[Haskell-cafe] Re: ANNOUNCE: deepseq-

Duncan Coutts duncan.coutts at googlemail.com
Wed Nov 18 09:39:10 EST 2009

On Wed, 2009-11-18 at 09:17 +0000, Simon Marlow wrote:

> So the main difference is that with the current formulation of deepseq, 
> you need to explicitly force the result in order to use it, either with 
> a pattern match, another seq, or a pseq.  If we used (a -> b -> b) then 
> the top-level forcing is "built-in".
> Let's look at an example instance; here (1) is the current deepseq, (2) 
> is deepseq :: a -> b -> b
> instance (DeepSeq a, DeepSeq b) => DeepSeq (a,b) where
>    -- (1) deepseq (a,b) = deepseq a `seq` deepseq b
>    -- (2) deepseq (a,b) = deepseq a . deepseq b
> They're both fairly similar.  Most instances follow this pattern, with 
> seq being replaced by (.).
> You could argue that (a -> b -> b) is "doing more" than (a -> ()), 
> because it has a kind of built-in continuation (Luke's point).  I buy 
> that, although (a -> ()) has a strange-looking unit type in the result 
> and you have to use it in conjunction with seq.

I think the most important thing is to make the public interface that
people use most frequently simple and easy to remember.

Thus I suggest the primary function people use should be

        deepseq :: DeepSeq a => a -> b -> b

because then all that users have to remember is:

        "deepseq --- like seq but more so!"

That's it. Users already know how to use seq, so now they know how to
use deepseq too.

> (1) generates slightly better code with GHC, because it compiles seq 
> directly into a case expression, whereas (.) involves a thunk.  If 
> deepseq is inlined all the way down, then it would turn into the same 
> thing either way.
> I don't feel terribly strongly, but I have a slight preference for the 
> current version.

If it so happens that it is more convenient or faster to make the class
and instances use the (a -> ()) style then that is fine. We can give the
class method a different name. Presumably people have to write Deepseq
instances much less frequently than they use deepseq.


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