On Tuesday 16 August 2005 22:29, Keean Schupke wrote:
Benjamin Franksen wrote:
On Tuesday 16 August 2005 21:56, Keean Schupke wrote:
You can even use existential types to create lists of things with a common interface, where you do not know in advance what types you may need:
data XWrap = XWrap (forall a . Show a => a) type ListXWrap = [XWrap]
You probably meant to write
data XWrap = forall a . Show a => XWrap a
or, in GADT style (which I find a bit more intuitive here):
data XWrap where XWrap :: Show a => a -> XWrap
Yes I always get confused by the notation Haskell uses...
Same here.
I used explicit universal quantification by mistake. I tried to think logically about the encapsulation existential types represent - and got the wrong form.
I for one would like to see the use of 'exists' as a keyword for existential types, after all different symbols are used in modal logic (upside-down-A for forall, and backwards-E for exists).
I once read a paper about type classes and existentials (can't remember exact title or author, was it Läufer?) where the proposal was to make existential quantification implicit (just as the universal one is in Haskell98). That is, any type variable that appears on the rhs of a data type, but not on the lhs, is implicitly existentially quantified, as in data XWrap = Show a => XWrap a I always thought this was a pretty nice idea. Ben