Make <end> UPARSE combinator invisible

Most cases using `<end>` or `to <end>` or `thru <end>` have no interest in
the value of the tail of the series.

If the tail is desired, that can be done with `<end> <here>`

Making `<end>` invisible means you can avoid writing `elide <end>` in the
much more common cases.

src/mezz/uparse.r

@@ -760,12 +760,12 @@ default-combinators: make map! reduce [
 
     <end> combinator [
         {Only match if the input is at the end}
-        return: "End position of the parse input"
-            [any-series?]
+        return: "Invisible"
+            [nihil?]
     ][
         if tail? input [
             remainder: input
-            return input
+            return nihil
         ]
         return raise "PARSE position not at <end>"
     ]

tests/parse/parse-bitset.test.reb

@@ -12,7 +12,7 @@
     count-up n 512 [
         if n = 1 [continue]
 
-        if not try parse (append copy "" codepoint-to-char n - 1) [
+        if not ok? parse (append copy "" codepoint-to-char n - 1) [
             c: any-char <end>
         ][
             fail "Parse didn't work"

tests/parse/parse-blank.test.reb

@@ -26,7 +26,7 @@
 
 (raised? parse [x] ['x blank])
 ('_ = parse [x _] ['x _])
-([] == parse [x] [try blank 'x <end>])
+('x == parse [x] [try blank 'x <end>])
 
 (raised? parse [] [blank blank blank])
 

tests/parse/parse-collect.test.reb

@@ -295,8 +295,8 @@
 
 ; Nested collect
 [
-    (did all [
-        [] == parse [1 2 3 4] [
+    (all [
+        [1 4] == parse [1 2 3 4] [
             a: collect [
                 keep integer!
                 b: collect [keep spread across repeat 2 integer!]
@@ -351,8 +351,8 @@ https://github.com/metaeducation/ren-c/issues/935
         x = ["aaa" "b"]
     ])
 
-    (did all [
-        "" == parse "aaabbb" [x: collect [keep across to "b"] to <end>]
+    (all [
+        ["aaa"] == parse "aaabbb" [x: collect [keep across to "b"] to <end>]
         x = ["aaa"]
     ])
 
@@ -428,8 +428,10 @@ https://github.com/metaeducation/ren-c/issues/935
 ;     red>> test
 ;     == ""
 ;
+; But in Ren-C, <end> is invisible.  So KEEP fails with no argument.
+;
 [https://github.com/red/red/issues/2561
-    ([""] = parse "" [collect [keep to <end>]])
+    ~???~ !! (parse "" [collect [keep to <end>]])
     ([""] = parse "" [collect [keep across to <end>]])
 ]
 

tests/parse/parse-get-group.test.reb

@@ -101,7 +101,7 @@
                     | #"]" :(f x)
                 ]
             ]
-            return "" == parse s [try some r <end>]
+            return ok? parse s [try some r <end>]
         ]
 
         f "420,]]"

tests/parse/parse-into.test.reb

@@ -68,7 +68,7 @@
 )
 
 [(
-    "" == parse "baaabccc" [
+    "a" == parse "baaabccc" [
         subparse [between "b" "b"] [some "a" <end>] to <end>
     ]
 )(
@@ -78,11 +78,11 @@
 )(
     raised? parse "baaabccc" [subparse [between "b" "b"] ["a"], to <end>]
 )(
-    "" == parse "baaabccc" [
+    "c" == parse "baaabccc" [
         subparse [between "b" "b"] ["a" to <end>], "c", to <end>
     ]
 )(
-    "" == parse "aaabccc" [subparse [across to "b"] [some "a"], to <end>]
+    "a" == parse "aaabccc" [subparse [across to "b"] [some "a"], to <end>]
 )]
 
 

tests/parse/parse-not.test.reb

@@ -35,7 +35,7 @@
         'a == parse [a] [not wb 'a]
     )
     (raised? parse [a a] [not ['a 'a] to <end>])
-    ([] == parse [a a] [not [some 'b] to <end>])
+    (~not~ == parse [a a] [not [some 'b] to <end>])
 ]
 
 [
@@ -49,7 +49,7 @@
         #a == parse "a" [not wb #a]
     )
     (raised? parse "aa" [not [#a #a] to <end>])
-    ("" == parse "aa" [not [some #b] to <end>])
+    (~not~ == parse "aa" [not [some #b] to <end>])
 ]
 
 [
@@ -63,5 +63,5 @@
         #{0A} == parse #{0A} [not wb #{0A}]
     )
     (raised? parse #{0A0A} [not [#{0A} #{0A}] to <end>])
-    (#{} == parse #{0A0A} [not [some #{0B}] to <end>])
+    (~not~ == parse #{0A0A} [not [some #{0B}] to <end>])
 ]

tests/parse/parse-some.test.reb

@@ -101,8 +101,8 @@
 ]
 
 [https://github.com/red/red/issues/3108
-    ([] == parse [1] [some further [to <end>]])
-    ([] == parse [1] [some further [to [<end>]]])
+    (void? parse [1] [some further [to <end>]])
+    (void? parse [1] [some further [to [<end>]]])
 ]
 
 (#c == parse "baaac" [<any> some [#a] #c])

tests/parse/parse-tag-end.test.reb

@@ -3,21 +3,24 @@
 ; In UPARSE, the <end> tag should be used.  This frees up the END
 ; word for variables (like start and end for ranges being copied, for
 ; example, or begin and end, etc.)
+;
+; It vanishes, because this is the overwhelmingly most useful behavior.
+; e.g. recognizing a single element in a block can be done with [<any> <end>].
 
 
 ; BLOCK! end tests from %parse-test.red
 [
     (
         block: [a]
-        (tail block) = parse block ['a <end>]
+        'a = parse block ['a <end>]
     )
     (raised? parse [a b] ['a <end>])
-    ([] == parse [a] [<any> <end>])
+    ('a == parse [a] [<any> <end>])
     (raised? parse [a b] [<any> <end>])
-    ([] == parse [] [<end>])
+    (void? parse [] [<end>])
     (
         be6: ~
-        did all [
+        all [
             1 == parse [] [<end> (be6: 1)]
             be6 = 1
         ]
@@ -28,12 +31,12 @@
 [
     (
         text: "a"
-        (tail text) == parse text [#a <end>]
+        #a == parse text [#a <end>]
     )
     (raised? parse "ab" [#a <end>])
-    ("" == parse "a" [<any> <end>])
+    (#a == parse "a" [<any> <end>])
     (raised? parse "ab" [<any> <end>])
-    ("" == parse "" [<end>])
+    (void? parse "" [<end>])
     (
         be6: ~
         did all [
@@ -47,12 +50,12 @@
 [
     (
         binary: #{0A}
-        (tail binary) == parse #{0A} [#{0A} <end>]
+        #{0A} == parse #{0A} [#{0A} <end>]
     )
     (raised? parse #{0A0B} [#{0A} <end>])
-    (#{} == parse #{0A} [<any> <end>])
+    (10 == parse #{0A} [<any> <end>])
     (raised? parse #{0A0B} [<any> <end>])
-    (#{} == parse #{} [<end>])
+    (void? parse #{} [<end>])
     (
         be6: ~
         did all [

tests/parse/parse-text.test.reb

@@ -25,7 +25,7 @@
 ("a" == parse "a" ["a"])
 ("ab" == parse "ab" ["ab"])
 ("abc" == parse "abc" ["abc"])
-("" == parse "abc" ["abc" <end>])
+("abc" == parse "abc" ["abc" <end>])
 
 ; Ren-C does not mandate that rules make progress, so matching empty strings
 ; works, as it does in Red.
@@ -92,7 +92,7 @@
 (
     test: to-binary {The C😺T Test}
     did all [
-        #{} == parse test [to {c😺t} x: across to space to <end>]
+        #{43F09F98BA54} == parse test [to {c😺t} x: across to space to <end>]
         x = #{43F09F98BA54}
         "C😺T" = to-text x
     ]
@@ -153,14 +153,16 @@
     (
         res: ~
         did all [
-            "" == parse str [thru "ipsum" <any> res: across to #" " to <end>]
+            "dolor" == parse str [
+                thru "ipsum" <any> res: across to #" " to <end>
+            ]
             res = "dolor"
         ]
     )
     (
         res: ~
         did all [
-            "" == parse str [thru #p res: <here> to <end>]
+            "sum dolor sit amet." == parse str [thru #p res: <here> to <end>]
             9 = index? res
         ]
     )

tests/parse/parse-thru.test.reb

@@ -4,17 +4,17 @@
 
 ; Edge case of matching END with THRU
 ;
-("" == parse "" [thru ["a" | <end>]])
-([] == parse [] [thru ["a" | <end>]])
+(void? parse "" [thru ["a" | <end>]])
+(void? parse [] [thru ["a" | <end>]])
 
 [
-    ([] == parse [] [thru <end>])
+    (void? parse [] [thru <end>])
     ('a == parse [a] [thru 'a])
-    ([] == parse [a] [thru 'a <end>])
+    ('a == parse [a] [thru 'a <end>])
     (raised? parse [a] [thru 'a <any>])
     ('b == parse [a b] [thru 'b])
     ('a == parse [a] [thru ['a]])
-    ([] == parse [a] [thru ['a] <end>])
+    ('a == parse [a] [thru ['a] <end>])
     (raised? parse [a] [thru ['a] <any>])
     ('b == parse [a b] [thru ['b]])
 ]
@@ -102,14 +102,14 @@
 )
 
 [
-    ("" == parse "" [thru <end>])
+    (void? parse "" [thru <end>])
     (#a == parse "a" [thru #a])
-    ("" == parse "a" [thru #a <end>])
+    (#a == parse "a" [thru #a <end>])
     (raised? parse "a" [thru #a <any>])
     (#b == parse "ab" [thru #a <any>])
     (#a == parse "aaba" [<any> thru #a repeat 2 <any>])
     (#a == parse "a" [thru [#a]])
-    ("" == parse "a" [thru [#a] <end>])
+    (#a == parse "a" [thru [#a] <end>])
     (raised? parse "a" [thru [#a] <any>])
     (#b == parse "ab" [thru [#a] <any>])
     (#a == parse "aaba" [<any> thru [#a] repeat 2 <any>])
@@ -133,14 +133,14 @@
         wa: [#{0A}]
         true
     )
-    (#{} == parse #{} [thru <end>])
+    (void? parse #{} [thru <end>])
     (#{0A} == parse #{0A} [thru #{0A}])
-    (#{} == parse #{0A} [thru #{0A} <end>])
+    (#{0A} == parse #{0A} [thru #{0A} <end>])
     (raised? parse #{0A} [thru #{0A} <any>])
     (11 == parse #{0A0B} [thru #{0A} <any>])
     (10 == parse #{0A0A0B0A} [<any> thru #{0A} repeat 2 <any>])
     (#{0A} == parse #{0A} [thru [#{0A}]])
-    (#{} == parse #{0A} [thru [#{0A}] <end>])
+    (#{0A} == parse #{0A} [thru [#{0A}] <end>])
     (raised? parse #{0A} [thru [#{0A}] <any>])
     (11 == parse #{0A0B} [thru [#{0A}] <any>])
     (10 == parse #{0A0A0B0A} [<any> thru [#{0A}] repeat 2 <any>])
@@ -157,19 +157,19 @@
     (
         res: ~
         did all [
-            #{} == parse bin [thru #{CAFE} <any> res: across to # to <end>]
+            #{BABE} == parse bin [thru #{CAFE} <any> res: across to # to <end>]
             res = #{BABE}
         ]
     )
     (
         res: ~
         did all [
-            #{} == parse bin [thru #{BABE} res: <here> to <end>]
+            #{00DEADBEEF00} == parse bin [thru #{BABE} res: <here> to <end>]
             9 = index? res
         ]
     )
 ]
 
 [https://github.com/red/red/issues/3427
-    (%"" == parse/part %234 ["23" thru [<end>]] 3)
+    ("23" == parse/part %234 ["23" thru [<end>]] 3)
 ]