Markdown writer: in YAML metadata, put double quotes around...

...strings that are numbers beginning with 0 or ending with 0 and
having a decimal point.  Otherwise they will read as YAML numbers
and potentially be modified (e.g. 3.10 -> 3.1).

It would be better to fix this on the reader side, but since
we use a standard YAML parser it's hard to see how.

Closes #11715.

src/Text/Pandoc/Writers/Markdown.hs

@@ -31,6 +31,7 @@ import qualified Data.Map as M
 import Data.Maybe (fromMaybe, mapMaybe, isNothing, isJust)
 import qualified Data.Set as Set
 import Data.Text (Text)
+import Text.Read (readMaybe)
 import Data.Char (isSpace)
 import qualified Data.Text as T
 import Text.HTML.TagSoup (Tag (..), isTagText, parseTags)
@@ -178,12 +179,20 @@ valToYaml (SimpleVal x)
       if hasNewlines x
          then hang 0 ("|" <> cr) x
          else case x of
-                Text _ t | isSpecialString t ->
+                Text _ t | isSpecialString t || looksLikeNumber t ->
                          "\"" <> fmap escapeInDoubleQuotes x <> "\""
                 _ | isNothing (foldM needsDoubleQuotes True x) ->
                          "\"" <> fmap escapeInDoubleQuotes x <> "\""
                   | otherwise -> x
     where
+      -- we need to put quotes around numbers that begin with 0
+      -- or have decimal points, because their YAML renderings
+      -- may differ. See #11715.
+      looksLikeNumber t = case readMaybe (T.unpack t) of
+                     Nothing -> False
+                     Just (_ :: Double) ->
+                       (T.any (== '.') t && T.takeEnd 1 t == "0") ||
+                       T.take 1 t == "0"
       isSpecialString t = Set.member t specialStrings
       specialStrings = Set.fromList
        ["y", "Y", "yes", "Yes", "YES", "n", "N",

test/command/11715.md

@@ -0,0 +1,10 @@
+```
+% pandoc -f native -t markdown -s
+Pandoc Meta {unMeta = fromList [ ("format_version", MetaString "3.10") ] } []
+^D
+---
+format_version: "3.10"
+---
+
+
+```