On 05/02/14 09:35, Erik Hesselink wrote:
Show is usually valid Haskell (at least derived instances are), and has instances for most types. This class would only be a conversion from types that *are* strings in some sense (like Text) to String.
Totally agree. On 05/02/14 08:13, Atze van der Ploeg wrote:
Isn't toString just show?
No, show is very different: show "hello" == "\"hello\"" While you would expect toString "hello" == "hello" Personally, I would always recommend this: * Make show represent the "programmer version" of your data type, e.g. close to what "deriving Show" would give you, and implement it for every data type: newtype MyString = MyS String instance Show MyString where show (MyS s) = "MyS " ++ show s * Use a different type class (e.g. a ToString) to represent the stringyness of your type: instance ToString MyString where show (MyS s) = s
From my experience, everything else makes programming really nasty, outcomes unpredictable, and debugging very hard. If you want an example, try GHC code / the GHC API, where many things unfortunately do not have a show instance.
We have a (very small) package for this called 'isstring' that we use internally. It has instances for String, Text (x2) and ByteString (x2) assuming UTF8 encoding. I'd be happy to open source it if people are interested.
Yes. And I think a `ToString` typeclass would be useful be in base. Also, this existed once but is now deprecate it: http://hackage.haskell.org/package/to-string-class I speculate that it got deprecated because it is not nice to maintain all the orphan instances - the class in base would fix that.