Encoded values as Params?

[0x777]

list :: Params [(LibPQ.Oid, Maybe ByteString)]

It is useful when dealing with dynamically generated queries.

[nikita-volkov]

I agree this might be useful to some.

However I'd rather go with the following:

raw :: Value (LibPQ.Oid, Maybe ByteString)

Then the definition of your list function becomes just contramany raw.

I won't promise to implement that soon, though. A PR would be welcome.

[0x777]

If we go with raw, what does it mean to use functions like arrayValue on raw?

[nikita-volkov]

Right. Actually it seems like our both variants are impossible. What we can do is something more like this:

-- | Given an OID and an according array OID
-- produces an encoder of raw bytes.
raw :: Oid -> Oid -> Value (Maybe ByteString)

[0x777]

How about this?

rawList :: Params [(LibPQ.Oid, Maybe ByteString)]
rawList = Params (Params.Params $ Op encoder)
  where
    encoder = DList.fromList . map (fmap const)

[0x777]

Or rather

rawList :: Params [(LibPQ.Oid, Bool -> Maybe ByteString)]
rawList = Params (Params.Params $ Op DList.fromList)

[nikita-volkov]

Oh, right! This you can do.

Can you describe your use-case though? Maybe we could find something better than exporting such a low-level functionality.

[0x777]

I work on a compiler which converts a JSON based query language to SQL. In a nutshell,

exec :: DatabaseSchema -> JSONQuery -> IO (Either Error Aeson.Value)

So, given a query, a Postgres prepared statement is generated, and with the DatabaseSchema, the arguments to this statement are converted to binary format (using encoders from postgresql-binary). This statement is then executed with a thin wrapper written on top of postgresql-libpq.

I would like to replace this wrapper with hasql. The starting point would be to use the statement function. By the time this function is used, the arguments to the prepared statement are already converted to postgres's binary format. Hence, the requirement for the Params [(LibPQ.Oid, Maybe ByteString)] encoder.