…prehaps most notably being the arrival of type classes in C++! More importantly though, designers of other languages can pick and choose what new innovations have appeared in Haskell. Therefore any advantage granted to Haskell by way of continual innovation also diminishes over time. But the same cannot be said about the cost of innovation.
As I noted eariler, each new language-extending innovation has to exist alongside all prior ones, which leads to an exponential number of language-extension combinations. In the absence of some wondrous advance in managing the ensuing complexity…the cost of continual language innovation will eventually be overwhelming. Any notion of “a balanced approach” to innovation must take that cost into consideration.
For the forseeable future, the only way to manage that cost is to limit the total number of language extensions, by:
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consolidation - widely-used extensions are brought into new Haskell standards,
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or elimination - the least-used extensions are eventually dropped.