This can break things (at compile time, unless you errantly mark something with an OVERLAPS pragma), which is why it's recommended that instances live where the

This can break things (at compile time, unless you errantly mark something with an OVERLAPS pragma), which is why it's recommended that instances live where the type is defined or where the typeclass is defined. Instances defined elsewhere are known as "orphan instances", and trigger a warning (which you can disable) in GHC. Edited to add: Note that the above applies mostly to libraries. There's not really a downside to defining orphan instances in an application so long as you guarantee consistency (and the best way to do that, IME, is to put all your orphans in one place, with only that file marked -fno-warn-orphans).