[Haskell-beginners] Organizing cmd line flow

[John Wiegley]

>>>>> Bryan Vicknair <bryanvick at gmail.com> writes:

> 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::

> 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