Extensible data types?

[Jose Emilio Labra Gayo]

> 
> The above code fails to type check due to instances
> overlapping. Hugs gives the following error message:
> 
In Hugs, there is a flag that you can set to allow overlapping instances

	:s +o

In GHC, you can also set

	-fallow-overlapping-instances

BTW, I use extensible union types in a "Language prototyping System" that
I am implementing and which compiles with GHC and Hugs (it is based
on Liang, Hudak and Jones paper). 
You can download the source code from 
  "http://lsi.uniovi.es/~labra/LPS/LPS.html"

Best regards, Jose Labra
http://lsi.uniovi.es/~labra