Yes, you’re right to observe that the types versus values distinction in Idris does not align with the compiletime-only versus runtime-and-compiletime distinction. That’s a good thing. It is useful to have values which exist only at compiletime, just as in program logics we have “ghost variables” used only in specifications. It is useful also to … Read more
You’re already familiar with “value-level” programming, whereby you manipulate values such as42 :: Int or ‘a’ :: Char. In languages like Haskell, Scala, and many others, type-level programming allows you to manipulate types like Int :: * or Char :: * where * is the kind of a concrete type (Maybe a or [a] are … Read more
This can be done (at least in some cases) with compiler internals import scala.language.experimental.macros import scala.reflect.internal.util import scala.reflect.macros.{blackbox, contexts} object Macros { def allImplicits[A]: List[String] = macro impl[A] def impl[A: c.WeakTypeTag](c: blackbox.Context): c.Tree = { import c.universe._ val context = c.asInstanceOf[contexts.Context] val global: context.universe.type = context.universe val analyzer: global.analyzer.type = global.analyzer val callsiteContext = context.callsiteTyper.context … Read more
Materialize the Value for a Type that has One Inhabitant
There are two separate questions here: Why does Shapeless use type members instead of type parameters in some cases in some type classes? Why does Shapeless include Aux type aliases in the companion objects of these type classes? I’ll start with the second question because the answer is more straightforward: the Aux type aliases are … Read more
Not a framework, but Jorge Ortiz (@JorgeO) mentioned some utilities he added to the tests for Foursquare’s Rogue library at NEScala in 2012 which support tests for non-compilation: you can find examples here. I’ve been meaning to add something like this to shapeless for quite a while. More recently, Roland Kuhn (@rolandkuhn) has added a … Read more