Esqueleto query that returns constant value

Question

Is it possible to use esqueleto to create a query that returns a constant value? Like SELECT 1 for example.

Try `select (return 1)` -- can't check it now, but it might work. — chi, Jul 09 '16 at 11:04
@chi: unfortunately not: `No instance for (Database.Esqueleto.Internal.Sql.SqlSelect Int r0)` — snøreven, Jul 09 '16 at 11:09

score 1 · Accepted Answer · answered Jul 19 '16 at 18:26

Try this:

import Database.Esqueleto

-- | We have to specialize `val` or else the type inferencer
-- will complain about the `Esqueleto` instance.
val_ :: Int -> SqlExpr (Value Int)
val_ = val

query :: SqlPersistT IO [Value Int]
query = select $ return (val_ 1)

@chi's comment was close, but the 1 needed to be lifted into a SqlExpr. val is written generically and depends on an Esqueleto class instance. Normally the type inferencer would grab this as soon as you used from and pulled in a SQL table, but since none of that is available here we have to specialize manually.

Overall a good example of how typeclasses can obfuscate meaning and force people to turn to documentation or forums.

Esqueleto query that returns constant value

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