[Haskell-cafe] File path programme

[Malcolm Wallace]

"Simon Marlow" <simonmar at microsoft.com> writes:

> Hmm, I deliberately didn't suggest using a type class.  Do you really
> want to expose Win32 paths and POSIX paths, and whatever else, as
> different types?  Maybe you do for extensibility, but you really want a
> single Path type too - otherwise portable code will have to use
> conditional compilation.

Good point.  Ideally, you want to be able to write all FilePath code
polymorphically at type (Path p => p), and have a decent defaulting
mechanism available to choose a particular instance at compile time
(perhaps conditionally, based on OS platform).

Sadly, in the absence of a better 'default' declaration semantics,
we are left having to resolve the ambiguity ourselves, or resort to
some other type-system extension, like as you say existentials.

> The Path type therefore has to be an
> existential wrapper:
> 
> data Path = (CPath p) => Path p
> 
> Maybe we do want to do this, but it causes portability issues with the
> Path library itself: nhc98 can't compile this code for example

Actually, nhc98 /can/ cope with existentials when written as:
  data Path = forall p . (CPath p) => Path p
but it doesn't do local universal quantification like:
  data Path = Path (forall p . p)

Regards,
    Malcolm