How can I query SQLite using the sqlight package?

Question

I have a simple todos table in SQLite that I'm trying to query using Gleam. My code is as follows:

pub type Todo {
  Todo(id: Int, title: String, completed: Int)
}

fn todo_encoder() -> decode.Decoder(Todo) {
  let encoder = {
    use id <- decode.field("id", decode.int)
    use title <- decode.field("title", decode.string)
    use completed <- decode.field("completed", decode.int)
    decode.success(Todo(id: id, title: title, completed: completed))
  }
  encoder
}

fn db_directory() {
  let assert Ok(priv_directory) = wisp.priv_directory("app")
  echo "Private directory: " <> priv_directory
  priv_directory <> "/data/"
}


fn open_db_conn() {
  let db_conn = sqlight.open(db_directory() <> "app.db")
  case db_conn {
    Ok(conn) -> {
      echo "Database connection established."
      Ok(conn)
    }
    Error(err) -> {
      Error(err)
    }
  }
}

pub fn get_todos() -> Result(List(Todo), sqlight.Error) {
  let assert Ok(conn) = open_db_conn()
  let todos = sqlight.query("SELECT * FROM todos", conn, [], todo_encoder())

  case todos {
    Ok(results) -> {
      echo "Retrieved todos successfully."
      Ok(results)
    }
    Error(error) -> {
      echo "Error retrieving todos: " <> error.message
      Error(error)
    }
  }
}

I keep seeing this error when I call get_todos:

"Error retrieving todos: Decoder failed, expected Dict, got Tuple of 3 elements in "

My decoder seems correct, and my call to open the db also looks correct. What am I doing wrong?

The error message suggests that your decoder is expecting a Dict (key-value pairs), but the SQLite query is returning a Tuple (list of values) instead. — Deepak Saini
– Deepak Saini, Commented Mar 29 at 4:29

Peter Mortensen · Accepted Answer · 2025-08-19 18:25:06Z

1

Updated the decoder code:

fn todo_decoder() -> decode.Decoder(Todo) {
  {
    use id <- decode.field(0, decode.int)
    use title <- decode.field(1, decode.string)
    use completed <- decode.field(2, decode.int)
    decode.success(Todo(id: id, title: title, completed: completed))
  }
}

edited Aug 19 at 18:25

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Mar 29 at 23:17

jwesonga

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