#8370: Ranked Instances -------------------------------------+------------------------------------ Reporter: wvv | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by wvv): Wow! Great book! Thank you very much! == Contribution == I have Windows without VM. And even compile GHC from 0 (even without any contribution) will be a big challenge )) So, maybe this is not an option right now. == Easy Implemetation == I'm not familiar with low-Haskell code, but yes, it is easy! We can divide changes to support code and significant code. == Support code == This is a dev's routine, I a newbie with these techniques. It is needed to - add extension name to extension - add parsing extension pragma - add line "use -XRankedInstances or -XOverlappingInstances" in error message if both RankedInstances and OverlappingInstances are off - I recommend deny to use both RankedInstances and OverlappingInstances in one module - add data Ranked-Instances, absolutely similar as data Overlapping- Instances - add parsing pattern to parser/Parser.y.pp, something like `| "instance" "rank" digits "." <rest>` . But forbid to parse without extension - extract number - and add "rank" field to all data Instance<Smth> and copy-paste this field everywhere it is needed. == Significant code == Fortunately, almost all code is already written by guys, who added Overlapping instances to GHC! The reason - Ranked Instances are "user control overlapping". http://ghc.haskell.org/trac/ghc/browser/ghc/compiler/types/InstEnv.lhs#L564 {{{ lookupInstEnv ... = pruned_matches = foldr insert_overlapping [] all_matches (safe_matches, safe_fail) = if length pruned_matches == 1 then check_safe (head pruned_matches) all_matches else (pruned_matches, False) }}} We need to change 2 functions - `insert_overlapping` and `check_safe`. ''' insert_overlapping ''' Let's look to `insert_overlapping` : http://ghc.haskell.org/trac/ghc/browser/ghc/compiler/types/InstEnv.lhs#L618 This function is fully satisfied, exept for `beats` sub-function. http://ghc.haskell.org/trac/ghc/browser/ghc/compiler/types/InstEnv.lhs#L641 {{{ (instA, _) `beats` (instB, _) = overlap_ok && isJust (tcMatchTys (mkVarSet (is_tvs instB)) (is_tys instB) (is_tys instA)) -- A beats B if A is more specific than B, -- (ie. if B can be instantiated to match A) -- and overlap is permitted }}} We cahange it to {{{ (instA, _) `beats` (instB, _) = (fromRank instA < fromRank instB) || {- We don't need to check if extension is on, all non-extension instances have a privilege - the lowest 0 rank -} overlap_ok && ... }}} We add (!) just 1 line of significant code. ''' check_safe ''' This a bit complicated. Devs, who add Overlapping instances add a guard : "We restrict code compiled in 'Safe' mode from overriding code compiled in any other mode." But we wish to allow over-rank instances through the modules. The simplest way - to forbid use Overlapped-Instances together with Ranked-Instances in one module. And add such lines from: http://ghc.haskell.org/trac/ghc/browser/ghc/compiler/types/InstEnv.lhs#L609 {{{ inSameMod b = let na = getName $ getName inst la = isInternalName na nb = getName $ getName b lb = isInternalName nb in (la && lb) || (nameModule na == nameModule nb) }}} into (this is not effective code, looks a bit hacky, but it works): {{{ inSameMod b = let na = getName $ getName inst la' = isInternalName na la = isRankedExtension || la' -- allowing Rank nb = getName $ getName b lb' = isInternalName nb lb = isRankedExtension || lb' -- allowing Rank in (la && lb) || (nameModule na == nameModule nb) }}} == Conclusion == Yes, Ranked Instances is very powerful extension! And yes, it is easy to implement! Yes, I'm confused, no one even reply at my proposal. I don't even speak about serious discussions. And yes, I regret, I have no one advocate through developers, who could add this extension less than a day in a test branch. I've showed, how to add 3 lines of significant code (sure, it is needed some routine code for support this extension.) I hope, it's become clearer. Am I missing something? Is it still overly complex in implementation? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8370#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler