Proposal: System.FilePath: current directory should be ".", not ""

wren ng thornton wren at
Mon Nov 16 03:20:38 EST 2009

Ben Moseley wrote:
> Duncan Coutts wrote:
>> My intuition with file paths in Cabal, is that this distinction would
>> catch more bugs. Ideally cabal's building code would be independent of
>> the current directory, but the fact that relative paths get
>> automagically completed to being relative to the current directory means
>> that it's very easy to break this property.
> [...]
>> There are various other distinctions one could try to make, the question
>> becomes which type distinctions are useful and when does it just become
>> too much.
> I think this is a very important point. It's definitely a balancing 
> act. I have
> ondered whether even capturing Abs/Rel was overkill, but my current 
> feeling
> (not yet based on much real-world experience) is that it is worth it.

Having done a lot of system administration and metaprogramming of system
administration, I can certainly say that making the relative/absolute
distinction is helpful for tracking down bugs. From the same set of
experience I'd say that Duncan's incomplete/complete distinction would
be even more helpful.

With a lot of sysadmin metaprogramming (which includes package
installers) what people are really working on is grafting different
trees together. It's important to distinguish which paths/trees are
being created and don't yet refer to an actual or potential file on the
disk, from those paths/trees which refer to actual/potential files on
the disk.

To think about why this distinction is helpful, consider where all we
may want to attach the paths/trees we're creating. The two obvious
places are / and . but there are other options too. Most programs
support some notion of a $PATH variable or allow flags to specify the
target directory for reading or writing. With the complete/incomplete
distinction we distinguish names which aren't yet grounded from names
that refer to files, regardless of where they're mounted. With the
relative/absolute distinction we don't know when a path is finished
being constructed, and we also have no real vocabulary for expressing
paths relative from some point other than the current directory.

Most sysadmins don't seem to realize they're doing tree grafting when
they mess with this stuff, so I can see why it might seem foreign, but
viewing it that way really does help clean code up and find bugs. Tree
grafting also forms a monad, which could be helpful for some folks.

Live well,

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