Hugo Pacheco:
If the equality does not hold, you should get a type error because
your program is not type correct. So, what is it that you would like
different?
I would simply like the compiler not to use that instance if the
equality constraint does not hold, like some another instance
On Fri, Mar 7, 2008 at 2:10 AM, Ryan Ingram [EMAIL PROTECTED] wrote:
I don't think the cost is that great; the compiler can easily flag
polymorphic functions that require type information for some or all
arguments and in many cases the type evidence can be passed
just-in-time when calling
Hugo Pacheco:
Just something I have been wondering.
I would like to implement somehting like:
type family F a :: * - *
...
class C a b where ...
instance (F a ~ F b) = C a b where ...
But apparently type equality coercions can not be used as a single
context. If I enable
If the equality does not hold, you should get a type error because
your program is not type correct. So, what is it that you would like
different?
I would simply like the compiler not to use that instance if the equality
constraint does not hold, like some another instance dependency
What I said is not true since overlapping instances are not that much
decidable.
Btw, in previous versions of GHC this worked well, but now I suppose order
does not suffices to define instances overlapping
How could I compile such an example, assuming that I want to use the
instance C String for
2008/3/6 Hugo Pacheco [EMAIL PROTECTED]:
How could I compile such an example, assuming that I want to use the
instance C String for Strings only and the more general instance for the
rest?
class C a where
c :: a
instance C Char where
c = 'a'
instance C a = C [a] where
c = [c
On Thu, Mar 6, 2008 at 7:52 PM, Ryan Ingram [EMAIL PROTECTED] wrote:
I wish there was some form of instance declaration that let you do
case analysis; something similar to the following:
instances C [a] where
instance C String where
c = a
instance C a = C [a] where
On Thu, Mar 6, 2008 at 9:52 PM, Ryan Ingram [EMAIL PROTECTED] wrote:
This is actually a general issue with the way typeclasses are defined
in Haskell; the accepted solution is what the Show typeclass does:
class C a where
c :: a
cList :: [a]
cList = [c,c]
instance C
On Thu, Mar 6, 2008 at 7:16 PM, Luke Palmer [EMAIL PROTECTED] wrote:
I agree that this would be nice. But I think that the situation is
stickier than it looks on the surface. Consider this:
instances GShow [a] where
instance [Char] where gShow = id
instance [a] where
Just something I have been wondering.
I would like to implement somehting like:
type family F a :: * - *
...
class C a b where ...
instance (F a ~ F b) = C a b where ...
But apparently type equality coercions can not be used as a single context.
If I enable -fallow-undecidable-instances,
Hi all,
I have recently tried to replicate some examples from in the articles about
type families but found some possible bugs.
In [2], the example
class C a where
type S a (k :: * - *) :: *
instance C [a] where
type S [a] k = (a,k a)
does not compile under the claim that the type
2008/3/3 Hugo Pacheco [EMAIL PROTECTED]:
class C a where
type S a (k :: * - *) :: *
instance C [a] where
type S [a] k = (a,k a)
does not compile under the claim that the type variable k is not in scope.
It's not entirely syntactical sugar; I believe that when a type family
is a
Hugo Pacheco:
I have recently tried to replicate some examples from in the
articles about type families but found some possible bugs.
In [2], the example
class C a where
type S a (k :: * - *) :: *
instance C [a] where
type S [a] k = (a,k a)
does not compile under the claim that the
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