No instance for ToField Cursor

[CGenie]

Hello,

I'd like to use cursors to perform an update of a very large table. Postgres allows for this kind of query:

UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;

(see https://www.postgresql.org/docs/current/plpgsql-cursors.html#PLPGSQL-CURSOR-USING)

However, I can't just do

execute [sql| UPDATE foo SET dataval = ? WHERE CURRENT OF ?|] ( myval, cursor )

because Cursor doesn't have a ToField instance. Also, I see that only the datatype of Cursor is exported

https://hackage.haskell.org/package/postgresql-simple-0.6.5/docs/src/Database.PostgreSQL.Simple.Cursor.html#declareCursor

and not the constructor, so I can't access the name field.

[CGenie]

I was able to successfully run the above query with this instance:

import qualified Data.Text.Encoding as T

instance ToField Cursor where
  toField (Cursor name _conn) = toField $ Identifier $ T.decodeUtf8 $ fromQuery name

[CGenie]

As a side note, it would be nice if the declareCursor function allowed to accept query parameters as well.

[CGenie]

This branch seems to work for me: https://github.com/CGenie/postgresql-simple/tree/cursor-to-field