#8370: Ranked Instances ------------------------------------+------------------------------------- Reporter: wvv | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This is a first part of 3 depended extensions: Ranked Instances => Inherit Instances => Newclasses This is very easy to implement. And it gives big advantages. And further developing. I suggest to add "rank" to instance. Now the system of instances is flat, and compiler takes all of instances and check if only 1 match. If it is not(less or more than 1) - compiler trow an error. With rankings compiler * set N=0 * took all N-ranked instances and * => try to find only one match instance. * -> If it found only 1 - this is RESULT: needed instance, * -> If it found many instances - throw an error * -> If compiler found NO instances, compiler set N=N+1 and repeat * If N=MaxRank and still no matches compiler throw an error We use `RankedInstances` '''before''' `OverlappingInstances` (which must include `RankedInstances`), and in many cases instead of. == Proposed grammar == We add "rank N" between "instance" word and ".", where N is a number {{{ {-# LANGUAGE RankedInstances #-} instance rank 1. C a => D a where ... }}} == Default Rank == If instance has no rank, this means it has 0 rank {{{ instance C a => D a where ... ~ instance rank 0. C a => D a where ... }}} This is a backward compatibility. '''Examples''' 1) {{{ instance rank 1. C Int a where ... -- (A) instance C a Bool where ... -- (B) rank 0 }}} 1+) {{{ instance rank 1. C Int a where ... -- (A) instance rank 1. C a Bool where ... -- (B) instance C Int Bool where ... -- (C) rank 0 }}} 2) {{{ instance rank 1. C Int [a] where ... -- (C) instance C Int [Int] where ... -- (D) }}} As we see, all instances are unambiguous! == Backward Compatibility == Without -XRankedInstances all instances with hight rank n, where n >0 - are not exported More information (about all 3 extensions) is here http://haskell.1045720.n5.nabble.com/Proposal-RankedInstances- tt5737152.html -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8370 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler