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Today's Topics:
1. Type class instance with Num (Galaxy Being)
2. Re: Type class instance with Num (Bob Ippolito)
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Message: 1
Date: Sat, 3 Apr 2021 23:26:09 -0500
From: Galaxy Being <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Type class instance with Num
Message-ID:
<cafahfsvd8spajofah5+voktgaqhzlejzeo4kleev3xav8mv...@mail.gmail.com>
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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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Message: 2
Date: Sat, 3 Apr 2021 21:39:23 -0700
From: Bob Ippolito <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Type class instance with Num
Message-ID:
<cacwmpm93hhk4-eyi9fxnmer2bm86xpzuz_h-6hmlkoczwye...@mail.gmail.com>
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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 <[email protected]> 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
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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