Configuration file feature

[ExternalReality]

As a user I would like to be able to specify whether the database type, store path and database connection string get read from a configuration file or from the environment.

Implementing this brings up a few behavioral issues:

1. What should Moo read by default, the environment or the config file?
   • Configuration file by default with option to specify something like --config-file=[path] or --env (for environment)
   • Environment default; --config-file=[path] optional argument
2. What command line options specify the desired behavior?
3. If the user chooses to read from a config file and does not specify a path where does Moo look?
4. What configuration format(s) are acceptable? (?? YAML ??)

DBM_DATABASE_TYPE:     "postgresql" 
DBM_DATABASE:          "a=beautiful; connection=string"
DBM_MIGRATION_STORE:   "Path to a migration store"

A simple implementation using Data.Yaml in the applicative. (Data.Yaml is based on Aeson and the names haven't changed yet.)

module Moo.ConfigParser where

import Data.Yaml
import Moo.Core


parseConfigOptions :: FilePath -> IO ConfigData
parseConfigOptions configFilePath = do
  configContents <- decodeFile configFilePath

  case configContents of
    Nothing  -> error "Failed Parsing Yaml"
    Just a   -> return a

module Moo.Core where

import Control.Applicative ( (<$>), (<*>) )
import Data.Yaml ( FromJSON( parseJSON ), Value ( Object ), (.:) )

...

-- |ConfigOptions are those options read from configuration file
--  or environment. 
data ConfigData = ConfigData { _dbTypeStr     :: String  
                             , _dbConnStr     :: String 
                             , _fileStorePath :: String
                             }


instance FromJSON ConfigData where
   parseJSON (Object v) = ConfigData                 <$>     
                          v .: "DBM_DATABASE_TYPE"   <*>   
                          v .: "DBM_DATABASE"        <*>
                          v .: "DBM_MIGRATION_STORE" 

   parseJSON _           = mzero

module Moo.CommandInterface

...

commandOptions :: [ OptDescr (CommandOptions -> IO CommandOptions) ]
commandOptions =  [ optionConfigFile
                  , optionTest
                  , optionNoAsk
                  ]


optionConfigFile :: OptDescr (CommandOptions -> IO CommandOptions)
optionConfigFile = Option "c" ["config-file"]
                   (ReqArg (\arg opt ->
                             return opt { _configFilePath = Just arg }) "FILE")
                   "Specify location of configuration file"

The rest would depend on the answers to the questions above I suppose.

[jtdaugherty]

On the default behavior, I'd say look at the environment unless --config is specified. If --config is given and the config file doesn't exist or can't be read for any reason, that should constitute an error; otherwise, transparent fallback to the environment could be very, very confusing. As for the format, I think YAML would be the least jarring change as it is already the format used by the migration files. I'd be open to other format options (such as ini-style files as supported by the hsini package), but at the moment I don't see how that could provide enough extra benefit to justify using two different formats.

[jtdaugherty]

Given your recent pull request, it looks like we can close this. Is there anything else that needs doing on this?

[ExternalReality]

Yes, this issue can be closed.

[jtdaugherty]

Great! Thanks for contributing the feature! :)