Re: [Haskell-beginners] Organizing cmd line flow

30 Jan 2013

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Bryan Vicknair  writes:
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Is there any way to get closer to the following?  I think this is much
clearer, but perhaps I'm missing a much larger point in how this should be
done in Haskell::
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createDb fpath = do
    checkFileExists fpath
    checkParentDirExists fpath
    con <- openCon (Config fpath)
    create con
    closeCon con
Here is one way:

    {-# LANGUAGE DeriveDataTypeable #-}

    import Control.Applicative
    import Control.Exception
    import Control.Monad
    import Control.Monad.Trans.Error
    import Data.Data
    import Data.Typeable
    import System.Directory

    data ConfirmationFailed = ConfirmationFailed String
                            deriving (Show, Data, Typeable)

    instance Exception ConfirmationFailed

    confirm desc test = do
        result <- test
        unless result $ throwIO (ConfirmationFailed desc)

    main = do
        confirm "/tmp exists" (doesDirectoryExist "/tmp")
        print "hello"
        confirm "/tmpx exists" (doesDirectoryExist "/tmpx")
        print "hello"

You can also use "assert" or "guard" for this, although neither will be as
descriptive.

-- 
John Wiegley
FP Complete                         Haskell tools, training and consulting
http://fpcomplete.com               johnw on #haskell/irc.freenode.net