§ How GHC does typeclass resolution

• As told to me by davean:

• Its like 5 steps
• Find all instances I that match the target constraint; that is, the target constraint is a substitution instance of I. These instance declarations are the candidates.
• If no candidates remain, the search fails. Eliminate any candidate IX
• for which there is another candidate IY such that both of the following hold: IY is strictly more specific than IX. That is, IY is a substitution instance of IX but not vice versa. Either IX is overlappable, or IY is overlapping. (This “either/or” design, rather than a “both/and” design, allow a client to deliberately override an instance from a library, without requiring a change to the library.)
• If all the remaining candidates are incoherent, the search succeeds, returning an arbitrary surviving candidate.
• If more than one non-incoherent candidate remains, the search fails.
• Otherwise there is exactly one non-incoherent candidate; call it the “prime candidate”.
• Now find all instances, or in-scope given constraints, that unify with the target constraint, but do not match it. Such non-candidate instances might match when the target constraint is further instantiated. If all of them are incoherent top-level instances, the search succeeds, returning the prime candidate. Otherwise the search fails.
• GHC manual