Enhanced version

src/prolog/clause_types.rs

@@ -264,6 +264,7 @@ pub enum SystemClauseType {
     WAMInstructions,
     WriteTerm,
     WriteTermToChars,
+    ScryerPrologVersion,
 }
 
 impl SystemClauseType {
@@ -418,6 +419,7 @@ impl SystemClauseType {
             &SystemClauseType::WAMInstructions => clause_name!("$wam_instructions"),
             &SystemClauseType::WriteTerm => clause_name!("$write_term"),
             &SystemClauseType::WriteTermToChars => clause_name!("$write_term_to_chars"),
+            &SystemClauseType::ScryerPrologVersion => clause_name!("$scryer_prolog_version"),
         }
     }
 
@@ -552,6 +554,7 @@ impl SystemClauseType {
             ("$wam_instructions", 3) => Some(SystemClauseType::WAMInstructions),
             ("$write_term", 6) => Some(SystemClauseType::WriteTerm),
             ("$write_term_to_chars", 7) => Some(SystemClauseType::WriteTermToChars),
+            ("$scryer_prolog_version", 1) => Some(SystemClauseType::ScryerPrologVersion),
             _ => None,
         }
     }

src/prolog/machine/system_calls.rs

@@ -3515,6 +3515,15 @@ impl MachineState {
                     unreachable!()
                 }
             }
+            &SystemClauseType::ScryerPrologVersion => {
+                use crate::git_version::git_version;
+                let version = self[temp_v!(1)];
+                let buffer =
+                    git_version!(cargo_prefix = "cargo:", fallback = "unknown");
+                let chars = buffer.chars().map(|c| Addr::Char(c));
+                let result = Addr::HeapCell(self.heap.to_list(chars));
+                self.unify(version, result);
+            }
         };
 
         return_from_clause!(self.last_call, self)

src/prolog/toplevel.pl

@@ -54,7 +54,8 @@ print_help(Args) :-
     halt.
 
 print_version(Args) :-
-    write('v0.8.120'), nl, % TODO: Something better is required here.
+    '$scryer_prolog_version'(Version),
+    write(Version), nl,
     halt.
 
 gather_goal(Type, Args0, Goals) :-