Add fuzz tester to catch cases where parser fails

tests/ParserRecoveryTests.elm

@@ -0,0 +1,93 @@
+module ParserRecoveryTests exposing (suite)
+
+import Expect exposing (Expectation)
+import Fuzz
+import Markdown.Block as Block exposing (..)
+import Markdown.Inline
+import Markdown.Parser as Markdown exposing (..)
+import Parser
+import Parser.Advanced as Advanced exposing ((|.), (|=))
+import Test exposing (..)
+
+
+type alias Parser a =
+    Advanced.Parser String Parser.Problem a
+
+
+parse : String -> Result (List (Advanced.DeadEnd String Parser.Problem)) (List Block)
+parse markdown =
+    markdown
+        |> Markdown.parse
+
+
+suite : Test
+suite =
+    describe "parser errors all have fallbacks so parsing always succeeds"
+        [ fuzz Fuzz.string "random strings are parseable" <|
+            \randomString ->
+                randomString
+                    |> Markdown.parse
+                    |> Expect.ok
+        , test "recover from invalid html" <|
+            \() ->
+                "<no-closing-tag maybe this can just be rendered as a regular paragraph and not as HTML"
+                    |> Markdown.parse
+                    |> Expect.equal
+                        (Ok
+                            [ Block.Paragraph
+                                [ Block.Text
+                                    "<no-closing-tag maybe this can just be rendered as a regular paragraph and not as HTML"
+                                ]
+                            ]
+                        )
+        , test "<< is treated as regular text, not HTML" <|
+            \() ->
+                "In Elm, there are two ways to pipe: <| or <<"
+                    |> Markdown.parse
+                    |> Expect.equal
+                        (Ok
+                            [ Block.Paragraph
+                                [ Block.Text
+                                    "In Elm, there are two ways to pipe: <| or <<"
+                                ]
+                            ]
+                        )
+        ]
+
+
+expectOk : List Block -> String -> Expectation
+expectOk expected input =
+    case input |> parse of
+        Ok actual ->
+            actual
+                |> Expect.equal expected
+
+        Err error ->
+            Expect.fail (Debug.toString error)
+
+
+plainListItem : String -> Block.ListItem Block.Inline
+plainListItem body =
+    Block.ListItem Block.NoTask [ Block.Text body ]
+
+
+unstyledText : String -> List Inline
+unstyledText body =
+    [ Block.Text body ]
+
+
+emphasisText : String -> List Inline
+emphasisText body =
+    [ Block.Emphasis <|
+        [ Block.Text body ]
+    ]
+
+
+parserError : String -> Expect.Expectation
+parserError markdown =
+    case parse markdown of
+        Ok _ ->
+            Expect.fail "Expected a parser failure!"
+
+        Err _ ->
+            Expect.pass