On January 10, 2013 13:56:02 Richard Eisenberg wrote:
Class instances that overlap are chosen among by order of specificity;
Sorry to jump in the middle here, but this caught my attention as this sort of specificity determination is exactly what I had in mind when I was working on my "The shape of things: a reason for type preference" paper. I would love to know what approach GHC is currently taking to determine this. The document gives the example of matching C Int [Int] against C Int a C a Bool C Int [a] C Int [Int] This is easy though. Obviously the last match is best because it is exact. What if we have something like C Int [[Int]] against C Int b C a [[b]] though. How is the best match determined in this case? Thanks! -Tyson