Ian Lynagh wrote:
On Tue, Dec 11, 2007 at 02:22:14PM +0000, Simon Marlow wrote:
Ian Lynagh wrote:
The two answers above together mean that the property
\x y -> let x' = normalise x y' = normalise y z = x' y' in z == normalise z'
doesn't hold (for x = ".", y = "foo"), but it is one I would expect to hold. I think you have an extra ' on the last line of the code above.
Yes, thanks.
But anyway, why do you expect that to hold?
It just seems obvious to me that operators on a datatype would maintain normalisedness of that type. If it's not obvious to you then I'm not sure I can explain why; it's like explaining to someone why you find something beautiful.
The only reason *not* to make "." p == p is that in some contexts (e.g. rawSystem) "foo" is not the same as "./foo". You could argue that rawSystem is therefore not taking a proper FilePath as an argument, it is taking something else, and we should be more explicit about it, data ExecutableSpec = FindOnPath String | Relative FilePath this certainly has some merit. However, if we don't change the API of rawSystem, then having "." p == p would make it somewhat harder to construct the argument to rawSystem, you would have to do it manually: "." ++ pathSeparator : "foo". I guess that seems reasonable, given that rawSystem is really the exception. Cheers, Simon