Slim instance declaration
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This is an excerpt from "Disclipline for re-use: Slim instances" http://www.haskell.org/pipermail/libraries/2006-September/005791.html
All instance declarations should be slim.
This means
- Instance declarations (and default definitions) should never contain non-trivial function definitions.
- Instance declarations (and default definitions) should only provide "plumbing" to make existing functions accessible via the type class resolution mechanism.
- The "plumbed" functions should always be exported (since instances are always exported).
Since the naming will not always be straight-forward, the last point is particularly important and would enable more re-use and less re-invention of mostly trivial wheels.