Data.Ix.unindex

Motivation

The Data.Ix.Ix class has an index method, for translating array coördinates into single-dimensional indices.

https://hackage.haskell.org/package/base-4.13.0.0/docs/Data-Ix.html#v:index

-- | The position of a subscript in the subrange.
index :: (a, a) -> a -> Int

The inverse operation is also useful, but is missing. (I just went to Data.Ix looking for the inverse operation and was surprised to find it wasn't there.)

Proposal

Add this typeclass method.

-- | The subscript of a position in the subrange. The inverse operation of 'index'.
unindex :: (a, a) -> Int -> a

Further

While we're at it, the documentation for Data.Ix should also explain clearly that this module can be used for multidimensional arrays, with examples. Unless a reader examines the class instances carefully, they will likely not notice that.