Added Transformers

Makefile

@@ -1,4 +1,4 @@
-PHONY: build docs
+.PHONY: build docs
 
 FILES=`find ./src -iregex '.*\.\(hs\)'`
 DOCFILES=`find . -iregex '.*\.\(hs\)'`

eta-prelude.cabal

@@ -2,10 +2,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ba97beeabbafc893bb8065143e121492afe977b1c7564d7c51a43a268b18bc77
+-- hash: 982169364dc7e1f96608940bfcc199bb998eabfbd3101daba27de462595ade15
 
 name:                eta-prelude
-version:             0.2.0
+version:             0.3.0
 synopsis:            The Eta prelude
 description:         Please see README.md
 homepage:            https://github.com/typelead/eta
@@ -36,6 +36,8 @@ library
       Eta.Classes.Integral
       Eta.Classes.IsString
       Eta.Classes.Monad
+      Eta.Classes.MonadTrans
+      Eta.Classes.MonadTrans.Except
       Eta.Classes.Monoid
       Eta.Classes.Num
       Eta.Classes.Ord
@@ -59,6 +61,7 @@ library
   default-extensions: NoImplicitPrelude
   build-depends:
       base >=4 && <5
+    , transformers
   hs-source-dirs:
       src
   default-language: Haskell2010

package.yaml

@@ -1,5 +1,5 @@
 name:                eta-prelude
-version:             0.2.0
+version:             0.3.0
 synopsis:            The Eta prelude
 description:         Please see README.md
 homepage:            https://github.com/typelead/eta
@@ -12,5 +12,7 @@ category:            Language
 library:
   exposed-modules:    Eta
   default-extensions: NoImplicitPrelude
-  dependencies:       base >= 4 && < 5
+  dependencies:
+    - base >= 4 && < 5
+    - transformers
   source-dirs:      src

src/Eta/Classes.hs

@@ -18,6 +18,7 @@ import Eta.Classes.Generic as Exported
 import Eta.Classes.Integral as Exported hiding (quot, rem, div, mod, quotRem, divMod)
 import Eta.Classes.IsString as Exported hiding (fromString)
 import Eta.Classes.Monad as Exported hiding ((>>=))
+import Eta.Classes.MonadTrans as Exported
 import Eta.Classes.Monoid as Exported
 import Eta.Classes.Num as Exported hiding (abs, signum)
 import Eta.Classes.Ord as Exported

src/Eta/Classes/MonadTrans.hs

@@ -0,0 +1,46 @@
+module Eta.Classes.MonadTrans
+  ( {-|
+A Monad Transformer is a typeclass that
+allows to execute functions of one Monad
+inside of another by providing a 'lift'
+operation:
+
+@
+wontWork :: Maybe (IO Int)
+wontWork = do
+  someString <- getLine    -- Compile error: getLine returns an IO action, not Maybe
+  if someString == ""
+     then Nothing
+     else length someString |> Just
+
+
+ok :: MaybeT IO Int
+ok = do
+  someString <- lift getLine
+  if someString == ""
+     then Nothing
+     else length someString |> Just
+@
+
+There are quite a lot of transformers available:
+
+* ExceptT
+* IdentityT
+* MaybeT
+* ReaderT
+* StateT
+* WriterT
+-}
+    module Control.Monad.Trans.Class
+  , module Exported
+  )
+where
+
+import Control.Monad.Trans.Class
+
+import Control.Monad.Trans.Except as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)
+import Control.Monad.Trans.Identity as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)
+import Control.Monad.Trans.Maybe as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)
+import Control.Monad.Trans.Reader as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)
+import Control.Monad.Trans.State as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)
+import Control.Monad.Trans.Writer as Exported hiding (liftCallCC, liftListen, liftPass, liftCatch)