type-foo

Not so long ago, I wrote a post about creating type-safe query builders in Scala from scratch. In it, I also suggested using shapeless library to do what was described. Now, I decided to write how it could be done. The code is in this repository.

Problem reminder

Without going into much details, the problem was to provide a type-safe way to build queries (to an abstract database, for instance) with parameters of different types. Something like

val query = beginQuery()
  .addParameter[String]("param1")
  .addParameter[Int]("param2")
  .addParameter[Boolean]("param3")
  .build()

// Compile:
query.execute("some string", 1, false)
// Won't compile:
query.execute(42, true, "another string")

Update 11.01.2016: the next post about type-safe query builders using shapeless.

Recently, I was hacking on a Scala library for queries to Cassandra database, phantom. At the moment, it was not able to build prepared statements, which I needed, so I added this functionality. However, phantom developers also implemented prepared statements quickly :) Nevertheless, I decided to write this post about the core idea of my implementation.

Caution: Don’t do this at home, use shapeless :)

My plan was to be able to prepare queries like this:

val query = select
  .where(typedParamPlaceholder1)
  .and(typedParamPlaceholder2)
  .and(typedParamPlaceholder3)
  .build()

and after that to execute this query: query.execute("string", 1, false). Moreover, I wanted this execution to be type-safe, so it was not possible to execute something like query.execute(1, "string", 0).

I will abstract from phantom queries and will focus on the general idea. The code is in this repository.