John Lato
writes:
On Mon, Dec 2, 2013 at 9:17 PM, AntC wrote:
... Importing an overlapping instance is trapped immediately; no risk of incoherence.
How can this possibly work with open type families? What happens in this case?
module A where type instance F a b c | b /~ c = Int module B where type instance F a b c | a /~ c = Bool
During compilation, neither A nor B is aware of the other. What happens in a module that imports both?
Thanks John, a good use case! The trapping is needed with imports for any approach to open instances (not just type families). Suppose I have NoOverlappingInstances everywhere:
module A where instance C a b c where ... module B where instance C a b c where ... module D where instance C Int Bool Char where ...
And a module that imports all three. Any importer has to validate all instances sometime or other. (Currently ghc sticks its head in the sand, and hopes there won't be a usage that trips over the ambiguity.) All we're talking about is _when_ we validate. I'd rather know at the point of declaring the instance, or of importing the instance. AntC