Documentation for Ix is contradictory around minimal definition

[goldfire]

The Haddock documentation for Ix ​here says that the minimal definition requires range, index, and inRange. But then it says that the minimal definition requires range and inRange. It seems that Haddock is inferring a MINIMAL definition where none is in the code. Perhaps a MINIMAL should be added.

[hvr]

so while range and inRange have no default implementation, index has one that is recursive:

        -- Must specify one of index, unsafeIndex

        -- 'index' is typically over-ridden in instances, with essentially
        -- the same code, but using indexError instead of hopelessIndexError
        -- Reason: we have 'Show' at the instances
    {-# INLINE index #-}  -- See Note [Inlining index]
    index b i | inRange b i = unsafeIndex b i
              | otherwise   = hopelessIndexError

    unsafeIndex b i = index b i

So the minimal pragma should specify that we minimally need

range && inRange && (index || unsafeIndex)

I'll make a patch rightaway...

[Herbert Valerio Riedel <hvr@…>]

In 7a2d65a4d93273c89fbb1d19e282d5933c67c7ca/ghc:

Define proper `MINIMAL` pragma for `class Ix`

Summary: This addresses #10142

Reviewers: goldfire, austin, ekmett

Reviewed By: austin, ekmett

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D709

GHC Trac Issues: #10142

[hvr]

landed to ghc-7.10 via 0d586136ce995f725bcaae064c98298d391ba178