Use Natchez tracing more generically with cats-tagless

Natchez Tagless

Semi-automatically derive Natchez trace instrumentation for algebras supported by cats-tagless.


All the examples below have the following imports:

import cats._, cats.syntax.all._, cats.effect._, cats.tagless._, cats.tagless.aop._, cats.tagless.syntax.all._, cats.effect.std._,, natchez.Trace.Implicits.noop, com.dwolla.tracing._

Programs as Values

Each instance of a Cats Effect IO is essentially a little program—this is the core of the concept of "programs as values." If we have an IO[Int], that represents a program that will result in an Int value being calculated when it's run.

scala> val x: IO[Int] = IO(45 - 3)
val x: cats.effect.IO[Int] = IO(…)

scala> x.unsafeRunSync()
val res0: Int = 42

It isn't until we actually run the x program that the result is calculated and 42 is returned. You could think of this as being similar to how there are lots of programs on your computer that will perform calculations and return results when executed—but not before!

Capturing Metadata

So an IO[_] is a program that can be run to yield a value or perform some effect in the world. Since we can assign it to variables, we could theoretically stick it inside another class that contains metadata about the program:

scala> case class IOMetadata(name: String, value: IO[Int])
class IOMetadata

scala> val metadata = IOMetadata("the answer", x)
val metadata: IOMetadata = IOMetadata(the answer,IO(…))

(Note that the string representation of the metadata variable printed to the console by the REPL shows the actual value of the name field, but the value field is shown as IO(…) since it hasn't been calculated yet.)

Use the Metadata to Enhance the Original Program

Maybe we want to always print the name of the program before executing our value: IO[_]. We could convert the IOMetadata into another IO that combines the two effects, first printing the name, and then calculating the value:

def printNameAndThenCalculate(m: IOMetadata): IO[Int] =
  IO.println( >> m.value

We can apply this to our metadata: IOMetadata value from above, and then execute it (because again, none of the IO values are actually executed until we run them!):

scala> val xWithName: IO[Int] = printNameAndThenCalculate(metadata)
val xWithName: cats.effect.IO[Int] = IO(…)

scala> xWithName.unsafeRunSync()
the answer
val res1: Int = 42

Or, if we have a Trace[IO] in scope, we could use the metadata to help define a new span in which to run the program:

def traceWithIOMetadata(m: IOMetadata): IO[Int] =
scala> traceWithIOMetadata(metadata)
val res2: IO[Int] = IO(…)

What if we don't always want to calculate Int values? We can tweak the definitions above so that they'll work with any type:

case class IOMetadata[A](name: String, value: IO[A])

def printNameAndThenCalculate[A](m: IOMetadata[A]): IO[A] =
  IO.println( >> m.value
scala> val metadata = IOMetadata("say hello", IO("hello").flatTap(IO.println(_)))
val metadata: IOMetadata[String] = IOMetadata(say hello,IO(...))

scala> printNameAndThenCalculate(metadata).unsafeRunSync()
say hello
val res3: String = hello

Note that the return types now contain String instead of Int as before!

Abstract Types

We've shown that we can attach metadata to an IO by putting both the metadata and the IO into a wrapper class, and how to use that wrapper class to run the IO in the context of a trace. But so far, this has been a pretty manual process—as the programmer, we had to define the string value to be used as the name and then place it and the IO into an IOMetadata instance. There's a way to largely avoid this boilerplate, though, and it involves using abstract higher kinded types and some metaprogramming implemented in the cats-tagless library.

First, a brief introduction to the abstract higher kinded types, and how this stuff fits together. Instead of hard-coding the IO effect type, we can introduce type variables into the methods we defined above, so they work with any type that has the right "shape." For example,

case class EffectMetadata[F[_], A](name: String, value: F[A])

def printNameAndThenCalculate[F[_] : FlatMap : Console, A](m: EffectMetadata[F, A]): F[A] =
  Console[F].println( >> m.value

Both the EffectMetadata case class and the printNameAndThenCalculate method define an F[_] type variable. This syntax means F[_] can be any type that itself requires another type to be fully defined. IO is one example, but some other examples from the Scala stdlib include Option and List. In all these cases, just knowing something is an IO, Option, or List isn't enough to fully define the type—you need to fill in those holes, like IO[Int], Option[String], or List[Boolean].

If F[_] can be literally any type that fits that shape, you can't do much with it as a programmer. For this reason, in printNameAndThenCalculate we specify two type constraints. F[_] : Foo essentially means "any type with a single hole for which exists an instance of Foo[F]."

In order for a call to printNameAndThenCalculate(EffectMetadata("name", fa: F[A]) to compile, the compiler must be able to find and identify an instance of FlatMap[F] and Console[F] for whatever type F[_] is. For example, there exists an instance of FlatMap[Option], but not an instance of Console[Option], meaning we couldn't call printNameAndThenCalculate and use Option as the F[_]. On the other hand, both FlatMap[IO] and Console[IO] exist, so we can call printNameAndThenCalculate with an IO[_].

Since we constrained the F[_] used for a call to printNameAndThenCalculate, inside the body of the method we can write code that assumes those constraints. Console[F].println( summons the instance of Console[F] and calls that instance's println method. >> is an alias for flatMap, which relies on the instance of FlatMap[F]. (This can be equivalently rewritten as FlatMap[F].flatMap(Console[F].println( => m.value) to show the summoning of FlatMap[F], but most Scala developers would not use that style.

Cats Tagless Transformations

Let's say we have an interface for calculating the answer to the Ultimate Question of Life, the Universe, and Everything!

trait DeepThought[F[_]] {
  def answer: F[Int]

And an implementation of that interface:

object DeepThought {
  def apply[F[_] : Applicative]: DeepThought[F] = new DeepThought[F] {
    def answer: F[Int] = 42.pure[F]

We can run the supercomputer using IO as our effect type, and it will return the answer:

scala> DeepThought[IO].answer.unsafeRunSync()
val res4: Int = 42

Cats Tagless defines a metadata wrapper called Instrumentation that captures information similar to our EffectMetadata class above:

final case class Instrumentation[F[_], A](value: F[A], algebraName: String, methodName: String)

(Don't worry about the word "algebra" here; there are deeper reasons why it's related to the algebra we learned in school, but for our purposes, we just mean an interface with a higher kinded type parameter, where each method returns a value of that higher kinded type parameter.)

An Instrumentation of the method call DeepThought[IO].answer would be

Instrumentation(DeepThought[IO].answer, "DeepThought", "methodName")

Cats Tagless also defines a typeclass called Instrument which transforms an instance of an algebra implemented in F[_] to an instance implemented in Instrumentation[F, *].

(Instrumentation[F, *] is a higher kinded type with a single hole—in other words, the same "shape" as F[_]. Instrumentation itself has two type parameters: F[_] and A. We can turn it into a higher kinded type with a single hole by filling in one of the two holes. This is also called partial application of the type parameters.)

We could implement an instance of Instrument for our DeepThought algebra:

implicit val deepThoughtInstrument: Instrument[DeepThought] = new Instrument[DeepThought] {
  def instrument[F[_]](af: DeepThought[F]): DeepThought[Instrumentation[F, *]] = new DeepThought[Instrumentation[F, *]] {
    def answer: Instrumentation[F, Int] = {
      val value: F[Int] = af.answer
      val algebraName: String = "DeepThought"
      val methodName: String = "answer"
      Instrumentation(value, algebraName, methodName)

You could imagine that with lots of algebras to instrument, or large algebras with lots of methods, manually writing out Instrument instances using brute force could get pretty tedious. Luckily, Cats Tagless comes with derivation macros that can usually do most of the work for us:

implicit val deepThoughtInstrument: Instrument[DeepThought] = cats.tagless.Derive.instrument

Ideally that implicit val would be placed in the companion object to the algebra (i.e., next to the def apply method above in object DeepThought) so that the compiler can find it during a search of the implicit scope.

Having a DeepThought[Instrumentation[F, *]] isn't super helpful on its own, because Instrumentation[F, *] just captures metadata. It becomes very useful when the Instrumentation[F, *] can be transformed back to an F[_] using something that can take advantage of the metadata. If we tweak our traceWithIOMetadata function above, it can do just that!

def traceWithInstrumentation[F[_] : Trace, A](fa: Instrumentation[F, A]): F[A] =

We can apply this method throughout the algebra to convert DeepThought[Instrumentation[F, *]] back to DeepThought[F]:

def traceInstrumentedDeepThought[F[_] : Trace](fa: DeepThought[Instrumentation[F, *]]): DeepThought[F] = new DeepThought[F] {
  def answer: F[Int] = {
    val instrumentation: Instrumentation[F, Int] = fa.answer

but again, that's a lot of boilerplate. Luckily, traceWithInstrumentation can be written as a natural transformation:

class TraceInstrumentation[F[_] : Trace] extends (Instrumentation[F, *] ~> F) {
  override def apply[A](fa: Instrumentation[F, A]): F[A] =

and then applied to our DeepThought[Instrumentation[F, *]] using mapK (which comes for free since we were able to define Instrument[DeepThought] above):

scala> DeepThought[IO]
val res5: DeepThought[cats.effect.IO] = DeepThought$$anon$4@5455ec6

scala> res5.instrument
val res6: DeepThought[[β$1$]cats.tagless.aop.Instrumentation[[+A]cats.effect.IO[A],β$1$]] = DeepThought$$anon$1$$anon$2@2a915583

scala> res6.mapK(new com.dwolla.tracing.TraceInstrumentation[IO])
val res7: DeepThought[cats.effect.IO] = DeepThought$$anon$1$$anon$3@252d03a9

and in fact, TraceInstrumentation is made available by this library (see com.dwolla.tracing.TraceInstrumentation).

Capturing Method Parameters and Return Values

The Instrumentation class only captures the primary effect value, algebra name, and method name, meaning method parameters are not available. Cats Tagless has a more powerful metadata class called Weave, which can capture inputs in certain contexts.

final case class Weave[F[_], Dom[_], Cod[_], A](algebraName: String,
                                                domain: List[List[Advice[Eval, Dom]]],
                                                codomain: Advice.Aux[F, Cod, A])

The domain value represents the inputs; it is modeled as a list-of-lists since methods can have multiple parameter lists. The Advice instances in the list are written in terms of Eval to support lazy parameters as well, using for strict parameters and Eval.always for lazy parameters.

The value assigned to the Dom[_] type parameter must be a typeclass that has instances for every type of input parameter of the methods of the algebra. The specific typeclass is parameterized because what is appropriate will vary depending on how you'll use the woven algebra, but it will typically be something like cats.Show to convert things to strings or io.circe.Encoder to convert values to JSON. We will use a typeclass that converts values to a format appropriate for attachment to a span as an attribute.

The Cod[_] type parameter is similar to Dom[_], but it must exist for the output type instead of the input types.

This library defines a ToTraceValue[_] typeclass which exists to convert things to Natchez's TraceValue type. We define a new typeclass specifically for converting to trace attributes lets you customize the behavior for each class (as compared to something like cats.Show, which is a more general-purpose typeclass for converting values to strings). For example, you may want to redact sensitive information, or use a JSON structure that can be parsed by your tracing backend. (If TraceValue was a typeclass and fit the "shape" of the Dom[_] or Cod[_] type parameters, we'd use it directly, but it doesn't fit, so we had to introduce ToTraceValue[_] to bridge the two.)

Cats Tagless also defines Trivial[_], which is always available, but doesn't provide any values. This is useful if you want to weave capturing only inputs but not outputs (for which you'd fix Cod[α] to Trivial[α]), or vica-versa.

To use Weave, define an implicit Aspect instance for each algebra to be woven. Often these can be semi-automatically derived:

object DeepThought {
  implicit val deepThoughtAspect: Aspect[DeepThought, ToTraceValue, ToTraceValue] = Derive.aspect[DeepThought, ToTraceValue, ToTraceValue]
  def apply[F[_] : Applicative]: DeepThought[F] = new DeepThought[F] {
    def answer: F[Int] = 42.pure[F]

Instances of ToTraceValue will need to exist for every input type. If you see errors like

On line 5: error: exception during macro expansion:
       scala.reflect.macros.TypecheckException: could not find implicit value for parameter G: com.dwolla.tracing.ToTraceValue[Foo]
        at scala.reflect.macros.contexts.Typers.$anonfun$typecheck$3(Typers.scala:44)

then implement ToTraceValue for the type describe in the error (in this case, Foo). You may have to do this several times until instances are available for all the input types.

Once an Aspect[DeepThought, ToTraceValue, ToTraceValue] is available, the traceWithInputs and traceWithInputsAndOutputs extension methods should also be available:

scala> DeepThought[IO].traceWithInputsAndOutputs
val res8: DeepThought[[+A]cats.effect.IO[A]] = DeepThought$$anon$1$$anon$3@6ad241cf

The automatic derivation isn't magic—writing out an Aspect instance is easy, but tedious, similar to the Instrument example above. Let's define a new trait with methods that have input parameters to demonstrate how it's done:

trait Foo[F[_]] {
  def foo(i: Int, s: => String): F[Boolean]

object Foo {
  implicit val fooAspect: Aspect[Foo, ToTraceValue, ToTraceValue] = new Aspect[Foo, ToTraceValue, ToTraceValue] {
    override def weave[F[_]](af: Foo[F]): Foo[Aspect.Weave[F, ToTraceValue, ToTraceValue, *]] =
      new Foo[Aspect.Weave[F, ToTraceValue, ToTraceValue, *]] {
        override def foo(i: Int, s: => String): Aspect.Weave[F, ToTraceValue, ToTraceValue, Boolean] =
            "Foo",                  // the algebra name
            List(                   // a list of parameter lists
              List(                 // the first list of parameters
                  "i",              // the parameter name
               // capture the parameter value, eagerly evaluated because i is a by-value parameter
                ),                  // ToTraceValue[Int] is implicitly passed here
                  "s",              // the parameter name
                  Eval.always(s)    // capture the parameter value, lazily evaluated because s is a by-name parameter
                ),                  // ToTraceValue[String] is implicitly passed here
              "foo",                // the method name
    , s)          // call the underlying method
            )                       // ToTraceValue[Boolean] is implicitly passed here

    override def mapK[F[_], G[_]](af: Foo[F])(fk: F ~> G): Foo[G] =
      new Foo[G] {
        override def foo(i: Int, s: => String): G[Boolean] = {
          val value =, s)  // call the underlying method
          fk(value)                 // convert F[_] to G[_]