[Haskell-beginners] Type class instance with Num

[Bob Ippolito]

You need something like this:

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

class YesNo a where
    yesno :: a -> Bool

instance (Num a, Eq a) => YesNo a where
    yesno = (/= 0)

The reason this doesn't work without turning on some "scary" flags is that
you can easily write code that is ambiguous since typeclasses are open.
Open means that some other file can define a data type that has an instance
of Num and an instance for YesNo and then there's no obvious choice which
instance should be used.

If you want a bit more detail, here's a relevant StackOverflow question:
https://stackoverflow.com/questions/8877541/how-to-write-an-instance-for-all-types-in-another-type-class


On Sat, Apr 3, 2021 at 9:26 PM Galaxy Being <borgauf at gmail.com> wrote:

> I'm following LYHFGG and I have this
>
> class YesNo a where
>     yesno :: a -> Bool
>
> instance YesNo Int where
>     yesno 0 = False
>     yesno _ = True
>
> but then I have to specify Int here
>
> > yesno (5 :: Int)
> True
>
> Just with 5 gives this error
>
> Ambiguous type variable ‘a0’ arising from the literal ‘5’
>       prevents the constraint ‘(Num a0)’ from being solved.
>       Probable fix: use a type annotation to specify what ‘a0’ should be.
>
> I tried this
>
> instance YesNo (Num a) where
>     yesno 0 = False
>     yesno _ = True
>
> but got cryptic errors. What can I do to make yesno take any of Num's
> numbers?
>
> LB
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20210403/473ae724/attachment.html>