26 Dec
2009
26 Dec
'09
9:30 a.m.
On 26 Dec 2009, at 11:58, haskell@kudling.de wrote:
class BarLike a where doSomething :: a -> Double
data Bar = forall a. BarLike a => Bar a
unwrapBar :: Bar -> a unwrapBar (Bar x) = x
How can i deconstruct the enclosed value of type a?
You can't write a function with a type that mentions existentially quantified "a". Period. But you can deconstruct the enclosed value temporarily: getSomething :: Bar -> Double getSomething b = case b of Bar a -> doSomething a