Lenses compile time insanely slow about shapeless

[milessabin]

How deep is the nesting? Also how are you constructing them? Is this using Nat indices?

[johanstenberg92]

Hi, The nesting is only 1 step deep, and I construct them as follows:

case class A(str: String)
case class B(a: A)

val strLens = lens[B] >> 'a >> 'str

What is Nat indices?

Cheers

[milessabin]

OK, thanks. Are you able to put together a simple source file which demonstrates the compile time blow up?

"Nat indices" refers to the earlier mechanism of specifying lenses by positional offsets of fields (ie. lens[Foo] >> _1 >> _2, etc.).

[johanstenberg92]

Here: http://pastebin.com/cNnuk8sg

Cheers

[milessabin]

Thanks for that. I needed to add case class GeoKeyDirectory() to get it to compile successfully. When it did it took 176 seconds for that file on it's own on my laptop ... is that comparable to what you're seeing with that file in isolation?

[johanstenberg92]

Hi, sorry I'm a scala rookie I must had forgot that class. It took me more time but then the file was in a large project and my computer is a bit slow.

However it's still way to long, any idea what would speed it up?

[milessabin]

Yes, I completely agree.

I'm fairly sure that this is the same issue which is at play in #50, which is due to compile time overheads in the use of singleton-typed Strings as field selectors. It's not clear to me at the moment if this is a Scala compiler issue or something which is fixable in shapeless.

A workaround is to use Nat field indices, ie.,

import shapeless.nat._

val artistLens = metaDataTagsLens >> _0
val copyrightLens = metaDataTagsLens >> _1
val dateTimeLens = metaDataTagsLens >> _2
val computerLens = metaDataTagsLens >> _3
val imageDescLens = metaDataTagsLens >> _4
val makerLens = metaDataTagsLens >> _5
val modelLens = metaDataTagsLens >> _6
val softwareLens = metaDataTagsLens >> _7

But this is, of course, very fragile if you are likely to reorder the elements of your case classes.

[johanstenberg92]

Okay, I'm trying out monacle now, if that doesn't work out I will switch back to shapeless with Nat field indices. Thank you for the quick response by the way!

Cheers

[milessabin]

Fixed in d4c3c71.