Martin Huschenbett wrote:
readValue :: Field - (forall s. SqlBind s = s) - Value
readValue _ = ...
That works just fine. But now I want a version of readValue that has a
Maybe wrapped around the second parameter and that shall call readValue
in the case of a Just and emptyValue in the
Hello Martin,
Friday, March 23, 2007, 11:37:16 PM, you wrote:
readValue' :: Field - Maybe (forall s. SqlBind s = s) - Value
Thank you very much, that's exactly what I wanted. After reading in the
GHC users guide about rank 2 polymorphism I thought that this is not
possible. Chapter 7.4.8.
apfelmus schrieb:
For me, the fourth trial works, at least on
f :: (forall s . Num s = Maybe s) - Int
f y = case y of
Just x - x
Nothing - 0
This works, because the compiler knows that x has to have type Int. But
if you want to apply a function g :: (forall a. Num a
Martin Huschenbett schrieb:
My thoughts were that for any class C the types
Maybe (forall a. C a = a) (I will call it T1 for short)
and
(forall a. C a = Maybe a) (I will call it T2 for short)
are isomorphic. Defining the isomorphism from T1 to T2 is quite simple:
iso1 :: Maybe (forall
Ian Lynagh schrieb:
readValue' :: Field - Maybe (forall s. SqlBind s = s) - Value
readValue' fld s =
if isJust s then readValue fld (fromJust s) else emptyValue fld
Thank you very much, that's exactly what I wanted. After reading in the
GHC users guide about rank 2 polymorphism I thought
Hello,
What Ian suggested is a very GHC 6.6 specific solution that uses much
more that simply rank-2 types. Here is another solution that uses
just rank-2 types (and, by the way, all type signatures are optional,
as in ordinary Haskell):
module Value where
class SqlBind a where
fromSqlValue
Hi again,
the solutions/proposals of Ian and Iavor seem to be exactly what I need
at a first glance. But looking at them more in detail reveals some other
problems.
I also have got a function
getFieldValueMB :: SqlBind s = Statement - String - Maybe s
To get Ians approach working I would
