Add FromField Char

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -9,6 +9,7 @@ module Database.PostgreSQL.Opium.FromField
 
 import Data.Attoparsec.Text
   ( Parser
+  , anyChar
   , choice
   , decimal
   , double
@@ -38,14 +39,22 @@ class FromField a where
   validOid :: Proxy a -> Oid -> Bool
   parseField :: Parser a
 
+-- | See https://www.postgresql.org/docs/current/datatype-character.html.
 instance FromField Text where
   validOid _ = Oid.text \/ Oid.character \/ Oid.characterVarying
   parseField = takeText
 
+-- | See https://www.postgresql.org/docs/current/datatype-character.html.
 instance FromField String where
   validOid _ = validOid @Text Proxy
   parseField = Text.unpack <$> parseField
 
+-- | See https://www.postgresql.org/docs/current/datatype-character.html.
+-- This instance accepts all character types but fails to decode fields that are not exactly one character.
+instance FromField Char where
+  validOid _ = validOid @Text Proxy
+  parseField = anyChar
+
 instance FromField Int where
   validOid _ = Oid.smallint \/ Oid.integer \/ Oid.bigint
   parseField = signed decimal

test/Database/PostgreSQL/Opium/FromFieldSpec.hs

@@ -30,6 +30,12 @@ newtype AString = AString
 
 instance FromRow AString where
 
+newtype AChar = AChar
+  { char :: Char
+  } deriving (Eq, Generic, Show)
+
+instance FromRow AChar where
+
 newtype AFloat = AFloat
   { float :: Float
   } deriving (Eq, Generic, Show)
@@ -90,6 +96,17 @@ spec = do
     it "Decodes character varying" $ \conn -> do
       shouldFetch conn "SELECT 'Hello, World!'::CHARACTER VARYING (20) AS string" [AString "Hello, World!"]
 
+  describe "FromField Char" $ do
+    it "Decodes text" $ \conn -> do
+      shouldFetch conn "SELECT 'X'::TEXT AS char" [AChar 'X']
+
+    it "Decodes character" $ \conn -> do
+      shouldFetch conn "SELECT 'XYZ'::CHARACTER(1) AS char" [AChar 'X']
+      shouldFetch conn "SELECT ''::CHARACTER(1) AS char" [AChar ' ']
+
+    it "Decodes character varying" $ \conn -> do
+      shouldFetch conn "SELECT 'X'::CHARACTER VARYING (20) AS char" [AChar 'X']
+
   describe "FromField Float" $ do
     it "Decodes real" $ \conn -> do
       shouldFetch conn "SELECT 4.2::real AS float" [AFloat 4.2]