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tutorials:intermediate:json_prolog [2019/04/24 11:37] – [Importing a package with queries] gkazhoyatutorials:intermediate:json_prolog [2022/01/25 16:35] (current) – [Prerequisites] gkazhoya
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 This tutorial requires CRAM, roslisp_repl and ROS to have already been installed. Go through the previous tutorials and the installation page if you don't have CRAM installed yet. This tutorial requires CRAM, roslisp_repl and ROS to have already been installed. Go through the previous tutorials and the installation page if you don't have CRAM installed yet.
 Make sure you have **Java 8** and **Gradle 5.1** already installed. Otherwise you won't be able to build KnowRob. Other versions could work, but have not been tested yet, so use them at your own risk!  Make sure you have **Java 8** and **Gradle 5.1** already installed. Otherwise you won't be able to build KnowRob. Other versions could work, but have not been tested yet, so use them at your own risk! 
- 
  
 === Installation === === Installation ===
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 === Launch === === Launch ===
-In addition to a **roscore** and the **roslisp_repl** you need to launch **json_prolog** in a Terminal with the following command: +In addition to a **roscore** and the **roslisp_repl** you need to launch **json_prolog** in a Terminal with the following command: 
 +<code bash> 
 +    $ roslaunch rosprolog rosprolog.launch 
 +</code> 
 +or if you're using an older version of KnowRob, then the following command:
 <code bash> <code bash>
     $ roslaunch json_prolog json_prolog.launch      $ roslaunch json_prolog json_prolog.launch 
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        (NIL)        (NIL)
 </code> </code>
 +
 +In JSON Prolog ''(NIL)'' means success, and ''NIL'' means failure. This is explained in detail [[tutorials/intermediate/json_prolog?&#nil_vs_nil|in the next section]].
          
 This allows us to use the ''knowrob_common'' package, which allows us to access many utility functions and also the most commonly used queries of KnowRob. This allows us to use the ''knowrob_common'' package, which allows us to access many utility functions and also the most commonly used queries of KnowRob.
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 <code lisp> <code lisp>
-    CL-USER (json-prolog:prolog-simple "lowercase('HELLOWORLD',Result)") +    CL-USER (json-prolog:prolog-simple "lowercase('HELLOWORLD',RESULT)") 
-       (((|?Result| . |''helloworld''|))+       (((?RESULT . |''helloworld''|))
         . #S(CRAM-UTILITIES::LAZY-CONS-ELEM         . #S(CRAM-UTILITIES::LAZY-CONS-ELEM
               :GENERATOR #<CLOSURE (LAMBDA () :IN JSON-PROLOG::PROLOG-RESULT->BDGS)               :GENERATOR #<CLOSURE (LAMBDA () :IN JSON-PROLOG::PROLOG-RESULT->BDGS)
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 </code> </code>
  
-This query converts the first elements upper case letters into lower case, and stores the obtained result in the ''Result'' variable.+This query converts the first elements upper case letters into lower case, and stores the obtained result in the ''RESULT'' variable.
  
-An overview of the currently available packages and their queries can be found [[http://www.knowrob.org/api | here]]. If when loading a package Emacs only returns ''NIL'' then it might be that the package does not exist or is not found on your system. In such cases, you can check the terminal in which json-prolog was launched. If it cannot be found, you will see the following error: +An overview of the currently available packages and their queries can be found [[http://www.knowrob.org/api|here]]. If when loading a package Emacs only returns ''NIL'' then it might be that the package does not exist or is not found on your system. In such cases, you can check the terminal in which json-prolog was launched. If it cannot be found, you will see the following error: 
 <code bash> <code bash>
    [rospack] Error: package 'knowrob_maps_tools' not found    [rospack] Error: package 'knowrob_maps_tools' not found
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 <code lisp> <code lisp>
- 
     Prolog query failed: PrologException: error(existence_error(procedure, '/'(u_load_episodes, 1)), context(':'(system, '/'(call, 1)), _1))     Prolog query failed: PrologException: error(existence_error(procedure, '/'(u_load_episodes, 1)), context(':'(system, '/'(call, 1)), _1))
     [Condition of type SIMPLE-ERROR]     [Condition of type SIMPLE-ERROR]
-     
 </code> </code>
          
-and if you check your terminal, in which the **json_prolog** node is running, there will be a huge java print as well, basically explaining the same thing.+and if you check your terminal, in which the ''json_prolog'' node is running, there will be a huge java print as well, basically explaining the same thing.
      
-     + 
-==== Some more information about Prolog in LISP ====+==== Case-sensitive variable names ==== 
 + 
 +In the former query we got a weird result from Prolog which looked like this:  
 +<code lisp> 
 +|''helloworld''| 
 +</code> 
 +What do the pipes stand for in this case? 
 + 
 +Lisp is case-insensitive, that means that variable of function names, or more correctly said, any symbol, can be written in any capitalization and will refer to the same symbol: 
 + 
 +<code lisp> 
 +CL-USER> (eq 'hello 'HELLO) 
 +T 
 +CL-USER> (eq 'hello 'Hello) 
 +T 
 +</code> 
 + 
 +Prolog, on the other hand, is case-sensitive. 
 +And in some cases, one would like to have a case-sensitive variable in Lisp as well. 
 +This is where the pipes come into play: 
 +<code lisp> 
 +CL-USER> (intern "HELLO") 
 +HELLO 
 +:INTERNAL 
 +CL-USER> (intern "Hello") 
 +|Hello| 
 +NIL 
 +</code> 
 +The function ''intern'' creates a symbol from a string. 
 +If the string is all capital, we get a normal usual symbol, in the other case we get the pipes. 
 + 
 +The same is with Prolog, if your variable or any other symbol is not all caps, you will get a symbol in Lisp which has pipes around it: 
 + 
 +<code lisp> 
 +CL-USER> (json-prolog:prolog-simple-1 "member(X, [1,2,3])") 
 +(((?X . 1))) 
 +CL-USER> (json-prolog:prolog-simple-1 "member(MYVAR, [1,2,3])") 
 +(((?MYVAR . 1))) 
 +CL-USER> (json-prolog:prolog-simple-1 "member(MY_VAR, [1,2,3])") 
 +(((?MY_VAR . 1))) 
 +CL-USER> (json-prolog:prolog-simple-1 "member(My_var, [1,2,3])") 
 +(((|?My_var| . 1))) 
 +</code> 
 + 
 +In that case, to get the value of the variable you should also use pipes: 
 + 
 +<code lisp> 
 +CL-USER> (cut:var-value '|?My_var| 
 +                        (car (json-prolog:prolog-simple-1 "member(My_var, [1,2,3])"))) 
 +1 
 +</code> 
 + 
 +Please note that in Prolog, all variables have to start with a capital letter and cannot contain dashes: 
 +<code lisp> 
 +CL-USER> (json-prolog:prolog-simple-1 "member(x, [1,2,3])") 
 +NIL 
 +CL-USER> (json-prolog:prolog-simple-1 "member(MY-VAR, [1,2,3])") 
 +NIL 
 +</code> 
 +==== (NIL) vs NIL ====
          
 If we want to check, if a certain number is member of the list, we must declare the variable accordingly. e.g. If we want to check, if a certain number is member of the list, we must declare the variable accordingly. e.g.