Is there a systematic way to discover which implicit defs are in scope, and which one is bound at a particular point?

Question

Often there's no need to pay any attention to implicit arguments in Scala, but sometimes it's very helpful to understand how the compiler is automatically providing them. Unfortunately, this understanding seems to be hard to obtain!

Is there a general method to discover how an implicit parameter has been provided, in a given piece of code?

Ideally, one day IDE integration would provide this information in some way, but I expect for now I'll have to dig deeper. Is there some way to ask the compiler to explain exactly which implicit definition it chooses at any given point? Can this be deciphered indirectly from other compiler output?

As an example, I'd like to know how to work out on my own where the implicit bf: CanBuildFrom[Repr, B, That] argument to TraversableLike.map comes from, without reading questions like this one on Stack Overflow!

Answer 1

Ideally, one day IDE integration would provide this information in some way, ...

That day is today in with JetBrains' IDEA. If you run the latest EAP of IDEA version 9 (9.0.3 EA #95.289) with a recent nightly release of the Scala plug-in, this capability is present. Every value expression may be selected and a command issued that displays a pop-up showing all applicable implicit conversions with the one the compiler will select highlighted.

And since there are apparently a few out there who don't yet know it, there is a free and open-source Community Edition of IDEA and it does support the Scala plug-in.

Answer 2

1. Add the option -Xprint:typer to the scalac command line. This prints the program tree just after the typer compiler phase. This works best with a short, self contained example. You can also pass this to scalac. This is a really huge step towards self-reliance in Scala!
2. As mentioned by Randall, IntelliJ shows in-scope and the selected Implicit View with CTRL-ALT-SHIFT-I. Wait a month or two and implicit arguments are likely to have similar support.