Dean Herington wrote:
deepSeq :: DeepSeq a => a -> b -> b
I should point out that deepSeq with this type is the composition of two simpler operations: deepSeq = seq . eval where eval :: DeepSeq a => a -> a eval ties a demand for a value to a demand for all its subvalues, while seq ties a demand for a value to a demand for another value of an unrelated type. Of course you can define eval x = x `deepSeq` x instead, so it's largely a matter of taste.
instance DeepSeq (IO a) where deepSeq = seq
This is an interesting instance (which is not to say I think it's wrong). It means the original poster's code won't work. He wanted to write foo <- eval makeFoo `catch` \e -> defaultFoo but makeFoo has a monadic type, so eval makeFoo === makeFoo. -- Ben