| The -XOverlappingInstances flag instructs GHC to allow more than one | instance to match, provided there is a most specific one. For example, | the constraint C Int [Int] matches instances (A), (C) and (D), but the | last is more specific, and hence is chosen. If there is no most-specific | match, the program is rejected." | What it doesn't says is how "most-specific match" is computed. An instance declaration (A) is "more specific" than another (B) iff the head of (A) is a substitution instance of (B). For example (A) instance context1 => C [Maybe a] (B) instance context2 => C [b] Here (A) is more specific than (B) because we can get (A) from (B) by substituting b:=Maybe a. Does that make it clear? If so I will add it to the manual. | not cases are so clear. For example, which of | | instance context1 => C Int b | instance context2 => C a [[b]] | | does C Int [[Int]] match best against? Neither is instance is more specific than the other, so C Int [[Int]] is rejected. | If there isn't currently a good | way to resolve this, I would like to suggest the type-shape measure I | proposed in that paper I wrote up awhile back could be used. I think the current design is reasonable, and I don't want to complicate it unless there is a very compelling reason to do so. Simon