refactor ExportActions

This will allow disabling exports for remotes that are not configured to
allow them. Also, exportSupported will be useful for the external
special remote to probe.

This commit was supported by the NSF-funded DataLad project

Command/Export.hs

@@ -69,6 +69,9 @@ exportKey sha = mk <$> catKey sha
 seek :: ExportOptions -> CommandSeek
 seek o = do
 	r <- getParsed (exportRemote o)
+	unlessM (exportSupported (exportActions r)) $
+		error "That remote does not support exports."
+
 	new <- fromMaybe (error "unknown tree") <$>
 		-- Dereference the tree pointed to by the branch, commit,
 		-- or tag.
@@ -113,29 +116,28 @@ startExport r ti = do
 	f = getTopFilePath $ Git.LsTree.file ti
 
 performExport :: Remote -> ExportKey -> Sha -> ExportLocation -> CommandPerform
-performExport r ek contentsha loc = case storeExport r of
-	Nothing -> error "remote does not support exporting files"
-	Just storer -> do
-		sent <- case ek of
-			AnnexKey k -> ifM (inAnnex k)
-				( metered Nothing k $ \m -> do
-					let rollback = void $ performUnexport r ek loc
-					sendAnnex k rollback
-						(\f -> storer f k loc m)
-				, do
-					showNote "not available"
-					return False
-				)
-			-- Sending a non-annexed file.
-			GitKey sha1k -> metered Nothing sha1k $ \m ->
-				withTmpFile "export" $ \tmp h -> do
-					b <- catObject contentsha
-					liftIO $ L.hPut h b
-					liftIO $ hClose h
-					storer tmp sha1k loc m
-		if sent
-			then next $ cleanupExport r ek
-			else stop
+performExport r ek contentsha loc = do
+	let storer = storeExport $ exportActions r
+	sent <- case ek of
+		AnnexKey k -> ifM (inAnnex k)
+			( metered Nothing k $ \m -> do
+				let rollback = void $ performUnexport r ek loc
+				sendAnnex k rollback
+					(\f -> storer f k loc m)
+			, do
+				showNote "not available"
+				return False
+			)
+		-- Sending a non-annexed file.
+		GitKey sha1k -> metered Nothing sha1k $ \m ->
+			withTmpFile "export" $ \tmp h -> do
+				b <- catObject contentsha
+				liftIO $ L.hPut h b
+				liftIO $ hClose h
+				storer tmp sha1k loc m
+	if sent
+		then next $ cleanupExport r ek
+		else stop
 
 cleanupExport :: Remote -> ExportKey -> CommandCleanup
 cleanupExport r ek = do
@@ -154,13 +156,12 @@ startUnexport r diff
 	f = getTopFilePath $ Git.DiffTree.file diff
 
 performUnexport :: Remote -> ExportKey -> ExportLocation -> CommandPerform
-performUnexport r ek loc = case removeExport r of
-	Nothing -> error "remote does not support removing exported files"
-	Just remover -> do
-		ok <- remover (asKey ek) loc
-		if ok
-			then next $ cleanupUnexport r ek
-			else stop
+performUnexport r ek loc = do
+	let remover = removeExport $ exportActions r
+	ok <- remover (asKey ek) loc
+	if ok
+		then next $ cleanupUnexport r ek
+		else stop
 
 cleanupUnexport :: Remote -> ExportKey -> CommandCleanup
 cleanupUnexport r ek = do