makeRelative package:pathtype

This function can be used to construct a relative path by removing the supplied AbsDir from the front. It is a runtime error if the supplied AbsPath doesn't start with the AbsDir.
Path.makeRelative (absDir "/tmp/somedir") (absFile "/tmp/somedir/anotherdir/file.txt") == Posix.relFile "anotherdir/file.txt"
Path.makeRelative (absDir "/tmp/somedir") (absDir "/tmp/somedir/anotherdir/dir") == Posix.relDir "anotherdir/dir"
Path.makeRelative (absDir "c:\\tmp\\somedir") (absFile "C:\\Tmp\\SomeDir\\AnotherDir\\File.txt") == Windows.relFile "AnotherDir\\File.txt"
Path.makeRelative (absDir "c:\\tmp\\somedir") (absDir "c:\\tmp\\somedir\\anotherdir\\dir") == Windows.relDir "anotherdir\\dir"
Path.makeRelative (absDir "c:tmp\\somedir") (absDir "c:tmp\\somedir\\anotherdir\\dir") == Windows.relDir "anotherdir\\dir"