modern: Still support ``\ foo`` for now.

extra/modern/modern-tests.factor

@@ -62,7 +62,7 @@ IN: modern.tests
     { { "SYNTAX:" { "\\AVL{" } } }
 } [ "SYNTAX: \\AVL{" string>literals >strings ] unit-test
 
-{ { "\\" } } [ "\\" string>literals >strings ] unit-test
+[ "\\" string>literals >strings ] must-fail ! \ alone should be legal eventually (?)
 { { "\\FOO" } } [ "\\FOO" string>literals >strings ] unit-test
 
 {
@@ -81,4 +81,10 @@ IN: modern.tests
 [ "char: {" string>literals >strings ] must-fail
 [ "char: \"" string>literals >strings ] must-fail
 { { { "char:" "\\\\" } } } [ "char: \\\\" string>literals >strings ] unit-test
-{ { { "char:" "\\" } } } [ "char: \\" string>literals >strings ] unit-test
+
+[ "char: \\" string>literals >strings ] must-fail ! char: \ should be legal eventually
+
+! \ backslash going away someday
+
+{ { { "\\" "(" } } } [ "\\ (" string>literals >strings ] unit-test
+

extra/modern/modern.factor

@@ -198,7 +198,15 @@ ERROR: colon-word-must-be-all-uppercase-or-lowercase n string word ;
     dup { [ "!" sequence= ] [ "#!" sequence= ] } 1||
     [ take-comment ] [ merge-slice-til-whitespace ] if ;
 
-ERROR: backslash-expects-whitespace slice ;
+ERROR: no-backslash-payload n string slice ;
+: read-backslash ( n string slice -- n' string obj )
+    merge-slice-til-whitespace dup "\\" tail? [
+        ! \ foo, M\ foo
+        [
+            skip-blank-from slice-til-whitespace drop
+            dup [ no-backslash-payload ] unless
+        ] dip swap 2array
+    ] when ;
 
 ! If the slice is 0 width, we stopped on whitespace.
 ! Advance the index and read again!
@@ -267,6 +275,7 @@ ERROR: mismatched-terminator n string slice ;
             [ "\"!:[{(<>\s\r\n" slice-til-either ] dip swap [ span-slices ] dip
             over "\\" head? [
                 drop
+                dup "\\" sequence= [ read-backslash ] when
             ] [
                 (lex-factor)
             ] if

extra/modern/slices/slices.factor

@@ -131,7 +131,9 @@ ERROR: unexpected-end n string ;
     ] if ; inline
 
 : merge-slice-til-whitespace ( n string slice --  n' string slice' )
-    [ slice-til-whitespace drop ] dip merge-slices ;
+    pick [
+        [ slice-til-whitespace drop ] dip merge-slices
+    ] when ;
 
 : merge-slice-til-eol ( n string slice --  n' string slice' )
     [ slice-til-eol drop ] dip merge-slices ;