* Petr Pudlák
If I define a function that matches on a single-constructor data type, such as (,), is there any reason against using ~? Like
f (a,b) = ...
instead of
f ~(a,b) = ...
? I've seen that not using ~ can lead to problems sometimes, but not the other way around.
(Of course changing the semantics of existing functions such as `swap` is problematic, my question targets the problem in general.)
The usual dangers of laziness apply. Consider, for example, the State monad. The difference between lazy and strict State is exactly the way of pattern matching on the tuple. Strict state monad allows you to keep your state evaluated. Lazy doesn't. Prelude Control.Monad.State.Strict> flip evalState () $ (put $! undefined) >> return () *** Exception: Prelude.undefined Prelude Control.Monad.State> flip evalState () $ (put $! undefined) >> return () () Roman