[Haskell-cafe] Is it possible to have constant-space JSON decoding?

[Albert Y. C. Lai]

On 12-12-05 12:48 AM, Jason Dagit wrote:
> I thought it was possible to get around this with lazy patterns such
> "Wadler's force function" [1]?
>
> (untested code)
>
> force y =
>    let Just x = y
>    in Just x
>
> lazyDecode :: FromJSON a => ByteString -> Maybe a
> lazyDecode = force . decode

This says, the type is Maybe, but the value is always Just. If there 
will be a parse error, you will get an async imprecise exception, not 
Nothing, at the later time when you look closer.

This respects the letter, but not the point, of the Maybe type. If you 
are to do this (I have no moral, I hold nothing against doing this, the 
async imprecise exception will give you enough grief), you may as well 
skip the Just wrapping and directly go ByteString -> a.

The point of the Maybe type is to communicate parse errors by a less 
async, less imprecise way.