mapPos provides wrong indices on submatrices

[jk977]

Overview

Using mapPos on submatrices does not generate the expected indices. I'm fairly sure this occurs because the function doesn't factor in the column offset when calling decode.

Versions

• matrix v0.3.6.1
• GHC v8.4.4

Example

The following gives unexpected results:

import qualified Data.Matrix as M
m = M.identity 3
sm = M.submatrix 2 3 1 2 m
unexpected = M.mapPos const sm

This assigns the following to unexpected:

┌             ┐
│ (2,2) (3,1) │
│ (4,1) (4,2) │
└             ┘

This could have one of two possible expected results. The first is to have the upper-left element be (1,1):

┌             ┐
│ (1,1) (1,2) │
│ (2,1) (2,2) │
└             ┘

The second is to have the upper-left element be (2,1), consistent with the submatrix offset:

┌             ┐
│ (2,1) (2,2) │
│ (3,1) (3,2) │
└             ┘

In my opinion, the first option provides more consistent results. It would prevent divergent behavior when, for example, calling mapPos on a 2x2 submatrix vs. a 2x2 matrix with no internal offsets.

[jk977]

Quick update: if the first option was chosen, I'm thinking the matrix would need to be forced before mapping, unless the callback was changed from type (Int, Int) -> a -> b to (Int, Int) -> a -> a. The former would sacrifice performance, but the latter is a breaking change to the API and would sacrifice flexibility. Although forcing would cause the function to take a performance hit, it could be mitigated slightly by checking if ncols is equal to vcols before forcing. The matrix is not a submatrix if they're equal, so no forcing would be necessary.