Yitzchak Gale wrote:
Roman Leshchinskiy wrote:
In general, if we are going to overload list literals then forcing the desugaring to always go through lists seems wrong to me. There are plenty of data structures where that might result in a significant performance hit.
These are literals. So the lists will almost always be quite short, and they will be evaluated only once. So I don't think there will be that much of a performance hit normally.
Calling them literals is misleading, IMO. They won't necessarily be only evaluated once: f x = [x] In DPH, it wasn't uncommon for certain benchmarks to spend 90% of the time constructing arrays from [:x,y,z:] terms until we made a significant effort to ensure that this doesn't happen. This is the only real data point related to this that I have but it does indicate that making the desugaring efficient is quite important.
That said, my extension that allows them to be desugared at compile time would solve that issue if it arises.
Personally, I don't like having desugaring depend on TH at all. I'm not sure think there is a real need for it. This would, IMO, already be better than fromList wrt efficiency: class Cons a where type Elem a empty :: a cons :: Elem a -> a -> a Roman