On 31-Jan-2001, Lennart Augustsson
Ashley Yakeley wrote:
data Any = forall a. Any a
get :: Any -> Maybe Char get (Any (c::Char)) = Just c -- bad get _ = Nothing --
...but as it stands, this is not legal Haskell, according to Hugs:
ERROR "test.hs" (line 4): Type error in application *** Expression : Any c *** Term : c *** Type : Char *** Does not match : _0 *** Because : cannot instantiate Skolem constant
This, of course, is because the '::' syntax is for static typing. It can't be used as a dynamic pattern-test.
Question: how big of a change would it be to add this kind of pattern matching? Is this a small issue, or does it have large and horrible implications?
It has large and horrible implications. To do dynamic type tests you need to carry around the types at runtime. This is not something that Haskell does (at least you don't have to).
But you can achieve a similar effect to the example above using the
Hugs/ghc `Dynamic' type. Values of type Dynamic do carry around the
type of encapsulated value.
data Any = forall a. typeable a => Any a
get :: Any -> Maybe Char
get (Any x) = fromDynamic (toDyn x)
This works as expected:
Main> get (Any 'c')
Just 'c'
Main> get (Any "c")
Nothing
Main> get (Any 42)
ERROR: Unresolved overloading
*** Type : (Typeable a, Num a) => Maybe Char
*** Expression : get (Any 42)
Main> get (Any (42 :: Int))
Nothing
--
Fergus Henderson