Re: Union Types for Haskell!?

24 Nov 2000


      At 2000-11-24 03:14, Fergus Henderson wrote:
...
...
Is there any reason for this restriction
in the Haskell type system? Does this lead to losing the principal type
property?
If you allow (2) above, there may be serious problems for
principal types.  For example, consider
f x = case x of
      Nothing -> False
      Just _ -> True
What's the most general type for `f'?
The type `f :: Maybe a -> Bool' is less general than
e.g. `f :: Union { Maybe a, ... } -> Bool',
but you certainly don't want to infer the latter type.
There are two different kinds of 'general' here. Informally, you want the 
type most general in the type-substitution sense, but probably most 
specific in the subtype sense.

-- 
Ashley Yakeley, Seattle WA