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tutorials:intermediate:json_prolog [2019/04/24 11:37] – [Importing a package with queries] gkazhoyatutorials:intermediate:json_prolog [2019/04/24 11:46] – gkazhoya
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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: ''|''helloworld''|''. 
 + 
 + 
 +==== (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.