[Haskell-beginners] Question about constraints, signatures
Philippe Sismondi
psismondi at arqux.com
Mon May 9 22:47:28 CEST 2011
On 2011-05-09, at 4:04 AM, David Virebayre wrote:
> 2011/5/8 Philippe Sismondi <psismondi at arqux.com>:
>
>> On 2011-05-06, at 12:12 PM, Brent Yorgey wrote:
>
>>> On Fri, May 06, 2011 at 11:34:47AM -0400, Philippe Sismondi wrote:
>>>> I am trying to understand the Typeclassopedia article by Brent Yorgey in Monad.Reader. I am simultaneously working my way through Real World Haskell, so I may be doing things a bit out of order.
>
>>>> I (more or less) understand that types and typeclasses are not the same.
>>>> But it seems to me that they do conflate in a way, if we think of types as defined by functions on them.
>>>> So why should we not be able to write:
>
>>>> f :: Num d -> String -> Int
>
> You can have multiple class constraints, so it's not clear how you
> would write them using your syntax; how would you write :
>
> f :: (Eq d, Ord d) => d -> String -> Int ?
>
> Also, you only have to tell the class constraint once, with your syntax
>
> g :: (Num d) => d -> d -> d
>
> would become:
>
> g :: Num d -> Num d -> Num d
>
> but that syntax lets you write nonsense :
>
> h :: Num d -> Eq d -> Ord d
>
>
> David.
David:
Thanks for your input. As I have quite a distance to go with Haskell, I shall take all these comments under advisement.
- Phil -
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