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...
On Sat, Apr 3, 2021 at 9:26 PM Galaxy Being
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
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