Add refineResult

library/Hasql/Private/Decoders.hs

@@ -50,6 +50,9 @@ Will raise the 'Hasql.Errors.UnexpectedAmountOfRows' error if it's any other.
 singleRow :: Row a -> Result a
 singleRow (Row row) = Result (Results.single (Result.single row))
 
+refineResult :: (a -> Either Text b) -> Result a -> Result b
+refineResult refiner (Result results) = Result (Results.refine refiner results)
+
 -- ** Multi-row traversers
 -------------------------
 

library/Hasql/Private/Decoders/Results.hs

@@ -82,3 +82,8 @@ dropRemainders =
           where
             checkErrors =
               ExceptT $ fmap (mapLeft ResultError) $ Result.run Result.noResult (integerDatetimes, result)
+
+refine :: (a -> Either Text b) -> Results a -> Results b
+refine refiner results = Results $ ReaderT $ \ env -> ExceptT $ do
+  resultEither <- run results env
+  return $ resultEither >>= mapLeft (ResultError . UnexpectedResult) . refiner

library/Hasql/Private/Decoders/Row.hs

@@ -1,6 +1,6 @@
 module Hasql.Private.Decoders.Row where
 
-import Hasql.Private.Prelude
+import Hasql.Private.Prelude hiding (error)
 import Hasql.Private.Errors
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 import qualified PostgreSQL.Binary.Decoding as A

library/Hasql/Private/Prelude.hs

@@ -17,7 +17,7 @@ where
 
 -- base-prelude
 -------------------------
-import BasePrelude as Exports hiding (assert, left, right, isLeft, isRight, fromLeft, fromRight, error, (<>), First(..), Last(..), new)
+import BasePrelude as Exports hiding (assert, left, right, isLeft, isRight, fromLeft, fromRight, (<>), First(..), Last(..), new)
 
 -- transformers
 -------------------------

library/Hasql/Statement.hs

@@ -1,6 +1,7 @@
 module Hasql.Statement
 (
   Statement(..),
+  refineResult,
   -- * Recipies
 
   -- ** Insert many
@@ -14,6 +15,7 @@ where
 import Hasql.Private.Prelude
 import qualified Hasql.Decoders as Decoders
 import qualified Hasql.Encoders as Encoders
+import qualified Hasql.Private.Decoders as Decoders
 
 {-|
 Specification of a strictly single-statement query, which can be parameterized and prepared.
@@ -60,6 +62,17 @@ instance Profunctor Statement where
   dimap f1 f2 (Statement template encoder decoder preparable) =
     Statement template (contramap f1 encoder) (fmap f2 decoder) preparable
 
+{-|
+Refine a result of a statement,
+causing the running session to fail with the `UnexpectedResult` error in case of refinement failure.
+
+This function is especially useful for refining the results of statements produced with
+<http://hackage.haskell.org/package/hasql-th the \"hasql-th\" library>.
+-}
+refineResult :: (a -> Either Text b) -> Statement params a -> Statement params b
+refineResult refiner (Statement template encoder decoder preparable) =
+  Statement template encoder (Decoders.refineResult refiner decoder) preparable
+
 
 {- $insertMany