diff options
Diffstat (limited to 'users/Profpatsch/my-prelude/src/Postgres/Decoder.hs')
| -rw-r--r-- | users/Profpatsch/my-prelude/src/Postgres/Decoder.hs | 94 |
1 files changed, 94 insertions, 0 deletions
diff --git a/users/Profpatsch/my-prelude/src/Postgres/Decoder.hs b/users/Profpatsch/my-prelude/src/Postgres/Decoder.hs new file mode 100644 index 0000000000..008b89b4ba --- /dev/null +++ b/users/Profpatsch/my-prelude/src/Postgres/Decoder.hs @@ -0,0 +1,94 @@ +module Postgres.Decoder where + +import Control.Applicative (Alternative) +import Data.Aeson qualified as Json +import Data.Aeson.BetterErrors qualified as Json +import Data.Error.Tree +import Data.Typeable (Typeable) +import Database.PostgreSQL.Simple (Binary (fromBinary)) +import Database.PostgreSQL.Simple.FromField qualified as PG +import Database.PostgreSQL.Simple.FromRow qualified as PG +import Json qualified +import Label +import PossehlAnalyticsPrelude + +-- | A Decoder of postgres values. Allows embedding more complex parsers (like a 'Json.ParseT'). +newtype Decoder a = Decoder (PG.RowParser a) + deriving newtype (Functor, Applicative, Alternative, Monad) + +-- | Parse a `bytea` field, equivalent to @Binary ByteString@ but avoids the pitfall of having to use 'Binary'. +bytea :: Decoder ByteString +bytea = fromField @(Binary ByteString) <&> (.fromBinary) + +-- | Parse a nullable `bytea` field, equivalent to @Binary ByteString@ but avoids the pitfall of having to use 'Binary'. +byteaMay :: Decoder (Maybe ByteString) +byteaMay = fromField @(Maybe (Binary ByteString)) <&> fmap (.fromBinary) + +-- | Turn any type that implements 'PG.fromField' into a 'Decoder'. Use type applications to prevent accidental conversions: +-- +-- @ +-- fromField @Text :: Decoder Text +-- @ +fromField :: PG.FromField a => Decoder a +fromField = Decoder $ PG.fieldWith PG.fromField + +-- | Turn any type that implements 'PG.fromField' into a 'Decoder' and wrap the result into the given 'Label'. Use type applications to prevent accidental conversions: +-- +-- @ +-- fromField @"myField" @Text :: Decoder (Label "myField" Text) +-- @ +fromFieldLabel :: forall lbl a. PG.FromField a => Decoder (Label lbl a) +fromFieldLabel = label @lbl <$> fromField + +-- | Parse fields out of a json value returned from the database. +-- +-- ATTN: The whole json record has to be transferred before it is parsed, +-- so if you only need a tiny bit of it, use `->` and `->>` in your SQL statement +-- and return only the fields you need from the query. +-- +-- In that case pay attention to NULL though: +-- +-- @ +-- SELECT '{"foo": {}}'::jsonb->>'foo' IS NULL +-- → TRUE +-- @ +-- +-- Also note: `->>` will coerce the json value to @text@, regardless of the content. +-- So the JSON object @{"foo": {}}"@ would be returned as the text: @"{\"foo\": {}}"@. +json :: Typeable a => Json.ParseT ErrorTree Identity a -> Decoder a +json parser = Decoder $ PG.fieldWith $ \field bytes -> do + val <- PG.fromField @Json.Value field bytes + case Json.parseValue parser val of + Left err -> + PG.returnError + PG.ConversionFailed + field + (err & Json.parseErrorTree "Cannot decode jsonb column" & prettyErrorTree & textToString) + Right a -> pure a + +-- | Parse fields out of a nullable json value returned from the database. +-- +-- ATTN: The whole json record has to be transferred before it is parsed, +-- so if you only need a tiny bit of it, use `->` and `->>` in your SQL statement +-- and return only the fields you need from the query. +-- +-- In that case pay attention to NULL though: +-- +-- @ +-- SELECT '{"foo": {}}'::jsonb->>'foo' IS NULL +-- → TRUE +-- @ +-- +-- Also note: `->>` will coerce the json value to @text@, regardless of the content. +-- So the JSON object @{"foo": {}}"@ would be returned as the text: @"{\"foo\": {}}"@. +jsonMay :: Typeable a => Json.ParseT ErrorTree Identity a -> Decoder (Maybe a) +jsonMay parser = Decoder $ PG.fieldWith $ \field bytes -> do + val <- PG.fromField @(Maybe Json.Value) field bytes + case Json.parseValue parser <$> val of + Nothing -> pure Nothing + Just (Left err) -> + PG.returnError + PG.ConversionFailed + field + (err & Json.parseErrorTree "Cannot decode jsonb column" & prettyErrorTree & textToString) + Just (Right a) -> pure (Just a) |