Enforce Value decoder's result to have a representational role

Fixes #137

.travis.yml

@@ -1,7 +1,6 @@
 if: tag IS blank
 
 env:
-  - ghc=8.4.2
   - ghc=8.6.1
   - ghc=8.8.1
   - ghc=8.10.2 benchmarks=1 tests=1

hasql.cabal

@@ -24,7 +24,7 @@ source-repository head
 
 library
   hs-source-dirs: library
-  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, RoleAnnotations, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
   default-language: Haskell2010
   exposed-modules:
     Hasql.Decoders
@@ -55,7 +55,7 @@ library
     Hasql.Private.Encoders.Params
   build-depends:
     attoparsec >=0.10 && <0.15,
-    base >=4.11 && <5,
+    base >=4.12 && <5,
     bytestring >=0.10 && <0.12,
     bytestring-strict-builder >=0.4.5.1 && <0.5,
     contravariant >=1.3 && <2,
@@ -64,7 +64,7 @@ library
     hashable >=1.2 && <2,
     hashtables >=1.1 && <2,
     mtl >=2 && <3,
-    postgresql-binary >=0.12.3 && <0.13,
+    postgresql-binary >=0.12.4 && <0.13,
     postgresql-libpq ==0.9.*,
     profunctors >=5.1 && <6,
     text >=1 && <2,
@@ -76,7 +76,7 @@ test-suite tasty
   type: exitcode-stdio-1.0
   hs-source-dirs: tasty
   main-is: Main.hs
-  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, RoleAnnotations, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
   default-language: Haskell2010
   other-modules:
     Main.DSL
@@ -97,7 +97,7 @@ test-suite threads-test
   hs-source-dirs: threads-test
   main-is: Main.hs
   default-extensions:
-    Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+    Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, RoleAnnotations, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
   default-language:
     Haskell2010
   other-modules:
@@ -114,7 +114,7 @@ benchmark benchmarks
   type: exitcode-stdio-1.0
   hs-source-dirs: benchmarks
   main-is: Main.hs
-  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, RoleAnnotations, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
   default-language: Haskell2010
   ghc-options:
     -O2
@@ -131,7 +131,7 @@ test-suite profiling
   type: exitcode-stdio-1.0
   hs-source-dirs: profiling
   main-is: Main.hs
-  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
+  default-extensions: Arrows, BangPatterns, ConstraintKinds, DataKinds, DefaultSignatures, DeriveDataTypeable, DeriveFoldable, DeriveFunctor, DeriveGeneric, DeriveTraversable, EmptyDataDecls, FlexibleContexts, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, LambdaCase, LiberalTypeSynonyms, MagicHash, MultiParamTypeClasses, MultiWayIf, NoImplicitPrelude, NoMonomorphismRestriction, OverloadedStrings, PatternGuards, ParallelListComp, QuasiQuotes, RankNTypes, RecordWildCards, RoleAnnotations, ScopedTypeVariables, StandaloneDeriving, TemplateHaskell, TupleSections, TypeFamilies, TypeOperators, UnboxedTuples
   default-language: Haskell2010
   ghc-options:
     -O2

library/Hasql/Private/Decoders.hs

@@ -157,6 +157,8 @@ Decoder of a value.
 newtype Value a = Value (Value.Value a)
   deriving (Functor)
 
+type role Value representational
+
 {-|
 Decoder of the @BOOL@ values.
 -}
@@ -339,7 +341,7 @@ Refine a value decoder, lifting the possible error to the session level.
 -}
 {-# INLINABLE refine #-}
 refine :: (a -> Either Text b) -> Value a -> Value b
-refine fn (Value v) = Value (Value.Value (ask >>= \b -> lift (A.refine fn (Value.run v b))))
+refine fn (Value v) = Value (Value.Value (\b -> A.refine fn (Value.run v b)))
 
 {-|
 A generic decoder of @HSTORE@ values.

library/Hasql/Private/Decoders/Value.hs

@@ -5,23 +5,21 @@ import qualified PostgreSQL.Binary.Decoding as A
 
 
 newtype Value a =
-  Value (ReaderT Bool A.Value a)
+  Value (Bool -> A.Value a)
   deriving (Functor)
 
-
 {-# INLINE run #-}
 run :: Value a -> Bool -> A.Value a
 run (Value imp) integerDatetimes =
-  runReaderT imp integerDatetimes
+  imp integerDatetimes
 
 {-# INLINE decoder #-}
 decoder :: (Bool -> A.Value a) -> Value a
 decoder =
   {-# SCC "decoder" #-} 
-  Value . ReaderT
+  Value
 
 {-# INLINE decoderFn #-}
 decoderFn :: (Bool -> ByteString -> Either Text a) -> Value a
 decoderFn fn =
-  Value $ ReaderT $ \integerDatetimes -> A.fn $ fn integerDatetimes
-
+  Value $ \integerDatetimes -> A.fn $ fn integerDatetimes