convert: convert stdin when no csv file is specified

MANUAL.md

@@ -697,12 +697,11 @@ you will get this record:
 #### Converting streams
 
 The convert command also supports converting standard input if you're
-streaming a CSV file from the web or another tool. Use `-` as the input
-file and hledger will read from stdin:
+streaming a CSV file from the web or another tool. Use `-` (or nothing) as
+the input file and hledger will read from stdin. You must specify the
+rules file in this case:
 
-    $ cat foo.csv | fixup | hledger convert --rules foo.rules -
-
-Note that a rules file is required when streaming.
+    $ cat foo.csv | fixup | hledger convert --rules foo.rules
 
 ### histogram
 

hledger/Hledger/Cli/Convert.hs

@@ -86,12 +86,12 @@ type CsvRecord = [String]
 -- using/creating a .rules file.
 convert :: CliOpts -> IO ()
 convert opts = do
-  let csvfile = headDef "" $ patterns_ $ reportopts_ opts
-  when (null csvfile) $ error' "please specify a csv data file."
-  let 
-    rulesFileSpecified = isJust $ rules_file_ opts
-    rulesfile = rulesFileFor opts csvfile
-    usingStdin = csvfile == "-"
+  let csvfile = case headDef "" $ patterns_ $ reportopts_ opts of
+                  "" -> "-"
+                  s -> s
+      usingStdin = csvfile == "-"
+      rulesFileSpecified = isJust $ rules_file_ opts
+      rulesfile = rulesFileFor opts csvfile
   when (usingStdin && (not rulesFileSpecified)) $ error' "please specify a files file when converting stdin"
   csvparse <- parseCsv csvfile
   let records = case csvparse of

hledger/Hledger/Cli/Options.hs

@@ -168,7 +168,7 @@ addmode = (commandmode ["add"]) {
 convertmode = (commandmode ["convert"]) {
   modeValue = [("command","convert")]
  ,modeHelp = "show the specified CSV file as hledger journal entries"
- ,modeArgs = Just $ flagArg (\s opts -> Right $ setopt "args" s opts) "CSVFILE"
+ ,modeArgs = Just $ flagArg (\s opts -> Right $ setopt "args" s opts) "[CSVFILE]"
  ,modeGroupFlags = Group {
      groupUnnamed = [
       flagReq ["rules-file"]  (\s opts -> Right $ setopt "rules-file" s opts) "FILE" "rules file to use (default: CSVFILE.rules)"