Simplify/fix reading of --metadata values on command line.

Previously we used HsYAML's decodeStrict to recognize
boolean values (treating everything else as a string).
This caused problems relating to hvr/HsYAML#7.

We now just check for the recognized boolean values
`true|True|TRUE|false|False|FALSE`, and avoid using
HsYAML.

Closes #5177.

src/Text/Pandoc/App.hs

@@ -55,7 +55,6 @@ import qualified Data.Text.Lazy as TL
 import qualified Data.Text.Lazy.Encoding as TE
 import qualified Data.Text.Encoding.Error as TE
 import qualified Data.Text.Encoding.Error as TSE
-import qualified Data.YAML as YAML
 import Network.URI (URI (..), parseURI)
 import System.Directory (getAppUserDataDirectory)
 import System.Exit (exitSuccess)
@@ -332,12 +331,14 @@ removeMetaKeys :: [(String,String)] -> Pandoc -> Pandoc
 removeMetaKeys kvs pdc = foldr (deleteMeta . fst) pdc kvs
 
 readMetaValue :: String -> MetaValue
-readMetaValue s = case YAML.decodeStrict (UTF8.fromString s) of
-                       Right [YAML.Scalar (YAML.SStr t)]
-                                             -> MetaString $ T.unpack t
-                       Right [YAML.Scalar (YAML.SBool b)]
-                                             -> MetaBool b
-                       _                     -> MetaString s
+readMetaValue s
+  | s == "true"  = MetaBool True
+  | s == "True"  = MetaBool True
+  | s == "TRUE"  = MetaBool True
+  | s == "false" = MetaBool False
+  | s == "False" = MetaBool False
+  | s == "FALSE" = MetaBool False
+  | otherwise    = MetaString s
 
 -- Transformations of a Pandoc document post-parsing:
 

test/command/5177.md

@@ -0,0 +1,12 @@
+This should not give a "Prelude.read: no parse" error:
+
+```
+% pandoc -M foo=1e -s -t markdown
+hi
+^D
+---
+foo: 1e
+---
+
+hi
+```