Are you talking about instances such as Show a => Show (Maybe a)? If the fields of a record are fixed types, it seems like GHC would be able to inline the dictionaries. On Fri, Jan 11, 2019 at 1:14 PM Brandon Allbery wrote:

Only up to a point. Recursive uses — such as show, which delegates to other Show instances for record fields, etc. — are problematic.

On Fri, Jan 11, 2019 at 1:04 PM Johannes Waldmann wrote:

...

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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