Re: proposal: add 'unsafeCoerce'

On 2006-11-21, Malcolm Wallace wrote:

"David House" wrote:

...

...

* cast that changes a phantom type, or changes a type that is not reflected by a part of the value, eg. 'unsafeCoerce (Left 3) :: Either Int a' should be fine for any 'a',

Couldn't we have the following for this case?

castEitherL :: Either a b -> Either a c castEitherL (Left x) = Left x castEitherL z = z

castEitherR :: Either a b -> Either c b castEitherR (Right x) = Right x castEitherR z = z

No actually. Try it! Any compiler will reject the catch-all cases ('z') because you are asking for the usage of z on the rhs to be at a different type than the pattern usage of z on the lhs.

How about:

castEitherL :: Either a b -> Either a c castEitherL (Left x) = Left x castEitherL _ = error "castEitherL on Right"

castEitherR :: Either a b -> Either c b castEitherR (Right x) = Right x castEitherR _ = error "castEitherR on Left"

-- Aaron Denney -><-