On 6/1/12 6:11 AM, Gábor Lehel wrote:

On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton wrote:

...

TypeFamilies (aka TFs) These are really nifty and they're all the rage these days. In a formal sense they're equivalent to fundeps, but in practice they're weaker than fundeps.

Is that still true? The reason used to be that we didn't have superclass equalities, but we do have them now since 7.2. The only drawbacks I know of relative to FDs are that it's sometimes more typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow OverlappingInstances (ick).

The superclass equalities was a big thing, but the disparity remains even still. The main problem is that type inference with fundeps remains more powerful than type inference for TFs. There are still patterns of use which infer/check easily with fundeps but which don't have an equivalent TF implementation. I haven't messed with TFs enough to know the details here, but there are other folks on the list who do. In addition there's the issue of overlapping instances, which is currently being discussed elsewhere. -- Live well, ~wren