Change `--template` to allow use of extensionless templates.

The intent is to allow bash process substitution: e.g.,
`--template <(echo "foo")`.

Previously pandoc *always* added an extension based on the
output format, which caused problems with the absolute filenames
used by bash process substitution (e.g. `/dev/fd/11`).

Now, if the template has no extension, pandoc will first
try to find it without the extension, and then add the
extension if it can't be found.

So, in general, extensionless templates can now be used.
But this has been implemented in a way that should not cause
problems for existing uses, unless you are using a template
`NAME.FORMAT` but happen to have an extensionless file `NAME` in
the template search path.

Closes #5270.

2 files changed, 20 insertions, 16 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index 14ea6e216..c6770da04 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -761,14 +761,16 @@ header when requesting a document from a URL:
 `--template=`*FILE*|*URL*
 
 :   Use the specified file as a custom template for the generated document.
-    Implies `--standalone`. See [Templates], below, for a description
-    of template syntax. If no extension is specified, an extension
-    corresponding to the writer will be added, so that `--template=special`
-    looks for `special.html` for HTML output.  If the template is not
-    found, pandoc will search for it in the `templates` subdirectory of
-    the user data directory (see `--data-dir`). If this option is not used,
-    a default template appropriate for the output format will be used (see
-    `-D/--print-default-template`).
+    Implies `--standalone`. See [Templates], below, for a
+    description of template syntax. If the template is not found,
+    pandoc will search for it in the `templates` subdirectory of
+    the user data directory (see `--data-dir`). If no extension
+    is specified and an extensionless template is not found,
+    pandoc will look for a template with an extension
+    corresponding to the writer, so that `--template=special`
+    looks for `special.html` for HTML output. If this option is
+    not used, a default template appropriate for the output
+    format will be used (see `-D/--print-default-template`).
 
 `-V` *KEY*[`=`*VAL*], `--variable=`*KEY*[`:`*VAL*]
 
diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index f8c27bee4..38cd35455 100644
--- a/src/Text/Pandoc/App/OutputSettings.hs
+++ b/src/Text/Pandoc/App/OutputSettings.hs
@@ -25,7 +25,7 @@ import qualified Data.Text as T
 import Text.DocTemplates (toVal, Context(..), Val(..))
 import qualified Control.Exception as E
 import Control.Monad
-import Control.Monad.Except (throwError)
+import Control.Monad.Except (throwError, catchError)
 import Control.Monad.Trans
 import Data.Char (toLower)
 import Data.List (find)
@@ -111,13 +111,15 @@ optToOutputSettings scriptingEngine opts = do
           _ | not standalone -> return Nothing
           Nothing -> Just <$> getDefault
           Just tp -> do
-            -- strip off extensions
-            let tp' = case takeExtension tp of
-                        "" -> tp <.> T.unpack format
-                        _  -> tp
-            getTemplate tp'
-              >>= runWithPartials . compileTemplate tp'
-              >>= fmap Just . templateOrThrow
+            let getAndCompile fp =
+                   getTemplate fp >>= runWithPartials . compileTemplate fp >>=
+                      fmap Just . templateOrThrow
+            catchError
+              (getAndCompile tp)
+              (\e ->
+                  if null (takeExtension tp)
+                     then getAndCompile (tp <.> T.unpack format)
+                     else throwError e)
 
   (writer, writerExts, mtemplate) <-
     if "lua" `T.isSuffixOf` format