[Haskell-cafe] Pattern Matching with Different Types

[Matt]

Hi Yotam,

A type variable `a` means that you know absolutely nothing about the
variable, and can't treat it specially. So you can't special-case your
function to "either Foo or Bar."

You can achieve your goal like this:

func :: Either Foo Bar -> (Int -> Int)
func (Left (Foo a _)) = a
func (Right (Bar a)) = a

The kind of runtime type inspection you might be used to from other
langauges is available via the Typeable class. It's extremely non-idiomatic
Haskell, and I strongly recommend against writing this sort of code. For
completeness, this also works:

func :: Typeable a => a -> Maybe (Int -> Int)
func a = maybeFoo <|> maybeBar
  where
    maybeFoo = case cast a of
      Just (Foo a _) -> Just a
      Nothing -> Nothing
    maybeBar = case cast a of
      Just (Bar a) -> Just a
      Nothing -> Nothing

Matt Parsons

On Tue, Oct 10, 2017 at 10:56 AM, Yotam Ohad <yotam2206 at gmail.com> wrote:

> Hello cafe,
> I am trying to do the following:
>
> data Foo = Foo { a1 :: Int -> Int, a2 :: Int -> Char }
> data Bar = Bar { a1 :: Int -> Int }
>
> func :: a -> Maybe (Int -> Int) -- a is either Foo or Bar
> func (x::(Bar/Foo) = Just $ a1 x
> func _ = Nothing
>
> I'm not sure how to implement this. All I know that the types are matching
> so I think it could be possible.
>
> Thanks for your help
> -Yotam
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20171010/538770b3/attachment.html>