Improved template API and fixed a bug. Closes #5979.

* Text.Pandoc.Templates [API change]

  + Add Monad wrappers `WithDefaultPartials` and `WithPartials`.
    Wrapping these around an instance of `PandocMonad` gives
    us different instances of `TemplateMonad`, with different
    search behavior in retrieving partials.
    To compile a template and limit partial search to pandoc's
    data files, use `runWithDefaultPartials (compileTemplate ...)`.
    To compile a template and allow partials to be found locally
    (either on the file system or via HTTP, in the event that
    the main template has an absolute URL), ue
    `runWithPartials (compileTemplate ...)`.

  + Export `getTemplate`, which seeks a template locally,
    or via HTTP if the template has an absolute URL, falling
    back to the data files if not found.

  + Export `compileDefaultTemplate` -- does `getDefaultTemplate`
    and compiles the result, raising an error on failure.

* Text.Pandoc.Class [API change]

  + Remove `TemplateMonad` instances for `PandocIO` and `PandocPure`.
    These were too limiting and caused a bug whereby a local
    partial could be used even when the default template was requested.
    We now rely on instances provided in the Templates module.

Text.Pandoc.App.OutputSettings

  + Simplify template retrieval code.

1 files changed, 7 insertions, 29 deletions

diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index 944f1b63b..9a8e9969f 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 (catchError, throwError)
+import Control.Monad.Except (throwError)
 import Control.Monad.Trans
 import Data.Char (toLower)
 import Data.List (find, isPrefixOf)
@@ -171,40 +171,18 @@ optToOutputSettings opts = do
                       setVariableM "dzslides-core" dzcore vars
                   else return vars)
 
-  templStr <- case optTemplate opts of
+  templ <- case optTemplate opts of
                   _ | not standalone -> return Nothing
-                  Nothing -> Just <$> getDefaultTemplate format
+                  Nothing -> Just <$> compileDefaultTemplate format
                   Just tp -> do
                     -- strip off extensions
                     let tp' = case takeExtension tp of
                                    "" -> tp <.> T.unpack format
                                    _  -> tp
-                    Just . UTF8.toText <$>
-                          ((do surl <- stSourceURL <$> getCommonState
-                               -- we don't want to look for templates remotely
-                               -- unless the full URL is specified:
-                               modifyCommonState $ \st -> st{
-                                  stSourceURL = Nothing }
-                               (bs, _) <- fetchItem $ T.pack tp'
-                               modifyCommonState $ \st -> st{
-                                  stSourceURL = surl }
-                               return bs)
-                           `catchError`
-                           (\e ->
-                               case e of
-                                    PandocResourceNotFound _ ->
-                                       readDataFile ("templates" </> tp')
-                                    _ -> throwError e))
-
-  let templatePath = fromMaybe "" $ optTemplate opts
-
-  templ <- case templStr of
-             Nothing -> return Nothing
-             Just ts -> do
-               res <- compileTemplate templatePath ts
-               case res of
-                 Left  e -> throwError $ PandocTemplateError $ T.pack e
-                 Right t -> return $ Just t
+                    res <- getTemplate tp' >>= runWithPartials . compileTemplate tp'
+                    case res of
+                      Left  e -> throwError $ PandocTemplateError $ T.pack e
+                      Right t -> return $ Just t
 
   let writerOpts = def {
           writerTemplate         = templ