set rl_startup_hook only once

src/prolog/read.rs

@@ -107,16 +107,6 @@ pub fn readline_initialize() {
 }
 
 pub fn read_batch(prompt: &str) -> Result<&'static str, SessionError> {
-    unsafe {
-        use std::ptr::null;
-        use std::mem;
-
-        // deactivate the startup hook that emits a "?- " to the
-        // beginning of the readline buffer.
-        let p: *const i8 = null();
-        rl_startup_hook = mem::transmute(p);
-    }
-    
     match readline_rl(prompt) {
         Some(input) => Ok(input),
         None => Err(SessionError::UserPrompt)
@@ -131,7 +121,10 @@ fn read_line(prompt: &str) -> Result<&'static str, SessionError> {
 }
 
 unsafe extern "C" fn insert_query_prompt() -> i32 {
+    if let LineMode::Single = LINE_MODE {
         insert_text_rl("?- ");
+    }
+
     0
 }