Re: [Pharo-users] difference between double dispatch and the method explains here

[Roelof Wobben]

yes,  this is a real  tests from the pharo track on exercism.io I understand what you mean but maybe I overthinking things. But if we have a robot facing north and the robot turns to the left  , im my oponion it faces now to the east. like this test is saying : test04_RotatesTheRobotsDirection90DegreesClockwiseChangesTheDirectionFromEastToSouth     | result |     result := robotSimulatorCalculator         moveDirection: 'east'         position:             (Dictionary new                 add: 'x' -> 0;                 add: 'y' -> 0;                 yourself)         instructions: 'R'.     self         assert: result         equals:             (Dictionary new                 add: 'direction' -> 'south';                 add:                     'position'                         ->                             (Dictionary new                                 add: 'x' -> 0;                                 add: 'y' -> 0;                                 yourself);                 yourself) but I cannot come to the same outcome with this code : pointToName: aPoint   ^aPoint x isZero      ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]      ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]] maybe exercism.io is not a good way to practice and learn smalltalk but I found not a better one. or smalltalk is not for me. Roelof Op 8-4-2019 om 16:44 schreef Richard O'Keefe: The basic issue here is abstraction. An instance of "Robot" in your program is not a physical object.  How could it possibly point North, South, or Nor-nor-west?  It cannot. Its location and direction are abstract values *metaphorically* related to real world notions like position vectors and velocity vectors. "North" in this program is not a real thing, it is an *idea* which could be represented by 'North', 'north', #North, #north, $N, $n, 'Raki',  'raki',  #Raki,  #raki,  $R, $r, 137, (0@ -1), a picture of the star Polaris, the colour red (the conventional colour for that end of a compass needle which points north), a sound recording of a lecture by Alfred North Whitehead, or anything you please, as long as, inside the program, it *acts* the way *you* want "north" to act (which is not necessarily the way the physical direction North acts, and in fact in this case it most certainly is not). Locations and movements in a 2D space are, in Smalltalk, commonly represented by Points.  "Represented by." As for this method: test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth     | result |     result := robotSimulatorCalculator         moveDirection: 'north'         position:             (Dictionary new                 add: 'x' -> 0;                 add: 'y' -> 0;                 yourself)         instructions: 'A'.     self         assert: result         equals:             (Dictionary new                 add: 'direction' -> 'north';                 add:                     'position'                         ->                             (Dictionary new                                 add: 'x' -> 0;                                 add: 'y' -> 1;                                 yourself);                 yourself) PLEASE tell me that is not what they are actually using. Let's start with   (Dictionary new)      add: k1 -> v1;      ...      add: kn -> vn;      yourself Did you know that sending #add: to a dictionary is not portable?  Storing actual Association objects inside Dictionaries was originally an encapsulation error and remains a performance error, so there are Smalltalks that do not make that mistake.  The *portable* way to make a Dictionary is     (Dictionary new)        at: k1 put: v1;        ...        at: kn put: vn;        yourself. And why in the name of sanity are the keys *strings* instead of *symbols*?  This is not Smalltalk.  It is _javascript_ in drag. Now exercism.io has a habit of insisting on particular implementations.  For example, I completed the SML track, and found that the test code ONLY worked with Poly and not with any of the three SML implementations I already had on my machine.  Since you are doing this in Pharo, I take it that exercism.io will insist on the Smalltalk track being done in Pharo, and in that case it is *nauseating* to use a Dictionary when you could use a Point.  Old-fashioned Smalltalk style would have been to return something like    #(<direction> <x> <y>) e.g. #(north 1 0), and I still prefer that. In fact *good* Smalltalk style for something like this would be test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth   robotSimulatorCalculator     moveTo: 0@0;     head: #north;     obey: 'A'.   self assert: robotSimulatorCalculator heading equals: #north.   self assert: robotSimulatorCalculator location equals: 0@1. -- We're starting to get the idea that identifiers like robotSimulatorCalculator are not a very good idea when simulatedRobot would do the job as well or better. (This is also pointing us towards Betrand Meyer's Command/Query Separation principle, but we shan't go there today.) This is important feedback to give to the exercism.io people.  The test code should use a SMALLTALK interface, not a warmed-over _javascript_ interface. Now, how do we map between direction *names* and direction *points*?  Well, we have to start by laying down clearly what we *mean* by the directions. To move North one step is to add 1 to y and 0 to x. (We know that from the appalling test case above.) To move South one step is to add -1 to y and 0 to x. (South is the opposite of North.) To move East one step, oh we have a problem. THIS NEEDS SPELLING OUT.  And one of the things the exercism.io exercises are HORRIBLY BAD AT is specifying the problem.  Nearly every single exercise I have tried, I have been unable to tell what the problem is without examining the test cases, and that is not the way exercises are supposed to work.  (Yeah, that's why I'm screaming about it.  I've taught a class using exercises like this that were not of my writing and vague specifications really upset the students.  People who had taken the class under someone else several years before were still angry about it.) The geometric classes in Smalltalk were written to support graphic user interfaces.  And in user interfaces, the y coordinate increases DOWN.  So if we take the compass rose and rotate it so that North is DOWN, it follows that West is right and East is left.  So To move East one step is to add -1 to x and 0 to y. To move West one step is to add 1 to x and 0 to y. The chances are excellent that the problem specification is inconsistent with this.  Sigh.  Let's proceed, though. North  0@1 South  0@ -1 East   -1@0 West   1@0 pointToName: aPoint   ^aPoint x isZero      ifTrue:  [aPoint y > 0 ifTrue: [#north] ifFalse: [#south]]      ifFalse: [aPoint x > 0 ifTrue: [#west ] ifFalse: [#east ]] nameToPoint: aSymbol   aSymbol = #north ifTrue: [^0 @ 1].   aSymbol = #south ifTrue: [^0 @ -1].   aSymbol = #west  ifTrue: [^1 @ 0].   aSymbol = #east  ifTrue: [^-1 @ 0].   aSymbol error: 'not a compass direction in lower case'. Another problem I had with exercism was a "Space-Age" exercise where the README.md capitalised the planet names but test_Space-Age.<whatever> insisted on lower case. That might well happen here. Just for grins, Dictionary>> asPoint   ^(self at: 'x') @ (self at: 'y') Point>> asDictionary   ^(Dictionary new)      at: 'x' put: self x;      at: 'y' put: self y;      yourself On Mon, 8 Apr 2019 at 22:15, Roelof Wobben <r.wob...@home.nl> wrote: Richard thanks. One thing I do not see direct. you said : A direction could be represented by a pair of integers dx, dy such that |dx|+|dy| = 1.  It could also be represented by a Point with integer components. for me a direction is the direction the robot is facing so something like north or east. the challenge also wants a output like this : test11_MovesTheRobotForward1SpaceInTheDirectionItIsPointingIncreasesTheYCoordinateOneWhenFacingNorth     | result |     result := robotSimulatorCalculator         moveDirection: 'north'         position:             (Dictionary new                 add: 'x' -> 0;                 add: 'y' -> 0;                 yourself)         instructions: 'A'.     self         assert: result         equals:             (Dictionary new                 add: 'direction' -> 'north';                 add:                     'position'                         ->                             (Dictionary new                                 add: 'x' -> 0;                                 add: 'y' -> 1;                                 yourself);                 yourself) so how do I "convert" the point you are using to the text. Or do I misunderstood you somewhere wrong. Roelof Op 8-4-2019 om 10:57 schreef Richard O'Keefe: One thing I have often seen and lamented is students writing excessively complicated code with way too many classes.  There is a huge difference between   "A Robot knows its position and direction." and   "A Robot has-a Position and has-a Direction." The first is the important one.  The second is an over-commitment to too many classses.  For a problem like this, you really really do not want a Direction class, and you certainly have no use for double dispatch. A position can be represented by a pair of integers x, y.  It could also be represented by a Point with integer components. A direction could be represented by a pair of integers dx, dy such that |dx|+|dy| = 1.  It could also be represented by a Point with integer components. For movement, you need to be able to add the direction to the location, which could be simply x := x + dx.  y := y + dy. or it could be position := position + direction. For turning, you need to be able to rotate a direction vector by ninety degrees.  Now it so happens that Point has methods #leftRotated and #rightRotated. So we can do the following:    a Robot has position (a Point) and direction (aPoint)    position := 0 @ 0.    direction := 0 @ 1. To move forward without turning:    position := position + direction. To turn left without moving:    direction := direction leftRotated. To turn right without moving:    direction := direction rightRotated. To obey a sequence of characters, commands:    commands do: [:each |       each caseOf: {          [$A] -> [--move forward--].          [$L] -> [--turn left--].          [$R] -> [--turn right--]       }]. One of the key ideas in extreme programming is "You Ain't Gonna Need It", abbreviated to YAGNI! The idea is *DON'T* generalise beyond your immediate needs.  In this case, for example, the likelihood of *this* program needing to deal with more general kinds of movement is ZERO.  And the only reason for using Point here instead of just using a few simple assignment statements is that Point already exists, so costs nothing to write, and as a familiar class, code using it should be easy to read. If someone challenges you to do something counter-productive, refuse the challenge. On Mon, 8 Apr 2019 at 17:21, Roelof Wobben <r.wob...@home.nl> wrote: I can try to explain what I trying to solve. I have a Robot which can turn left,  turn right or moveForward. now I have a string like 'LAR' that means the robot needs to turn left (l) , move forward one place (A) and turn left. and I have to keep track to which direction the robot is facing and on which coordinate it stands. so to summarize with the above string lets say the robot is facing north on coordinate (0,0) then it has to turn left , so its facing east and still on coordinate (0,0) then it has to move forward, so its still  facing east but are on coordinate(0,1) then it has to turn right, so its facing north and on coordinate (0,1) and TimMacKinnon has challenged me to do this with double dispatch. So I think now I need a object Direction, a sub object North and a sub - sub object TurnLeft, turnRight and moveForward. So I can use double dispath first the direction North, East, South, West and then use double dispatch to find the right move. Roelof Op 8-4-2019 om 06:50 schreef Richard O'Keefe: It would really REALLY **REALLY** help if we knew what the heck you were trying to do.  There is an excellent chance that it is MUCH simpler than you think.  If you cannot show us the Smalltalk version of the problem, can you show us the version for some other language? On Sun, 7 Apr 2019 at 20:15, Roelof Wobben <r.wob...@home.nl> wrote: Op 6-4-2019 om 15:15 schreef K K Subbu: > On 06/04/19 4:49 PM, Roelof Wobben wrote: >> Hello, >> >> I just learned double dispatch. >> And now for the Robot challenge of exercism Tim has pointed me to >> this >> article(https://blog.metaobject.com/2019/04/accessors-have-message-obsession.html) >> >> but I fail to see how the move method looks like in that article. >> I had a conversation with Tim in the exercism channel and the way he >> explains it, it looks like double dispatch for me. >> >> Am I on the right track or do I oversee something here. > unary methods like moveRight perform specific ops and are not > parametric, so only a single dispatch, depending on the receiver, is > needed. > > If you change it to move: aDistanceOrAngle, then performing requests > like "move: 3 cms" or "move: 30 degrees" will depend not only on the > receiver but also on the class of the argument. This would need double > dispatch (aka multiple polymorphism). The first dispatch would be > based on the receiver and the receiver's method would then dispatch it > based on the class of the argument (i.e. Distance>>move or Angle>>move ) > > HTH .. Subbu > > hmm, still stuck I have now a class Direction with as instance variables north, south, east, west and made the accessors. then I thought I need a initialize like this : initialize     north = Direction( 0, -1).     east  = Direction( 1,  0).     south = Direction( 0,  1).     west  = Direction(-1,  0). but the Direction (0,-1)  is a problem . the compiler does not like the (0,-1) part to give you the big picture. I have a Robot which can turnRight , turnLeft and moveForward and I try to understand how the page would work in my case. So I have a object Direction as described above and a Object MoveForward which is a subobject of Direction. MoveForward has only 1 method : IsMove     ^  'A' Roelof