io: change prompt after line continuation

This ensures that line continuations inside of e.g. integer and float
expressions properly show a line continuation prompt.

Fixes #1992

src/io.c

@@ -220,6 +220,13 @@ Char GET_NEXT_CHAR(void)
             // if we see a backlash without a line terminator after it, stop
             break;
         }
+
+        // if we get here, we saw a line continuation; change the prompt to a
+        // partial prompt from now on
+        if (!SyQuiet)
+            STATE(Prompt) = "> ";
+        else
+            STATE(Prompt) = "";
     }
 
     return *STATE(In);

tst/testinstall/linecontinuation.tst

@@ -8,11 +8,17 @@ gap> START_TEST("linecontinuation.tst");
 gap> x:="foo\
 > bar";
 "foobar"
+gap> "foo\
+> bar";
+"foobar"
 
 # in triple quoted string
-gap> x:="""haha\
-> !""";
-"haha!"
+gap> x:="""foo\
+> bar""";
+"foobar"
+gap> """foo\
+> bar""";
+"foobar"
 
 # break keywords and operators like :=, <=, >= etc. in the middle
 gap> 1 m\
@@ -35,6 +41,19 @@ gap> {x.\
 > ..}->x;
 function( x... ) ... end
 
+# inside float expressions
+gap> 1.2e\
+> 0;
+1.2
+gap> 1.1\
+> ;
+1.1
+
+# inside integer expressions
+gap> 12\
+> 3;
+123
+
 # however, in comments, you cannot use line continuations:
 gap> # 1234\
 gap> 5;