[Haskell-beginners] Idiomatic way to avoid type class instance definitions for Int and Integer separately
Amitava Shee
amitava.shee at gmail.com
Tue Mar 15 20:21:40 CET 2011
While reading "Learn You a Haskell for Great Good!" I came across the YesNo
type class
I tried a minimal version as below
module Kind where
class Yesno a where
yesno :: a -> Bool
instance Yesno Int where
yesno 0 = False
yesno _ = True
I was surprised to get an error
*Kind> :load kind.hs
[1 of 1] Compiling Kind ( kind.hs, interpreted )
Ok, modules loaded: Kind.
*Kind> yesno 10
<interactive>:1:6:
Ambiguous type variable `t' in the constraints:
`Num t' arising from the literal `10' at <interactive>:1:6-7
`Yesno t' arising from a use of `yesno' at <interactive>:1:0-7
Probable fix: add a type signature that fixes these type variable(s)
Turns out 10 in this instance is an Integer and I have not defined Yesno
over Integer
Easy fix - just define an instance over Integer
instance Yesno Integer where
yesno 0 = False
yesno _ = True
My question - Is there a way to avoid this kind of boilerplate? What is the
idiomatic way?
Thanks & Regards,
Amitava Shee
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