Ranked Instances

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

  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

  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

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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