[Haskell-beginners] Idiomatic way to avoid type class instance definitions for Int and Integer separately

[Amitava Shee]

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 at 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 at 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 at 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
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