At run-time, only the branch (Left/Right) is "picked" at run-time.
For each branch, the "show" instance is determined at compile-time.

The problem is that such instances are represented by dictionaries,
and this adds extra cost (extra arguments) to function calls -
but only in those cases where these dictionaries
are not inlined (that is, removed) statically.

But I am thinking that GHC has become very good at inlining.

- J.W.

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