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Today's Topics:
1. Why is the type constraint different from signature?
(Lai Boon Hui)
2. Re: Why is the type constraint different from signature?
(David McBride)
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Message: 1
Date: Mon, 29 Aug 2016 22:56:46 +0800
From: Lai Boon Hui <laibo...@gmail.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] Why is the type constraint different from
signature?
Message-ID:
<cajdqggkdiy_r425xttnd5gygoeicp9gecogku8bpxsl3aff...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi All,
can some one explain to me why
ghci> let f:: (Ord a, Num b) => a -> b -> a ; f=undefined
ghci> :t f 1 2
ghci> f 1 2 :: (Num a, Ord a) => a
The initial type signature just required *a* to be a type that is an
instance of Ord but after it had the additional constraint of Ord as well???
--
Best Regards,
Boon Hui
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Message: 2
Date: Mon, 29 Aug 2016 11:08:23 -0400
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Why is the type constraint different
from signature?
Message-ID:
<CAN+Tr41Crr5Zba=58emffao5gt1bo-cgb_qpjkg0eemn6ga...@mail.gmail.com>
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>:t f
f :: (Num b, Ord a) => a -> b -> a
a is an instance of Ord
b is an instance of Num
>:t 1
1 :: Num a => a
>:t f 1
f 1 :: (Num a, Num b, Ord a) => b -> a
the literal 1 is an instance of Num, therefore
a must be an instance of Ord but now also of Num
What we know about a is that it must be a Num (because we assigned it the
literal 1 which is a Num) and that it must also be an Ord (because your
original type signature specified that it must also be an Ord).
On Mon, Aug 29, 2016 at 10:56 AM, Lai Boon Hui <laibo...@gmail.com> wrote:
> Hi All,
>
> can some one explain to me why
>
> ghci> let f:: (Ord a, Num b) => a -> b -> a ; f=undefined
> ghci> :t f 1 2
> ghci> f 1 2 :: (Num a, Ord a) => a
>
> The initial type signature just required *a* to be a type that is an
> instance of Ord but after it had the additional constraint of Ord as well???
>
>
> --
> Best Regards,
> Boon Hui
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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