Welcome to the Grid!
Introduction · QuickStart · Library Predicates · User Shell · Outline
This document is an introduction to the GridWorld platform used in the Prolog labs of the 3rd year AI unit COMS30014 (and associated assessments COMS30013 and COMS30062).
1 Introduction
The purpose of Grid World lab platform is to provide an inspiring and fun environment to help you develop your practical Prolog programming skills and thereby obtain a deeper conceptual understanding of the underpinning theory. These labs are vital for all students because the skills and concepts you learn will be tested in both the exam and coursework assessments on this unit.
Formally, these Prolog labs are split into the following three parts, which introduce the foundational features of the GridWorld Webserver and Wikipedia library functionality:
lab grid – (week 2) interact with an empty grid by writing predicates to spiral your agent from an outside corner into the centre.
lab identity – (week 3) interact with Wikipedia by writing predicates to infer a secret actor identity using clues given by a disembodied (o-grid) oracle.
lab search – (week 7) interact with a non-empty grid by writing predicates to nd a path that allows your agent to visit an embodied (on-grid) oracle.
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In these labs, you will not need to understand or even look at the library code — although you are welcome to try and do so if you wish!
These labs assume you have a running version of SWI Prolog and are developing a basic knowledge of logic programs obtained by working through the recommended Learn Prolog Now! tutorial. You should be starting to get comfortable running programs in SWIPL and editing code in a text editor (e.g. in PceEMacs, Atom, etc). You should also be starting to use the online manual and exploring the use of the built-in debugger.
In order develop good coding practice you are strongly advised to: comment your code to explain the meaning of each argument and the behaviour of each predicate; format your code to make the logical ow clear (using informative variable and predicate names); test your code to make sure it compiles without errors and that predicates terminate properly which means that wherever possible they should be (semi-)deterministic in the sense that they should terminate after succeeding once (or failing). You should especially try to avoid non-termination and run-time exceptions or predicates that return duplicate solutions or leave unnecessary choice points.
2 QuickStart
a) First you’ll need to install the GridWorld library on your machine:
Download the GridWorld library ZIP le
Extract the GridWorld library les to a convenient location on your machine Move into the GridWorld library root directory in which you should see:
three skeleton answer les (lab_grid_12345.pl, lab_identity_12345.pl and lab_search_12345.pl) where you’ll write your solutions for each lab
one lab runner le (ailp.pl) which is responsible for loading the library functions and solution le for each of the respective labs
Rename the skeleton les by replacing 12345 with your student number
While this last step of renaming the skeleton les it is not strictly necessary for these labs, it is good practice, especially if you plan to go on to do the coursework assessment option (where this will be required).
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BUT, if you do rename these les, then please make sure to use your (7-digit) student number and not your username (which contains letters that will disrupt the loading mechanism) or your candidate number (which should only be used when you are taking exams)!
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b) Then you’ll need to invoke one of the labs using the loader:
Open a terminal window (e.g. bash, cmd, powershell, etc.)
Run one of the labs using a command of the form swipl ailp.pl lab X, where X stands for one of the three Prolog terms grid or identity or search.
On Linux or Mac you may also be able to use slightly shorter commands like ./ailp.pl lab grid if you make the loader executable using chmod +x ailp.pl
On Windows you may also be able to double click on aipl.pl in an explorer window and then enter a term like lab grid at the Prolog prompt. Or you could type swipl-win ailp.pl lab grid in a (cmd or powershell) terminal.
These instructions assume the SWI installation directory is on your system path (which will be the case if you follow the installation advice given in prior lectures).
c) In the grid and search labs (but not in the identity lab) you’ll need to open a GridWorld webserver by running the following library commands at the Prolog prompt:
start.
then hit the “y” key (or any key except “n” or “N”) at the prompt to open a browser
and click the “Run” button at the bottom left of the resulting browser window join_game(A).
reset_game.
start_game.
To save pressing “y” you can instead launch the webserver with start. .
To save some typing you can instead run the last three commands with the command shell. followed by the macro setup.
In the grid and search labs, you’ll only be able to add a single agent which will always be given the identier A=1. In the identity lab, you won’t be able to run a GridWorld server but will use a special agent oscar that exists o grid!
In order to see the “Run” button, you may need to scroll down past an initially empty space (where the grid will be subsequently drawn once a game is started)
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Please note that nothing will happen below until you click “Run” in the browser!
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d) To interact with the GridWorld use the library and macro commands dened in Section 3 and Section 4 below! For example:
in lab grid you can try agent_do_moves(1,[p(1,2),p(1,3),p(1,4)]). when your agent is located at the initial position p(1,1) in the top left corner.
in lab identity you can try agent_ask_oracle(oscar,o(1),link,L). to get a link from the Wikipedia page of some secret actor you are trying to identify.
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e) In the grid and search labs (but not in the identity lab) it is good practice to close the GridWorld webserver by running the following library commands at the Prolog prompt:
leave_game.
stop.
To quit Prolog altogether use the command halt.
To abort a computation that seems to be hanging, you can try hitting ctrl-c, ctrl-
d or ctrl-x a few times possibly followed by hitting the a key
3 Library Predicates
These labs comprise a set of three consecutive games that involve interacting (over http using the ailp library predicates described below for working with) either with a localhost grid server (in the grid and search labs in weeks 2 and 7) or with live Wikipedia pages (in the identity lab in week 3).
The two grid-based games will involve you writing Prolog code to navigate a single agent around a 10×10 square grid rendered in a browser window. The location of each cell a grid will be represented by a Prolog term p(X,Y) where X and Y are natural numbers denoting the horizontal and vertical osets (rightwards and downwards) from the top left corner (as labelled on the grid). The contents of each cell in the grid is represented by exactly one of the following Prolog terms (where N is an integer identier of the corresponding object):
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After you make any changes to your code in the skeleton answer les don’t forget to run the commands make followed by reset_game/start_game (at which point your GridWorld browser window should automatically refresh)
Also, please make sure the game is not paused in the browser when you call reset_game or the server may hang due to a bug in the way http responses are assumed to be sequenced.
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Welcome to the Grid!
Term Colour
a(N) random empty green o(N) red t(N) black
Object
agent
empty space
oracle
Meaning
a user-controllable agent is located at this position (colour and shape chosen at random)
an agent adjacent to this cell can move here
an agent adjacent to this cell can ask this oracle a question
these represent “walls” or “obstacles” that your agent cannot move through
thing
Your agent will be allowed move one step at a time to any empty adjacent (on-grid) cell to
the immediate South, East, North or West of its current position. You will control your agent using the following library predicates.
Note that, in the remainder of this documentation, predicates are often annotated with their respective arities (name/arity) and arguments are often annotated with their intended modes (+In, -Out or ?Any).
BUT these arity and mode decorations should not be typed in any actual code – there are included in the documentation to show how the predicates should be used. As explained in the SWI manual:
an input argument (+) must be instantiated to a correctly typed term when the predicate is called,
an output argument (-) will become instantiated (if it wasn’t already) when the predicate succeeds,
and partial arguments (?) may be variable, ground or partially instantiated when the predicate is called and/or succeeds.
Please note that (SWI built-in and GridWorld library) predicates are only guaranteed to work properly (or even at all) when used in the correct way!
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Predicate Meaning
my_agent(-A)
return in A the integer id of the last Agent joined to the game (which will usually be 1 though you shouldn’t rely on that in code!)
ailp_grid_size(-S)
return in S the integer Size of (height and width) of the grid (which will usually be 10 though you shouldn’t rely on that in code!)
get_agent_position(+A,-Pos) return the specied Agent’s current Position
agent_do_moves(+A,+Path)
move the specied Agent on the grid along the specied Path (which should be a list of grid locations consecutively accessible from the current position) or fail at the rst point where a move is found to be invalid
say(+Message,+A)
Table 1: Library Predicates for Lab Grid
print the given Message string next to the specied Agent’s current position on the grid (which maybe useful for debugging?)
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Predicate Meaning
wp(+T)
print to stdout an encoding of the Wiki- pedia page with the given Title (which may be an actor name written as a Prolog term like ‘Billy Bob Thornton’)
wp(+T,-WT)
return the WikiText of the Wikipedia page with the given Title
wt_link(+WT,-Link)
successively return each Link contained inside the given WikiText
actor(-N)
successively return each of 12 predened possible secret actors Names
link(-L)
successively return each of 15 predened possible Links from their Wikipedia pages
agent_ask_oracle(oscar,o(1),link,-L)
the agent oscar can (repeatedly) request the oracle o(1) to return a random Link from a secret actor’s Wiki-pedia page
test
Table 2: Library Predicates for Lab Identity
Table 3: Library Predicates for Lab Search (additional to Table 1)
Test the solution predicate nd_identity(A) succeeds for all possible secret identities
Predicate Meaning
map_adjacent(+Pos,-Adj,-Obj)
given a grid Position, return any Adjacent on-grid cell location along with the Object it contains (or the term empty)
Note that map_adjacent/3 with modes map_adjacent(+Pos,-Adj,-Obj) must have an instantiated rst argument when you call it. So, given a Position, it returns Adjacent positions and Objects, but not vice versa!
Although you, the user, will be able to see the layout of the grid in the browser window, please remember your agent can only obtain that information by making calls to the relevant cells using map_adjacent/3 (which are comparatively expensive as they operate over http); Thus, the tasks of eciently nding an
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optimal path to a given location or nding the location of a given object are non- trivial (given the restrictions imposed by the above mode declarations)
Although you won’t need to exploit this fact in these labs, the library allows multiple clients to interact with the server over http via an internal predicate query_world/2 that enables one or more Prolog threads running on your machine to join agents to a game, move them around and query the grid. The only thing you need to know is that, because this all happens over http, calls that involve looking up the contents of a cell or moving an agent will have a signicant time overhead as compared to standard Prolog queries.
You may also look at http interactions in your browser by looking in the networking tab of the dev menu for your web browser. This can usually be opened using F12.
4 User Shell
In order to further facilitate user interaction during a session, the GridWorld also provides an interactive user shell that can be invoked with the following command that allows the user to run the set of macros below (which can reduce the amount of typing you need to do):
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Command
?-shell.
Macro
?help.
?stop.
?setup.
?reset.
?status.
?whoami.
?position.
?search.
?call(+G).
Meaning
open interactive shell providing the following macros % labs grid & search only
Meaning
% display a list of the macros below
% exit from this command shell ?-join_game(A),reset_game,start_game. ?-reset_game,start_game. ?-query_world(game_status,[A]) % e.g. running/ready ?-my_agent(A) ?-my_agent(A),get_agent_position(A,P)
?-search_bf % lab search only ?-findall(G,call(G),L).
Table 4: Possible shell macros.
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You can enter and leave the user shell at any time; and you can run non-shell commands from within the shell by wrapping them up as an argument to a call macro – which will implicitly nd all solutions of the specied goal.
If you want to make your head hurt, try running call(shell) from within the shell!
5 Outline
lab grid identity search
Week2 3 7
Solution lename
lab_grid_12345.pl
lab_identity_12345.pl
lab_search_12345.pl
Solution predicate
spiral/1
find_identity/1
search_bf/0
Needs internet?
No
Yes (access Wikipedia)
No
Needs localhost?
Yes (access grid)
No
Yes (access grid)
Provides shell?
Yes
No
Yes
Agent Name
a(1)
oscar
a(1)
Oracle Name
not applicable
o(1)
o(1)
Table 5: Outline of labs.
formatted by Markdeep 1.14
Note how the shell prompt “?” omits the dash in the standard Prolog prompt “?-“.
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