2.4 Porting World Programs to Universe

Matthias Felleisen, Robby Findler

2.4.1 The World is Not Enough

With the June 2009 release, we started deprecating the world teachpack; instead we recommended the use of the universe teachpack. With the January 2010 release, we are also introducing a new image teachpack and, in support of this second teachpack, we have separated out the image functionality from the functionality for world programs.

In this document, we explain how to port programs that assume the old world teachpack into this new setting, one step at a time. Most importantly, programs must now import two teachpacks insteead of one:

World Style

Universe Style

(require htdp/world)

(require 2htdp/universe)
(require htdp/image)

The table shows the old style on the left and the new style on the right. If your programs imported teachpacks via the drscheme teachpack menu, we recommend that you use the require form from now on; alternatively, you use the drscheme menu twice to import the functions from two teachpacks.

In the next section, we first explain how to port world programs so that they use the universe teachpack and the old image teachpack. In the section after that, we list suggestions for changing programs so that they no longer rely on the old image functionality but the new one.

In order to distinguish between the various pieces of functionality, we uniformly prefix old functionality with "htdp:" and new functionality with "2htdp:". There is no need to use these prefixes in your programs of course.

2.4.2 Porting World Programs

Here is the first program from the documentation for the world teachpack:

(require htdp/world)

; Number -> Scene
(define (create-UFO-scene height)
  (htdp:place-image UFO
                    50 height
                    (htdp:empty-scene 100 100)))

; Scene
(define UFO
  (htdp:overlay
    (htdp:circle 10 'solid 'red)
    (htdp:rectangle 40 4 'solid 'red)))

; — run program run
(htdp:big-bang 100 100 (/1 28) 0)
(htdp:on-tick-event add1)
(htdp:on-redraw create-UFO-scene)

This program defines a function for placing a UFO into a 100 by 100 scene, where UFO is a defined image. The world program itself consists of three lines:

the first one creates the 100 by 100 scene, specifies a rate of 28 images per second, and 0 as the initial world description;
the second one says that for each clock tick, the world (a number) is increased by 1; and
the last line tells drscheme to use create-UFO-scene as the function that renders the current world as a scene.

Let us now convert this program into the universe setting, step by step, staring with the require specification, which is converted as above:

World Style

Universe Style

(require htdp/world)

(require 2htdp/universe)
(require htdp/image)

The function that renders the world as a scene remains the same:

World Style

Universe Style

; Number -> Scene
(define (create-UFO-scene height)
  (htdp:place-image
    UFO
    50 height
    (htdp:empty-scene 100 100)))

For the image constant we switch from symbols to strings:

World Style

Universe Style

; Scene
(define UFO
  (htdp:overlay
    (htdp:circle
     10 'solid 'red)
    (htdp:rectangle
     40 4 'solid 'red)))

; Scene
(define UFO
  (htdp:overlay
    (htdp:circle
     10 "solid" "red")
    (htdp:rectangle
     40 4 "solid" "red")))

Strictly speaking, this isn’t necessary, but we intend to replace symbols with strings whenever possible because strings are more common than symbols.

The most important change concerns the lines that launch the world program:

World Style

Universe Style

(htdp:big-bang 100 100 (/1 28) 0)
(htdp:on-tick-event add1)
(htdp:on-redraw create-UFO-scene)

(2htdp:big-bang
  0
  (on-tick add1)
  (on-draw create-UFO-scene))

They are turned into a single expression that comes with as many clauses as there are lines in the old program. As you can see, the big-bang expression from the universe teachpack no longer requires the specification of the size of the scene or the rate at which the clock ticks (though it is possible to supply the clock rate if the default is not satisfactory). Furthermore, the names of the clauses are similar to the old names but shorter.

The other big change concerns key event handling and mouse event handling. The respective handlers no longer accept symbols and chars but strings only. Here is the first key event handler from the documentation of the world teachpack:

World Style

Universe Style

(define (change w a-key-event)
   (cond
     [(key=? a-key-event 'left)
      (world-go w -DELTA)]
     [(key=? a-key-event 'right)
      (world-go w +DELTA)]
     [(char? a-key-event)
      w]
     [(key=? a-key-event 'up)
      (world-go w -DELTA)]
     [(key=? a-key-event 'down)
      (world-go w +DELTA)]
     [else
      w]))

(define (change w a-key-event)
   (cond
     [(key=? a-key-event "left")
      (world-go w -DELTA)]
     [(key=? a-key-event "right")
      (world-go w +DELTA)]
     [(= (string-length a-key-event) 1)
      w]
     [(key=? a-key-event "up")
      (world-go w -DELTA)]
     [(key=? a-key-event "down")
      (world-go w +DELTA)]
     [else
      w]))

Note how the char? clause changed. Since all chars are now represented as strings containing one “letter”, the program on the right just checks the length of the string. Otherwise, we simply change all symbols into strings.

If you ever recorded your programs’ work via an animated gif, you can still do so. Instead of adding a fifth argument to big-bang, however, you will need to add a clause of the shape (record? x).

Finally, the universe teachpack implements a richer functionality than the world teachpack.

2.4.3 Porting Image Programs

The universe library also comes with a new image library, 2htdp/image. Using the old image library still works fine with 2htdp/universe, but the new image library provides a number of improvements, including faster image comparison (especially useful in check-expect expressions), rotating images, scaling images, curves, a number of new polygon shapes, and more control over line drawing.

To use the new image library in isloation:

World Style
Universe Style
(require htdp/image)
(require 2htdp/image)

and to use the new image library with the universe teachpack:

World Style
Universe Style
(require htdp/world)
(require 2htdp/universe)
(require 2htdp/image)

Overlay vs Underlay

The htdp:overlay function places its first argument under its second (and subsequent) arguments and so in 2htdp/image, we decided to call that function 2htdp:underlay.

World Style
Universe Style
(htdp:overlay
(htdp:rectangle
  10 20 "solid" "red")
(htdp:rectangle
  20 10 "solid" "blue"))
(2htdp:underlay
(2htdp:rectangle
  10 20 "solid" "red")
(2htdp:rectangle
  20 10 "solid" "blue"))

No more pinholes

The concept of pinholes from htdp/image has no correspondance in 2htdp/image (we do expect to bring back pinholes in 2htdp/image eventually, but they will not be as pervasive as they are in htdp/image).

Instead of a special position in the image that overlay operations are sensitive to, 2htdp/image has a family of overlay operations, that overlay images based on their centers or their edges.

Since the default position of the pinhole is in the center for most images and the default for overlaying and underlaying images in 2htdp/image is based on the center, simple examples (like the one above) behave the same in both libraries.

But, consider this expression that overlays two images on their upper-left corners, written using both libraries.

World Style
Universe Style
(htdp:overlay
(htdp:put-pinhole
  (htdp:rectangle 10 20 "solid" "red")
  0 0)
(htdp:put-pinhole
  (htdp:rectangle 20 10 "solid" "blue")
  0 0))
(2htdp:underlay/align
"left"
"top"
(2htdp:rectangle
  10 20 "solid" "red")
(2htdp:rectangle
  20 10 "solid" "blue"))

In the 2htdp/image version, the programmer uses 2htdp:underlay/align to specify where the images should be lined up, instead of using the pinhole.

Outlines in different places

The outline style shapes are now shifted by one pixel for 2htdp/image images as compared to htdp/image. This means that these two rectangles draw the same sets of pixels.

World Style
Universe Style
(htdp:rectangle
11 11 "outline" "black")
(2htdp:rectangle
10 10 "outline" "black")

Star changed

The 2htdp:star function is a completely different function from htdp:star. Both produce stars based, on polygons, but 2htdp:star always produces a five-pointed star. See also 2htdp:star-polygon for more general star shapes.

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2.1	Batch Input/ Output: "batch-io.ss"
2.2	Images: "image.ss"
2.3	Worlds and the Universe: "universe.ss"
2.4	Porting World Programs to Universe

2.4.1	The World is Not Enough
2.4.2	Porting World Programs
2.4.3	Porting Image Programs

World Style	Universe Style
(require htdp/image)	(require 2htdp/image)