Pin nixpkgs

Reorder the modules in level04 to make the order more clear.

Add explicit exports to the AppM modules in 5,6, and 7 to help make it clearer
that you don't need to use the explicit constructors.

Update type signatures for config file functions in level 6 to use the newly
generalised AppM to take advantage of the variable error type.

shell.nix

@@ -1,20 +1,28 @@
-{ nixpkgs ? import <nixpkgs> {}
-, compiler ? "default"
+{ compiler ? "default"
 }:
 let
-  inherit (nixpkgs) pkgs;
+  inherit (import <nixpkgs> {}) fetchFromGitHub;
+
+  nixpkgs = fetchFromGitHub {
+    owner  = "NixOS";
+    repo   = "nixpkgs-channels";
+    rev    = "4f3446f29910d21eb0fb942bd03091b089cdad63";
+    sha256 = "0dqjkhhhckp881mns69qxn4dngcykal1gqrpaf9qy2vja4i41ay5";
+  };
+
+  pkgs = import nixpkgs {};
 
   # Grab our course derivation
-  course = import ./. { inherit nixpkgs compiler; };
+  course = import ./. { nixpkgs = pkgs; inherit compiler; };
 
   # Override the basic derivation so we can have a more fully feature
   # environment for hacking on the course material
   courseDevEnv = (pkgs.haskell.lib.addBuildTools course
     [ # Include the SQLite Database application
-      nixpkgs.sqlite
+      pkgs.sqlite
 
       # 'ghcid' auto reloading tool
-      nixpkgs.haskellPackages.ghcid
+      pkgs.haskellPackages.ghcid
     ]
   # We don't want nix to build the thing, we want the environment so we can
   # build the thing.

src/Level04/README.md

@@ -65,11 +65,11 @@ function into the module scope.
 ## Steps for this level:
 
 The steps for this level:
-1) ``src/Level04/DB/Types.hs``
-2) ``src/Level04/DB.hs``
-3) ``src/Level04/Types/Topic.hs``
-4) ``src/Level04/Types/CommentText.hs``
-5) ``src/Level04/Types.hs``
+1) ``src/Level04/Types/Topic.hs``
+2) ``src/Level04/Types/CommentText.hs``
+3) ``src/Level04/DB/Types.hs``
+4) ``src/Level04/Types.hs``
+5) ``src/Level04/DB.hs``
 6) ``src/Level04/Core.hs``
 
 For the sake of simplicity, any configuration requirements will be hardcoded in

src/Level05/AppM.hs

@@ -1,7 +1,11 @@
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE InstanceSigs          #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-module Level05.AppM where
+module Level05.AppM
+  ( AppM
+  , liftEither
+  , runAppM
+  ) where
 
 import           Control.Monad.Except   (MonadError (..))
 import           Control.Monad.IO.Class (MonadIO (..))

src/Level06/AppM.hs

@@ -1,14 +1,22 @@
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE InstanceSigs          #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-module Level06.AppM where
+module Level06.AppM
+  ( AppM
+  , App
+  , liftEither
+  , runAppM
+  , runApp
+  ) where
 
 import           Control.Monad.Except   (MonadError (..))
 import           Control.Monad.IO.Class (MonadIO (..))
 
+import           Data.Bifunctor         (Bifunctor (..))
+
 import           Level06.Types          (Error)
 
--- | We're going to upgrade the capability of our AppM by generalising the type of the errors that
+-- We're going to upgrade the capability of our AppM by generalising the type of the errors that
 -- it handles. This means that we'll be able to reuse our 'AppM' in more places that maybe have an
 -- overabundance of 'IO (Either e a)' types.
 --
@@ -19,20 +27,20 @@ newtype AppM e a = AppM
   { runAppM :: IO (Either e a)
   }
 
--- | Predominantly our application has only one error type: 'Error'. It would be tedious to have to
+-- Predominantly our application has only one error type: 'Error'. It would be tedious to have to
 -- declare that on every signature. We're able to use a type _alias_ to avoid this problem. We can
 -- define this type alias to make the error type variable concrete as 'Error'.
 --
 type App = AppM Error
 
--- | We need to refactor the 'runAppM' function as now the name conflicts, and it needs to suit the
+-- We need to refactor the 'runAppM' function as now the name conflicts, and it needs to suit the
 -- specialised 'App' type. The definition is even simpler than before. If someone near you is up to
 -- the same section, try to explain to each other why this works.
 --
 runApp :: App a -> IO (Either Error a)
 runApp = runAppM
 
--- | You may copy your previously completed AppM instances here and then refactor them to suit the
+-- You may copy your previously completed AppM instances here and then refactor them to suit the
 -- more generalised type of AppM.
 
 -- | -----------------------------------------------------------------------------------------------
@@ -68,7 +76,14 @@ instance MonadError e (AppM e) where
   catchError :: AppM e a -> (e -> AppM e a) -> AppM e a
   catchError = error "catchError for (AppM e) not implemented"
 
---  This is a helper function that will `lift` an Either value into our new AppM
+-- The 'Bifunctor' instance for 'Either' has proved useful several times
+-- already. Now that our 'AppM' exposes both type variables that are used in our
+-- 'Either', we can define a Bifunctor instance and reap similar benefits.
+instance Bifunctor AppM where
+  bimap :: (e -> d) -> (a -> b) -> AppM e a -> AppM d b
+  bimap = error "bimap for AppM not implemented"
+
+-- This is a helper function that will `lift` an Either value into our new AppM
 -- by applying `throwError` to the Left value, and using `pure` to lift the
 -- Right value into the AppM.
 --

src/Level06/Conf.hs

@@ -46,7 +46,7 @@ makeConfig =
 --
 parseOptions
   :: FilePath
-  -> IO (Either ConfigError Conf)
+  -> AppM ConfigError Conf
 parseOptions =
   -- Parse the options from the config file: "files/appconfig.json"
   -- Parse the options from the commandline using 'commandLineParser'

src/Level06/Conf/File.hs

@@ -16,6 +16,7 @@ import           Waargonaut                 (Json, parseWaargonaut)
 import qualified Waargonaut.Decode          as D
 import           Waargonaut.Decode.Error    (DecodeError (ParseFailed))
 
+import           Level06.AppM               (AppM)
 import           Level06.Types              (ConfigError (BadConfFile),
                                              PartialConf (PartialConf))
 -- $setup
@@ -33,15 +34,15 @@ import           Level06.Types              (ConfigError (BadConfFile),
 --
 readConfFile
   :: FilePath
-  -> IO (Either ConfigError ByteString)
+  -> AppM ConfigError ByteString
 readConfFile =
   error "readConfFile not implemented"
 
 -- | Construct the function that will take a ``FilePath``, read it in, decode it,
 -- and construct our ``PartialConf``.
 parseJSONConfigFile
   :: FilePath
-  -> IO (Either ConfigError PartialConf)
+  -> AppM ConfigError PartialConf
 parseJSONConfigFile =
   error "parseJSONConfigFile not implemented"
   where

src/Level07/AppM.hs

@@ -2,7 +2,13 @@
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE InstanceSigs          #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
-module Level07.AppM where
+module Level07.AppM
+  ( AppM (..)
+  , App
+  , Env (..)
+  , liftEither
+  , runApp
+  ) where
 
 import           Control.Monad.Except   (MonadError (..))
 import           Control.Monad.IO.Class (MonadIO (..))

src/Level07/Core.hs

@@ -69,8 +69,9 @@ runApplication = do
     runWithDBConn env =
       appWithDB env >> DB.closeDB (envDB env)
 
-    appWithDB env =
-      Ex.finally (run ( confPortToWai $ envConfig env ) (app env)) (DB.closeDB (envDB env))
+    appWithDB env = Ex.finally
+      (run ( confPortToWai . envConfig $ env ) (app env))
+      $ DB.closeDB (envDB env)
 
 -- | Our AppM is no longer useful for implementing this function. Can you explain why?
 --