title: Doobie cannot find or construct a Read instance for type T
body: <p>I'm using doobie to query some data and everything works fine, like this:</p>
  
  <pre><code>case class Usuario(var documento: String, var nombre: String, var contrasena: String)
  
    def getUsuario(doc: String) =
       sql"""SELECT documento, nombre, contrasena FROM "Usuario" WHERE "documento" = $doc"""
        .query[Usuario]
        .option
        .transact(xa)
        .unsafeRunSync()
  </code></pre>
  
  <p>But if I declare a function with type restriction like this: </p>
  
  <pre><code> def getOption[T](f: Fragment): Option[T] = {
      f.query[T]
       .option
       .transact(xa)
       .unsafeRunSync()
  </code></pre>
  
  <p>}</p>
  
  <p>I got these errors:</p>
  
  <pre><code>Error:(42, 12) Cannot find or construct a Read instance for type:
    T
  This can happen for a few reasons, but the most common case is that a data
  member somewhere within this type doesn't have a Get instance in scope. Here are
  some debugging hints:
  - For Option types, ensure that a Read instance is in scope for the non-Option
    version.
  - For types you expect to map to a single column ensure that a Get instance is
    in scope.
  - For case classes, HLists, and shapeless records ensure that each element
    has a Read instance in scope.
  - Lather, rinse, repeat, recursively until you find the problematic bit.
  You can check that an instance exists for Read in the REPL or in your code:
    scala&gt; Read[Foo]
  and similarly with Get:
    scala&gt; Get[Foo]
  And find the missing instance and construct it as needed. Refer to Chapter 12
  of the book of doobie for more information.
      f.query[T].option.transact(xa).unsafeRunSync()
  
  Error:(42, 12) not enough arguments for method query: (implicit evidence$1: doobie.util.Read[T], implicit h: doobie.LogHandler)doobie.Query0[T].
  Unspecified value parameter evidence$1.
      f.query[T].option.transact(xa).unsafeRunSync()
  </code></pre>
  
  <p>Does anyone know how to make what I want? I think it's something with implicits but I don't know how to fix it.</p>