Debug long compile times in Scala and SBT

Question

In my Scala/SBT project, I have one file that takes up to 5(!) minutes to compile. All the other ones can compile in a few seconds. This makes development pretty painful.

I'm sure that I'm abusing some Scala constructs, but I have no idea how to go about debugging it. How does one debug long compile times in Scala?

I'm using Scala 2.9.2 and SBT 0.11.2

Answer 1

You can try the following Scala compiler options:

• -Ystatistics Print compiler statistics

Find a phase that takes the most time. Then, try those:

• -Xprint:<phase> Print out program after or "all"
• -Yshow-trees Show detailed trees when used in connection with -print:phase
• -Ydebug Output debugging messages
• -Ypmat-debug Trace all pattern matcher activity.

To enable these settings directly from the sbt console, you can use set scalacOptions in ThisBuild += "-Ystatistics", or for more than one, set scalacOptions in ThisBuild ++= Seq("-Yshow-trees", "-Ydebug")