>: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 <laiboonh@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

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