CODEWITHSUNDEEP
haskell
Clinton
Clinton

Reputation: 23135

Pattern matching on types

Is there nice way to write the following "x is of type t" parts? (I suspect I should be using Data.Type.Equality but I'm not sure exactly how)

f :: a -> Int
g :: b -> Int

h :: Typeable t => t -> Maybe Int
h x = case x of
  (x is of type a) -> Just (f x)
  (x is of type b) -> Just (g x)
  _ -> Nothing

Follow up question

Upvotes: 1

Views: 70

Answers (1)

Benjamin Hodgson
Benjamin Hodgson

Reputation: 44603

This is a job for the "type safe cast" bits of Data.Typeable. cast :: (Typeable a, Typeable b) => a -> Maybe b pulls the runtime type information out of the Typeable dictionaries for a and b and compares them; if a and b are the same type then it returns Just its argument, otherwise it fails.

So, with cast and Maybe's Alternative instance in hand, we have:

h x = f <$> cast x
  <|> g <$> cast x

As far as I know, there's no way to avoid the repetitious calls to cast, since they occur at different types.

Upvotes: 3

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