normalise should not alter paths that start with \\?\

[Rufflewind]

From https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx#maxpath :

They indicate that the path should be passed to the system with minimal modification, which means that you cannot use forward slashes to represent path separators, or a period to represent the current directory, or double dots to represent the parent directory.

My take from this is that whenever \\?\ is at the beginning of a path, then everything afterward must interpreted literally. Specifically:

• / should be understood as a normal character, not a path separator
• . and .. should be understood as normal path segments, not special directories

This means whenever normalise sees a path that starts with \\?\, it should basically do nothing at all.

As of filepath-1.4.1.0, normalise "\\\\?\\C:\\.\\Windows/" == "\\\\?\\C:\\Windows\\", which is incorrect according to MSDN. Instead, it should remain as is.

[ndmitchell]

If we do that, can things like takeExtension work? Since they are implicitly playing around with knowledge of the structure of \\?\ paths.

I am inclined to agree that treating \\?\ specially for normalise sounds like a good idea.

[Rufflewind]

If we do that, can things like takeExtension work? Since they are implicitly playing around with knowledge of the structure of \\?\ paths.

I guess the problem runs deeper than just normalise. For example, the various split functions likely have the same problem.

[ndmitchell]

Yep. I think we have to do what is pragmatic, rather than necessarily semantically precise. My inclination is normalise does nothing, everything else works as is.

[hasufell]

Another oddity I discovered:

*System.FilePath W> W.normalise "D:file"
"D:file"

*System.FilePath W> W.normalise "\\\\?\\D:file"
"\\\\?\\D:\\file"

In the second case it actually changes semantics.

[ndmitchell]

As I got into the details of normalise I found out it was hilariously slow and had about a million really crazy corner cases. I rewrote it for Shake as normaliseEx - https://hackage.haskell.org/package/shake-0.19.6/docs/Development-Shake-FilePath.html#v:normaliseEx. My intention was to replace this one wholesale with that, including the test suite for normaliseEx which is in Shake.