Summary:
This patch is only to make the code easier to read.

In addition, this is the first patch I send with the arc/differential workflow.
So I start with something very small.

Test Plan: I have not even tried to compile it yet.

Reviewers: simonmar, austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D189