Comment by harpocrates | Hacker News Reader

Comment by harpocrates | Hacker News Reader

I heartily recommend [splain][0] to anyone debugging non-trivial implicits. It is a scalac compiler plugin that, among other things, will swap out the horribly unhelpful "implicit not found" or "diverging implicit expansion" messages for an indented and annotated representation of the search tree that the compiler went through before giving up.

`-Xlog-implicits` is good to use every now and then, but it quickly becomes unreadable for any decent sized project.

  [0]: https://github.com/tek/splain

Well grown libraries are using the `@implicitNotFound` annotation to generate better library-controlled compiler errors, with advice given for what to do. It has its limitations of course, but proved very useful in many situations.

A search tree is cool, but it's not going to explain how to get a value when it has restrictions (dependencies).

`@implicitNotFound` still only gives you a top-level failure message. Also, my comment was aimed more at developers of libraries with complex implicit derivations not the consumers of such libraries.

I can't understand why this is not built into Scala.

It is utterly essential if you are doing anything with implicits. In particular with libraries like Circe.

I wish I knew about this when I wrote Scala (albeit briefly). It's one of those "ill posed problems" when debugging - what is the implicit from a third party library that is not in scope that makes this work?

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