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--- tutorials:intermediate:json_prolog [2019/04/24 11:43] – [Importing a package with queries] gkazhoya
+++ tutorials:intermediate:json_prolog [2022/01/25 16:35] (current) – [Prerequisites] gkazhoya
@@ Line 9: / Line 9: @@
 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!
 === Installation ===
@@ Line 28: / Line 27: @@
 === 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>
     $ roslaunch json_prolog json_prolog.launch
@@ Line 207: / Line 210: @@
 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.
+==== 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])")))
+
+</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 ====