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Yotam Ohad

10 Oct 2017 10 Oct '17

12:56 p.m.

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

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Adam Bergmark

10 Oct 10 Oct

1:33 p.m.

Hi Yotam, One way to do this is to take an `Either Foo Bar` as an argument instead, then you can pattern match on its Left and Right. This is preferable if the function is only meant to work for a limited number of types. If you want it to be extensible to any number of types you can write a typeclass that contains `func` with the same type signature as you have now. There are also some extensions built in to GHC to do this automatically for you, but I don't recall exactly which ones to enable... HTH, Adam On Tue, 10 Oct 2017 at 18:58 Yotam Ohad 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.

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Matt

1:39 p.m.

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 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.

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Eric Mertens

11 Oct 11 Oct

7:10 p.m.

If you want a function's behavior to vary by the types at which it is used, you'd need to use a type class. For example: data Foo = Foo { a1 :: Int -> Int, a2 :: Int -> Char } data Bar = Bar { a3 :: Int -> Int } class Func a where func :: a -> Maybe (Int -> Int) instance Func Foo where func = Just . a1 instance Func Bar where func = Just . a3 On Tue, Oct 10, 2017 at 10:42 AM Matt wrote:

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 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.

_______________________________________________ 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.

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participants (4)

Adam Bergmark
Eric Mertens
Matt
Yotam Ohad