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Implement Semigroup and Monoid instances for the ST monad

Since IO implements Semigroup and Monoid, is there a good reason for ST not to implement those, too? I discovered the lack of those instances while using foldMap for code that used to use IO as the underlying monad.

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Trac field Value
Version 8.2.1
Type FeatureRequest
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Core Libraries
Test case
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