I just cannot figure out how to print the value of X. Here is what I tried in the toplevel:
59 ?- read(X). |: 2. X = 2. 60 ?- write(X). _G253 true. What is _G253? I dont want the index number, I want the value X is bound to. What should I do in order to print the value of X?
2 Answers
When you type write(X). at the interactive prompt, and nothing else, X is not bound to anything in particular. If you want to read X from the user and then write it, try typing read(X), write(X). at the prompt.
?- read(X), write(X). |: 28. 28 X = 28. SWI Prolog does keep a history of top-level bindings; type help. to go into the manual, then search for bindings or just navigate to section 2.8 of the manual, 'Reuse of top-level bindings'. There, you can learn that the most recent value of any variable bound in a successful top-level goal is retained, and can be referred to using the name of the variable, prefixed with a dollar sign. So interactions like the following are possible:
?- read(X). |: 42. X = 42. ?- write($X). 42 true. But a top-level goal that just happens to use the variable name X will be interpreted as using a fresh variable; to do otherwise would violate the normal semantics of Prolog.
prolog - take as input and print the value of a variable.
go:- write('Enter a name'),nl, read(Name),nl, print(Name). print(Name):- write(Name),write(', Hello !!!'). 
