Writing custom queries

[hasufell]

I read these both pages:

1. https://hackage.haskell.org/package/beam-core-0.9.2.0/docs/Database-Beam-Query-CustomSQL.html
2. https://haskell-beam.github.io/beam/user-guide/extensibility/#custom-expressions

And have still no idea how to write a custom query. I followed the first page, which points to the non-existent page https://haskell-beam.github.io/beam/user-guide/extensibility/extensibility. Then I followed the second page and I end up getting errors:

    • No instance for (IsCustomSqlSyntax
                         (Database.Beam.Backend.SQL.SQL92.Sql92SelectTableExpressionSyntax
                            (Database.Beam.Backend.SQL.SQL92.Sql92SelectSelectTableSyntax
                               (Database.Beam.Backend.SQL.SQL92.Sql92SelectSyntax
                                  (Database.Beam.Backend.SQL.BeamSqlBackendSyntax ctxt)))))
        arising from a use of ‘customExpr_’
    • In the expression: customExpr_ myFuncImpl
      In an equation for ‘instr_’:
          instr_
            = customExpr_ myFuncImpl
            where
                myFuncImpl :: (Monoid a, IsString a) => a -> a -> a
                myFuncImpl str needle = "instr(" <> str <> ", " <> needle <> ")"

The page says:

Your backend's syntax must implement the IsSqlCustomExpressionSyntax type class.

I don't know what that means. How to implement this? Where's an example?

[kmicklas]

The instance it's complaining about is just the constraint you need to add to the declaration of instr_. It's implemented by the backend (Postgres or SQLite).

The Haddocks definitely don't make this clear enough, and we should probably have an example of this in the markdown docs too.

[kmicklas]

Actually it turns out we do have an example in the docs, but it's in the context of a query with a concrete backend, so we don't demonstrate the necessary context for defining a custom combinator.