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A Note about Coercions

Safe coercions in GHC are a very powerful feature. However, they are not perfect; and already many years ago I was also thinking about how we could make them more expressive.

For language like GHC Haskell where being zero-cost is non-negotiable requirement, this simple approach doesn't work. Because the language is simple (i.e. not dependent), we can define typing rules and small step semantics independently. Such uni-directional version of Coercible amounts to explicit inclusive subtyping and is more complicated than our current symmetric system.

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