[Haskell-cafe] RFC: demanding lazy instances of Data.Binary

[Don Stewart]

nicolas.frisby:
> I've noticed a few posts on the cafe, including my own experience,
> where the spine-strictness of the Binary instance for lists caused
> some confusion. I'd like to suggest an approach to preventing this
> confusion in the future, or at least making it easier to resolve.
> 
> Having decided that it is indeed appropriate for some standard
> instances to be strict by default [1], I think it would be beneficial
> to standardize an approach for expressing that a lazy instance is
> expected. I propose the following newtype be added to Data.Binary. A
> demonstration immediately follows.
> 
> > newtype Lazily a = Lazily { unLazily :: a }
> 
> > -- example instance
> > instance Binary a => Binary (Lazily [a]) where
> >     -- lazy get and put
> 
> Now
> 
> > [1..] = (unLazily . decode . encode . Lazily) [1..]
> 
> instead of
> 
> > _|_ = (decode . encode) [1..]
> 
> This email is a request for comments on this concept. I think it is a
> minimal way of expressing the intent that the serialisation be lazy
> [2]. Please share any concerns or suggestions. I'll submit a patch
> once the discussion is complete... or if it becomes inactive ;)

I think this is a good compromise: allowing laziness for those who need
it, in a clean manner. How about we provie

    Data.Binary.Lazy

with the Lazy newtype, and lazy instances for types that make sense to
be so?

For now, this can be developed as a single module depending on
Data.Binary. What do you think, Nick?

> 
> 1 - One solution is to make all standard Binary instances lazy
> wherever possible, but I presume that in most cases it's not needed
> and the compactness gained through default strictness (such as the []
> instance) is quite significant.

Yes, that's the argument.

-- Don