You can use GADTs for this. For example: data Showable where Showable :: Show a ... | Hacker News

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brentshields on March 30, 2014 | parent | context | favorite | on: Class Hierarchies? Don't Do That

You can use GADTs for this. For example:

  data Showable where
    Showable :: Show a => a -> Showable

This allows you to create a polymorphic container like [Showable 5, Showable "hello"] where the polymorphic type is constrained to be a member of the Show typeclass.

tel on March 30, 2014 [–]

Note that GADTs are a bit overkill for this. All you really need is ExistentialQuantification. GADTs are ExistentialQuantification + TypeEqualities.

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