[Haskell-beginners] Question about constraints, signatures

[David Virebayre]

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.