On 11-May-1998, Simon L Peyton Jones <[EMAIL PROTECTED]> wrote:
>
> Since GHC keeps the types right through the compiler, it
> really can't do CSE on two terms of type
>
> Either Int Int
> Either Bool Int
>
> even if they are both applications of Right.
>
> Actually, GHC does finally discard type information right at the
> end, so we could do an extra bit of CSE there, but frankly I doubt
> it would buy very much.
A simpler way of doing this is for the CSE pass to just insert a call
to a compiler builtin function `$unsafe_cast' if the types don't
match. At the very end when you finally discard type information you
can then optimize away the call to `$unsafe_cast'. This provides the
same benefits without the need for an extra pass.
It probably doesn't buy a lot, but IMHO it's easy enough that it's worth
doing anyway.
> Incidentally, I don't think it would be sensible to change
> the type system to allow the
>
> demo1 :: (a -> b) -> Either a c -> Either b c
> demo1 f (Left a) = Left (f a)
> demo1 _ r@(Right c) = r
>
> What type does r have? Either a c.
> What type does the result of the fn have? Either b c.
> Different types.
I agree, this code should be disallowed.
Note that the different types can lead to different semantics.
Consider the following code, which is similar to the code above:
foo :: (Int -> Float) -> Either Int Char -> Either Float Char
foo f (Left a) = Left (f a)
foo _ r@(Right c) = classmethod r
class Demo t where
classmethod :: t -> Either Float Char
instance Demo Either Int Char where
classmethod = m1
instance Demo Either Float Char where
classmethod = m2
Here `r' has type `Either Int Char', not `Either a Char',
and this determines which class method is called.
Note that writing
foo _ r@(Right c) = classmethod (Right c)
would result in compile error (or worse) due to an uninstantiated
type variable.
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
