Haskell analog instance?

Question

I'm new to Haskell, so my question is probably stupid.

I need a function

show2 :: (Show a) => a -> String

which will return show afor anyone a, but aif a is itself String. How to implement it?

PS great if this function is already implemented somewhere, but I still want to see an example implementation.

Answer 1

You can do this with this piece of dirty and dangerous code:

class Showable a where
  show2 :: a -> String

instance Showable String where
  show2 = id

instance (Show a) => Showable a where
  show2 = show

You need -XOverlappingInstances -XFlexibleInstances -XUndecidableInstancesto compile and use it.

*Main> show2 "abc"
"abc"
*Main> show2 3
"3"

Answer 2

You can use castfromData.Typeable

show2 :: (Typeable a, Show a) => a -> String
show2 s = maybe (show s) id ms
  where ms = cast s :: Maybe String

Answer 3

, Haskell, instanceof. Haskell ​​ , Haskell : , .

, Haskell-Lee, , , Haskell , , ( , Java, , - !).

, - instanceof . - . ( ):

• " " java
• Java
• http://smsohan.com/blog/2011/11/01/using-instanceof-is-mostly-code-smell/
• http://sourcemaking.com/refactoring/replace-conditional-with-polymorphism
• http://www.javapractices.com/topic/TopicAction.do?Id=31
• http://www.artima.com/interfacedesign/PreferPoly.html

: , , : , , , , instanceof, , .

Haskell:

• , , .
• , , .

, - :

class ToString a where
    toString :: a -> String

instance ToString String where
    toString str = str

instance ToString Integer where
    toString i = show i

-- ...