The suggested command for adding the stack source repository to Ubuntu is:

echo 'deb http://download.fpcomplete.com/ubuntu/wily stable main'|sudo tee /etc/apt/sources.list.d/fpco.list

But, from the Debian documentation:

The 'distribution' can be either the release code name / alias (wheezy, jessie, stretch, sid) or the release class (oldstable, stable, testing, unstable) respectively. If you mean to be tracking a release class then use the class name, if you want to track a Debian point release, use the code name.

This semantics carries over to Ubuntu.

What I would expect instead:

http://download.fpcomplete.com/ubuntu/wily stable main

Should be

http://download.fpcomplete.com/ubuntu wily main

This may seem completely trivial, but it does cause certain issues with system tools - for example, deploying stack via puppet on Ubuntu is unusually inconvenient.