kind.hs:7:0:
    Illegal instance declaration for `Yesno t'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are type *variables*,
         and each type variable appears at most once in the instance head.
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Yesno t'
Failed, modules loaded: none.

So, I added the suggested Pragma
{-# LANGUAGE FlexibleInstances #-}
module Kind where
....

Prelude> :l kind.hs
[1 of 1] Compiling Kind             ( kind.hs, interpreted )

kind.hs:7:0:
    Constraint is no smaller than the instance head
      in the constraint: Num t
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Yesno t'
Failed, modules loaded: none.

Adjusted pragma to
{-# LANGUAGE FlexibleInstances,
UndecidableInstances #-}

Prelude> :l kind.hs
[1 of 1] Compiling Kind             ( kind.hs, interpreted )
Ok, modules loaded: Kind.

*Kind> yesno 10
True
*Kind> yesno 0
False

I am not sure if I understand the implications here. Did I introduce a bug?

-Amitava


On Tue, Mar 15, 2011 at 3:28 PM, aditya siram <aditya.siram@gmail.com> wrote:
Untested, but you might try:

instance (Num t) => YesNo t where  ....

-deech

On Tue, Mar 15, 2011 at 2:21 PM, Amitava Shee <amitava.shee@gmail.com> wrote:
> 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
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>



--
Amitava Shee
Software Architect

There are two ways of constructing a software design. One is to make it so simple that there are obviously no deficiencies; the other is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult.
-- C. A. R. Hoare The Emperor's Old Clothes, CACM February 1981