a few typos in universe.scrbl fixed
svn: r13138
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@ -126,7 +126,7 @@ Your program may deal with such events via the @emph{designation} of
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installation of three event handlers: @scheme[on-tick], @scheme[on-key],
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and @scheme[on-mouse]. In addition, a @tech{world} program may specify a
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@scheme[draw] function, which is called every time your program should
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visualize the current world, and a @scheme[_stop?] predicate, which is used
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visualize the current world, and a @scheme[done] predicate, which is used
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to determine when the @tech{world} program should shut down.
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Each handler function consumes the current state of the @tech{world} and
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@ -835,7 +835,7 @@ The @scheme[on-receive] clause of a @scheme[big-bang] specifies the event handle
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The diagram below summarizes the extensions of this section in graphical form.
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@image["world.png"]
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@image["universe.png"]
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A registered world program may send a message to the universe server
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at any time by returning a @tech{Package} from an event handler. The
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@ -964,7 +964,7 @@ A @tech{server} keeps track of information about the @tech{universe} that
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a server tracks and how the information is represented depends on the
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situation and the programmer, just as with @tech{world} programs.
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@deftech{UniverseState} @scheme[any/c] represents the server's state For running
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@deftech{UniverseState} @scheme[any/c] represents the server's state. For running
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@tech{universe}s, the teachpack demands that you come up with a data
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definition for (your state of the) @tech{server}. Any piece of data can
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represent the state. We just assume that you introduce a data definition
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@ -1368,7 +1368,7 @@ The second step of the design recipe calls for functional examples:
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;; an example for receiving a message from the active world:
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(check-expect
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(switch (list world1 world2) '* world1 'it-is-your-turn)
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(switch (list world1 world2) '* world1 'done)
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(make-bundle (list world2 world1)
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'*
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(list (make-mail world2 'it-is-your-turn))))
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@ -1421,16 +1421,20 @@ Similarly, the protocol says that when @emph{switch} is invoked because a
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that there is at least this one world on this list. It is therefore
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acceptable to create a mail for this world.
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Start the server now.
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@schemeblock[(universe '* (on-new add-world) (on-msg switch))]
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Exercise: The function definition simply assumes that @emph{wrld} is
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@scheme[world=?] to @scheme[(first univ)] and that the received message
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@emph{m} is @scheme['it-is-your-turn]. Modify the function definition so that it
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@emph{m} is @scheme['done]. Modify the function definition so that it
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checks these assumptions and raises an error signal if either of them is
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wrong. Start with functional examples. If stuck, re-read the section on
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checked functions from HtDP. (Note: in a @tech{universe} it is quite
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possible that a program registers with a @tech{server} but fails to stick
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to the agreed-upon protocol. How to deal with such situations properly
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depends on the context. For now, stop the @tech{universe} at this point,
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but consider alternative solutions, too.)
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depends on the context. For now, stop the @tech{universe} at this point by
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returning an empty list of worlds. Consider alternative solutions, too.)
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Exercise: An alternative state representation would equate
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@tech{UniverseState} with @emph{world} structures, keeping track of the
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@ -1533,7 +1537,7 @@ the scene every time @scheme['it-is-your-turn] is received. Design this function
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(define (receive w m)
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(cond
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[(symbol=? 'resting w) HEIGHT]
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[(symbol? w) HEIGHT] ;; meaning: @scheme[(symbol=? w 'resting)]
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[else w]))
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))
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@ -1573,7 +1577,7 @@ the scene every time @scheme['it-is-your-turn] is received. Design this function
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[(number? x) (if (<= x 0) (make-package 'resting 'done) (sub1 x))]))
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))
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Exercise: what could happen if we had designed @emph{receive} so that it
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Exercise: what could happen if we had designed @emph{receive} so that it
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produces @scheme['resting] when the state of the world is @scheme[0]? Use
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your answer to explain why you think it is better to leave this kind of
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state change to the tick event handler instead of the message receipt
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@ -1591,7 +1595,7 @@ Finally, here is the third function, which renders the state as a scene:
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(check-expect (render 'resting)
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(place-image (text "resting" 11 'red) 10 10 MT))
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(define (render name)
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(define (render w)
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(place-image
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(text name 11 'black) 5 85
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(cond
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Binary file not shown.
Before Width: | Height: | Size: 15 KiB After Width: | Height: | Size: 17 KiB |
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@ -10,7 +10,8 @@
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" (on-draw render WIDTH HEIGHT)"
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" (on-tick tock RATE)"
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" (on-mouse click)"
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" (on-key react))"
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" (on-key react)"
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" (stop-when done))"
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))
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(define program
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@ -114,6 +115,6 @@
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(draw-pict the-image image-dc 0.0 0.0)
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(send image-bm save-file "world.png" 'png)
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(send image-bm save-file "nuworld.png" 'png)
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the-image
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@ -12,6 +12,7 @@
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" (on-mouse click)"
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" (on-key react)"
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" (on-receive receive)"
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" (stop-when done)"
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" (register LOCALHOST 'jimbob))"))
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