case class default values are non-lazy

[dontgitit]

import pureconfig._
import pureconfig.generic.auto._

case class SampleConf(foo: String = ???)

ConfigSource.string("{ foo: bar }").load[SampleConf]

the above produces:

scala.NotImplementedError: an implementation is missing
  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:347)
  at SampleConf$.apply$default$1(<console>:1)
  at anon$exportedReader$macro$16$1.inst$macro$1$lzycompute(<console>:1)
  at anon$exportedReader$macro$16$1.inst$macro$1(<console>:1)
  ... 35 elided

I would expect it to produce Right(SampleConf(bar)) since foo is defined. It seems the default values are being invoked even if they're unneeded. This example is a little egregious (using ???) but the default value might involve a complex operation that we don't want to be computed if it's unnecessary.

[dontgitit]

P.S. It breaks even when it's nested inside a nullable field;

import pureconfig._
import pureconfig.generic.auto._

case class Foo(bar: String, baz: Option[String] = ???)

case class Blah(hello: String, world: Option[Foo])

ConfigSource.string("{ hello: hey }").load[Blah]

which again makes:

scala.NotImplementedError: an implementation is missing
  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:347)
  at Foo$.apply$default$2(<console>:1)
  at anon$exportedReader$macro$99$1.inst$macro$49$lzycompute(<console>:1)
  at anon$exportedReader$macro$99$1.inst$macro$49(<console>:1)
  at anon$exportedReader$macro$99$1.$anonfun$inst$macro$21$1(<console>:1)
  at shapeless.Lazy$$anon$1.value$lzycompute(lazy.scala:123)
  at shapeless.Lazy$$anon$1.value(lazy.scala:123)
  at shapeless.lazily$.apply(lazy.scala:142)
  at anon$exportedReader$macro$99$1.inst$macro$21$lzycompute(<console>:1)
  at anon$exportedReader$macro$99$1.inst$macro$21(<console>:1)
  at anon$exportedReader$macro$99$1.$anonfun$inst$macro$20$1(<console>:1)
  at shapeless.Lazy$$anon$1.value$lzycompute(lazy.scala:123)
  at shapeless.Lazy$$anon$1.value(lazy.scala:123)
  at pureconfig.generic.MapShapedReader$$anon$2.fromWithDefault(MapShapedReader.scala:53)
  at pureconfig.generic.MapShapedReader$$anon$2.fromWithDefault(MapShapedReader.scala:47)
  at pureconfig.generic.MapShapedReader$$anon$2.fromWithDefault(MapShapedReader.scala:65)
  at pureconfig.generic.MapShapedReader$$anon$2.fromWithDefault(MapShapedReader.scala:47)
  at pureconfig.generic.DerivedConfigReader1$$anon$3.$anonfun$from$7(DerivedConfigReader.scala:70)
  at scala.util.Either$RightProjection.flatMap(Either.scala:757)
  at pureconfig.generic.DerivedConfigReader1$$anon$3.from(DerivedConfigReader.scala:70)
  at pureconfig.ConfigSource.$anonfun$load$1(ConfigSource.scala:67)
  at scala.util.Either$RightProjection.flatMap(Either.scala:757)
  at pureconfig.ConfigSource.load(ConfigSource.scala:67)
  at pureconfig.ConfigSource.load$(ConfigSource.scala:66)
  at pureconfig.ConfigObjectSource.load(ConfigSource.scala:91)
  ... 35 elided

I would expect this to never even try instantiating a Foo since Blah defines world: Option[Foo] and there's no world in the provided config, right?

[ruippeixotog]

Hi @dontgitit, thanks for filing this issue. Indeed, it looks like we are instantiating defaults earlier than we need to and that might be a problem (though I wouldn't recommend setting any kind of heavy logic, particularly stateful logic, in the defaults of config classes).

I'm not sure we can avoid computing all defaults in all cases, but I'll look into it.

[dontgitit]

I wouldn't recommend setting any kind of heavy logic, particularly stateful logic, in the defaults of config classes

Absolutely!
For context, I was considering having some default values loaded from EC2 instance tags or metadata API [in production, if config value missing] and was surprised when the code ran in dev [when the config was provided].

[ruippeixotog]

Hi @dontgitit, a very late update on this: at the time I investigated the possibility of making default values lazy, but unfortunately it seems to be out of our control. We rely on shapeless's Default case class for this, which does not provide us enough control to compute defaults only on demand. Getting default implementations seem to be possible only using macros that rely on internal compiler implementation details, so I don't think we can replicate this on the PureConfig side either...

I guess my only recommendation would be really to use only simple side-effect free fields as defaults. With a bit of extra boilerplate, one can always achieve the same functionality and keep impure logic separate, e.g.:

case class MyConfig(myTag: Option[String]) {
  def myTagValue: String = myTagValue.getOrElse(getFromAWS())
}

[dontgitit]

That makes sense. Thanks for taking a look!

[ruippeixotog]

This was fixed in #1636 for Scala 3. We have no plans to back port this to Scala 2 due to shapeless limitations, so I'm going to close this as completed.