gistfile1.scala

// build.sbt
scalaVersion := "2.11.6"

libraryDependencies ++= Seq(
  "org.scalaz" %% "scalaz-core"       % "7.1.1",
  "org.scalaz" %% "scalaz-effect"     % "7.1.1",
  "org.scalaz" %% "scalaz-concurrent" % "7.1.1",

  "org.tpolecat" %% "doobie-core"               % "0.2.1",
  "org.tpolecat" %% "doobie-contrib-postgresql" % "0.2.1",
  "org.tpolecat" %% "doobie-contrib-specs2"     % "0.2.1",
  
   "org.specs2" %% "specs2-core" % "2.4.15" % "test"
)

// test case
import org.specs2.mutable._
import doobie.imports._
import doobie.contrib.specs2.analysisspec._
import scalaz.concurrent.Task

class SqlSpec extends Specification with AnalysisSpec {
      val host = sys.env("pghost")
      val name = sys.env("pgdb")
      val user = sys.env("pguser")
      val pass = sys.env("pgpass")

      val transactor = DriverManagerTransactor[Task](
        "org.postgresql.Driver",
        s"jdbc:postgresql://${host}/${name}",
        user,
        pass
      )
      
      def select(tab : String, key : String) =
        sql"""
          SELECT val
          FROM   $tab
          WHERE  key = $key
        """.query[String]

      val q = select("foo", "bar")
      
      check(q)
}

// error
[info] SqlSpec
[info]
[info] Query0[String] defined at postgres.scala:69
[info]
[info]   SELECT val
[info]   FROM   ?
[info]   WHERE  key = ?
[info]
[info] x SQL Compiles and Typechecks
[error]    x ERROR: syntax error at or near "$1"
[error]     Position: 40 (specs2.scala:60)
[info]
[info]
[info] Total for specification SqlSpec
[info] Finished in 13 ms
[info] 1 example, 1 failure, 0 error
[error] Failed: Total 1, Failed 1, Errors 0, Passed 0

Just for the record, you can implement select as follows, and it will have the same final type you want.

def select(tab: String, key: String) =
  Query[String, String](s"""
    SELECT val
    FROM   $tab
    WHERE  key = ?
  """, None).toQuery0(key)