Change behavior with binary format output to stdout.

Previously, for binary formats, output to stdout was disabled
unless we could detect that the output was being piped (and not
sent to the terminal).  Unfortunately, such detection is not
possible on Windows, leaving windows users no way to pipe binary
output.  So we have changed the behavior in the following way:

* If the -o option is not used, binary output is never sent
  to stdout by default; instead, an error is raised.
* IF '-o -' is used, binary output is sent to stdout, regardless
  of whether it is being piped. This works on Windows too.

MANUAL.txt

@@ -110,8 +110,8 @@ If no *input-file* is specified, input is read from *stdin*.
 Otherwise, the *input-files* are concatenated (with a blank
 line between each) and used as input.  Output goes to *stdout* by
 default (though output to *stdout* is disabled for the `odt`, `docx`,
-`epub2`, and `epub3` output formats).  For output to a file, use the
-`-o` option:
+`epub2`, and `epub3` output formats, unless it is forced using
+`-o -`).  For output to a file, use the `-o` option:
 
     pandoc -o output.html input.txt
 
@@ -328,8 +328,8 @@ General options
 `-o` *FILE*, `--output=`*FILE*
 
 :   Write output to *FILE* instead of *stdout*.  If *FILE* is
-    `-`, output will go to *stdout*.  (Exception: if the output
-    format is `odt`, `docx`, `epub`, or `epub3`, output to stdout is disabled.)
+    `-`, output will go to *stdout*, even if a non-textual format
+    (`docx`, `odt`, `epub2`, `epub3`) is specified.
 
 `--data-dir=`*DIRECTORY*
 

src/Text/Pandoc/App.hs

@@ -91,10 +91,6 @@ import Text.Pandoc.Shared (headerShift, isURI, openURL,
 import qualified Text.Pandoc.UTF8 as UTF8
 import Text.Pandoc.XML (toEntities)
 import Text.Printf
-#ifndef _WINDOWS
-import System.Posix.IO (stdOutput)
-import System.Posix.Terminal (queryTerminal)
-#endif
 
 data LineEnding = LF | CRLF | Native deriving (Show, Generic)
 
@@ -124,7 +120,7 @@ parseOptions options' defaults = do
 convertWithOpts :: Opt -> IO ()
 convertWithOpts opts = do
   let args = optInputFiles opts
-  let outputFile = optOutputFile opts
+  let outputFile = fromMaybe "-" (optOutputFile opts)
   let filters = optFilters opts
   let verbosity = optVerbosity opts
 
@@ -245,18 +241,14 @@ convertWithOpts opts = do
                   (\(syn,dep) -> (T.unpack syn ++ " requires " ++
                     T.unpack dep ++ " through IncludeRules.")) xs)
 
-
-
-#ifdef _WINDOWS
-  let istty = True
-#else
-  istty <- queryTerminal stdOutput
-#endif
-  when (istty && not (isTextFormat format) && outputFile == "-") $
+  -- We don't want to send output to the terminal if the user
+  -- does 'pandoc -t docx input.txt'; though we allow them to
+  -- force this with '-o -'.
+  when (not (isTextFormat format) && optOutputFile opts == Nothing) $
     E.throwIO $ PandocAppError $
             "Cannot write " ++ format ++ " output to stdout.\n" ++
-            "Specify an output file using the -o option."
-
+            "Specify an output file using the -o option, or " ++
+            "use '-o -' to force output to stdout."
 
   let convertTabs = tabFilter (if optPreserveTabs opts || readerName == "t2t"
                                   then 0
@@ -303,7 +295,8 @@ convertWithOpts opts = do
     variables <-
         withList (addStringAsVariable "sourcefile")
                  (reverse $ optInputFiles opts)
-                 (("outputfile", optOutputFile opts) : optVariables opts)
+                 (("outputfile", fromMaybe "-" (optOutputFile opts))
+                  : optVariables opts)
                  -- we reverse this list because, unlike
                  -- the other option lists here, it is
                  -- not reversed when parsed from CLI arguments.
@@ -562,7 +555,7 @@ data Opt = Opt
     , optTemplate              :: Maybe FilePath  -- ^ Custom template
     , optVariables             :: [(String,String)] -- ^ Template variables to set
     , optMetadata              :: [(String, String)] -- ^ Metadata fields to set
-    , optOutputFile            :: FilePath  -- ^ Name of output file
+    , optOutputFile            :: Maybe FilePath  -- ^ Name of output file
     , optInputFiles            :: [FilePath] -- ^ Names of input files
     , optNumberSections        :: Bool    -- ^ Number sections in LaTeX
     , optNumberOffset          :: [Int]   -- ^ Starting number for sections
@@ -638,7 +631,7 @@ defaultOpts = Opt
     , optTemplate              = Nothing
     , optVariables             = []
     , optMetadata              = []
-    , optOutputFile            = "-"    -- "-" means stdout
+    , optOutputFile            = Nothing
     , optInputFiles            = []
     , optNumberSections        = False
     , optNumberOffset          = [0,0,0,0,0,0]
@@ -889,7 +882,7 @@ options =
 
     , Option "o" ["output"]
                  (ReqArg
-                  (\arg opt -> return opt { optOutputFile = arg })
+                  (\arg opt -> return opt { optOutputFile = Just arg })
                   "FILE")
                  "" -- "Name of output file"