[Haskell-beginners] Builds with cabal, but not with GHC

28 Sep 2011

      I'm getting some weird errors when I try to compile this with GHC, or load it
in GHCi, but it compiles (and runs) just fine using cabal. Here's the file...

**********
FILE Test.hs:
**********
module Main where

import Control.Monad.Error ( runErrorT, ErrorT, join, liftIO )
import Data.ConfigFile ( get, CPError, emptyCP, readfile )

data Configuration = Configuration
  {
    popDir :: FilePath,
    username :: String,
    sleepTime :: Int
  } deriving Show

parseConfig :: ErrorT CPError IO Configuration
parseConfig = do
  cp <- join $ liftIO $ readfile emptyCP "/etc/creatur-daemon.conf"
  u <- Data.ConfigFile.get cp "DEFAULT" "user"
  pd <- Data.ConfigFile.get cp "DEFAULT" "dir"
  st <- Data.ConfigFile.get cp "DEFAULT" "sleeptime"
  return $ Configuration { popDir = pd, username = u, sleepTime = st }

main :: IO ()
main = do
  rv <- runErrorT parseConfig
  case rv of
       Left (_, msg) -> putStrLn msg
       Right config -> print config
**********

When I compile it with ghc, I get the following:

$ ghc -hide-package monads-fd Test.hs
[1 of 1] Compiling Main             ( Test.hs, Test.o )

Test.hs:20:9:
    No instance for (mtl-1.1.1.1:Control.Monad.Error.Class.MonadError
                       CPError (ErrorT CPError IO))
      arising from a use of `get'
    Possible fix:
      add an instance declaration for
      (mtl-1.1.1.1:Control.Monad.Error.Class.MonadError
         CPError (ErrorT CPError IO))
    In a stmt of a 'do' expression: st <- get cp "DEFAULT" "sleeptime"
    In the expression:
      do { cp <- join
               $   liftIO $ readfile emptyCP "/etc/creatur-daemon.conf";
           u <- get cp "DEFAULT" "user";
           pd <- get cp "DEFAULT" "dir";
           st <- get cp "DEFAULT" "sleeptime";
           .... }
    In an equation for `parseConfig':
        parseConfig
          = do { cp <- join
                     $   liftIO $ readfile emptyCP "/etc/creatur-daemon.conf";
                 u <- get cp "DEFAULT" "user";
                 pd <- get cp "DEFAULT" "dir";
                 .... }

Test.hs:21:3:
    No instance for (Error (CPErrorData, String))
      arising from a use of `return'
    Possible fix:
      add an instance declaration for (Error (CPErrorData, String))
    In the expression: return
    In the expression:
        return $ Configuration {popDir = pd, username = u, sleepTime = st}
    In the expression:
      do { cp <- join
               $   liftIO $ readfile emptyCP "/etc/creatur-daemon.conf";
           u <- get cp "DEFAULT" "user";
           pd <- get cp "DEFAULT" "dir";
           st <- get cp "DEFAULT" "sleeptime";
           .... }

I tried adding the instance declarations, but I don't think I did it right
because I then had to add a bunch of imports, and the problems just snowballed.

With this cabal file, I can do "cabal install", and the program compiles and
runs just fine.

**********
FILE: Creatur.cabal
**********

Name:               Creatur
Version:             2.0
Description:       Créatúr
License:             OtherLicense
License-file:       LICENSE
Author:              Amy de Buitléir
Maintainer:        amy@nualeargais.ie
Build-Type:          Simple
Cabal-Version:    >=1.2

Executable amy-test
  Main-Is:         Test.hs
  GHC-Options:     -Wall -Werror
  Build-Depends:     base >= 4 && < 5, mtl ==1.1.*, ConfigFile ==1.0.*
**********

Can anyone tell me how to modify the code so it will compile? Thank you in 
advance.