Re: [Haskell-cafe] Re: How to generalize executing a series of commands, based on a list?

18 Nov 2010


      Thank you very much for the help. Good tips for improving my code design.
I'm new to sequence, mapM, and mapM_; I've seen mapM a lot while reading
code and wanted to learn it; now I have a reason! Thanks.
-- Peter


On Thu, Nov 18, 2010 at 5:40 AM, steffen wrote:
...
1. Write one routine, which does all the work for just one command.
2. use sequence or mapM, mapM_ from Control.Monad (depending on your
needs),
  to apply your function to a list of commands
accumulating results you may want to process the output of "sequence"
or use the WriterT Monad Transformer.
If you want to stop processing the rest of the list on error, either
write a recursive function yourself or use foldM or use ErrorT Monad
Transformer.
On Nov 18, 3:03 am, Peter Schmitz  wrote:
...
I am able to use System.Cmd (system) to invoke a shell command
and interpret the results.
Please see the code below that works okay for one such command.
(I invoke a program, passing two args.)
I am wondering how to generalize this to do likewise for a
series of commands, where the varying args (filenames, in this
case) are in a list ('inOutLeafs').
I will also want to accumulate some results; probably just a
failure count at this time.
Any advice or pointers to examples would be much appreciated.
Thanks in advance,
-- Peter
...
run :: ... -> IO (Int)    -- will return a fail count
run
   -- some args to this function here...
   = do
      -- ... set up: inputLeafs, outputLeafs, etc.
...
-- zip two lists of filenames:
      let inOutLeafs = zip inputLeafs outputLeafs
...
-- the first pair for the first command:
      let (inFile1,outFile1) = head inOutLeafs
...
-- build 1st command using 1st pair of filenames:
      let cmd1 = ...
...
exitCode <- system cmd1
      case (exitCode) of
         ExitSuccess -> do
            putStrLn $ "-- OK."
            return 0
         ExitFailure failCnt -> do
            putStrLn $ "-- Failed: " ++ show failCnt
            return 1
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