Do you want to request a feature or report a bug?

Feature

What is the current behavior?
Extraneous dependencies are pruned on yarn install. This is a problem for my lerna mono-repo when I use --hoist. Shared dependencies get hoisted to the root of the mono-repo, and subsequently running yarn install prunes them.

What is the expected behavior?
I see the value in having yarn install prune extraneous dependencies, but it would be great if there was an option to not prune yarn install --no-prune.

Please mention your node.js, yarn and operating system version.
Node.js: v7.5.0
yarn: 0.21.3
OS: OS X El Capitan