[#272] DerivingVia for Generic table codecs

Resolves #272

CHANGELOG.md

@@ -60,6 +60,8 @@ The changelog is available [on GitHub][2].
   Export a function for parsing TOML keys.
 * [#311](https://github.com/kowainik/tomland/issues/311):
   Reimplement custom `TomlState` instead of using `MaybeT` and `State`.
+* [#272](https://github.com/kowainik/tomland/issues/272):
+  Add `TomlTable` newtype to be used in generic `DerivingVia`.
 
 ## 1.2.1.0 — Nov 6, 2019
 

examples/Main.hs

@@ -1,12 +1,15 @@
 {-# OPTIONS -Wno-unused-top-binds #-}
+
 {-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingVia    #-}
 
 module Main (main) where
 
 import Control.Applicative ((<|>))
 import Control.Arrow ((>>>))
 import Data.Hashable (Hashable)
 import Data.HashSet (HashSet)
+import Data.ByteString (ByteString)
 import Data.IntSet (IntSet)
 import Data.List.NonEmpty (NonEmpty)
 import Data.Map.Strict (Map)
@@ -18,6 +21,7 @@ import GHC.Generics (Generic)
 import Toml (TomlCodec, TomlParseError (..), pretty, (.=), (<!>))
 import Toml.Type (TOML (..), Value (..))
 import Toml.Type.Edsl (mkToml, table, (=:))
+import Toml.Codec.Generic (TomlTable (..), stripTypeNameCodec, HasCodec (..), ByteStringAsBytes (..))
 
 import qualified Data.Text.IO as TIO
 
@@ -177,6 +181,24 @@ testT = Test
              <!> Toml.match (Toml._Right >>> Toml._Double)
                ) "either"
 
+data GenericPerson = GenericPerson
+    { genericPersonName    :: !Text
+    , genericPersonAddress :: !Address
+    } deriving stock (Generic)
+
+data Address = Address
+    { addressStreet :: !Text
+    , addressHouse  :: !Int
+    } deriving stock (Generic)
+      deriving HasCodec via (TomlTable Address)
+
+testGeneric :: TomlCodec GenericPerson
+testGeneric = stripTypeNameCodec
+
+newtype MyByteString = MyByteString
+    { unMyByteString :: ByteString
+    } deriving HasCodec via ByteStringAsBytes
+
 main :: IO ()
 main = do
     TIO.putStrLn "=== Printing manually specified TOML ==="
@@ -194,6 +216,12 @@ main = do
         Left msgs  -> Toml.prettyTomlDecodeErrors msgs
         Right test -> Toml.encode testT test
 
+    TIO.putStrLn "=== Testing Deriving Via ==="
+    genericFile <- TIO.readFile "examples/generic.toml"
+    TIO.putStrLn $ case Toml.decode testGeneric genericFile of
+        Left msg   -> Toml.prettyTomlDecodeErrors msg
+        Right test -> Toml.encode testGeneric test
+
 myToml :: TOML
 myToml = mkToml $ do
     "a" =: Bool True

examples/generic.toml

@@ -0,0 +1,5 @@
+name = "foo"
+
+[address]
+    addressStreet = "Bar"
+    addressHouse = 42

src/Toml/Codec/Generic.hs

@@ -125,6 +125,9 @@ module Toml.Codec.Generic
        , ByteStringAsBytes (..)
        , LByteStringAsText (..)
        , LByteStringAsBytes (..)
+
+         -- * Deriving Via
+       , TomlTable (..)
        ) where
 
 import Data.ByteString (ByteString)
@@ -669,10 +672,6 @@ instance HasCodec a => HasCodec (Last a) where
     hasCodec = Toml.last (hasCodec @a)
     {-# INLINE hasCodec #-}
 
-{-
-TODO: uncomment when higher-kinded roles will be implemented
-* https://github.com/ghc-proposals/ghc-proposals/pull/233
-
 {- | @newtype@ for generic deriving of 'HasCodec' typeclass for custom data
 types that should we wrapped into separate table. Use it only for data types
 that are fields of another data types.
@@ -712,7 +711,6 @@ instance (Generic a, GenericCodec (Rep a)) => HasCodec (TomlTable a) where
 
 instance (Generic a, GenericCodec (Rep a)) => HasItemCodec (TomlTable a) where
     hasItemCodec = Right $ Toml.diwrap $ genericCodec @a
--}
 
 {- $bytestring
 There are two ways to encode 'ByteString' in TOML:
@@ -726,9 +724,7 @@ To handle all these cases, @tomland@ provides helpful newtypes, specifically:
 * 'ByteStringAsBytes'
 * 'LByteStringAsText'
 * 'LByteStringAsBytes'
--}
 
-{- TODO: uncomment when the same GHC issue as above is resolved:
 As a bonus, on GHC >= 8.6 you can use these newtypes with the @DerivingVia@
 extensions for your own 'ByteString' types.
 

tomland.cabal

@@ -142,8 +142,12 @@ executable readme
 
 executable play-tomland
   import:              common-options
+  -- We are using DerivingVia that works only with > 8.6
+  if impl(ghc < 8.6)
+    buildable: False
   main-is:             Main.hs
   build-depends:       tomland
+                     , bytestring
                      , containers
                      , hashable
                      , text