RFC: Extended version-range descriptions for Cabal

[Thomas Schilling]

Hi librarians,

I'd like to propose a an extension to Cabal's dependency description
syntax.  Cabal currently supports version range descriptions using the
unary operators >, >=, <, <=, ==, and binary operators && and ||.  This
can lead to rather awkward descriptions of common version ranges, for
example,

  base >= 1 && < 2 

and could lead to subtle errors like

  base > 1 && < 2

or

  base >= 1 && <= 2


I therefore propose two extensions.  Both are just syntactic sugar.

(1) Wildcards:

  1.*   ~~>   >= 1 && < 2

in general

  m.n.o.*   ~~>    >= m.n.o && < m.n.(o+1) 

Note, that the translation is not ">= 1.0 && < 2", because that would
exclude version "1", as [1] < [1,0] == True.


(2) Ranges:

  1 - 2.1   ~~>    >= 1 && <= 2.1

in general

  m - n   ~~>   >= m && <= n

If the upper limit is supposed to be exclusive, this can be combined
with (1):

  1 - 2.0.*  ~~>  >= 1 && < 2.1



A possible alternatve syntax (subsuming both (1) and (2)) could be:

  [1, 2.1]   ~~>   >= 1 && <= 2.1 
  [1, 2.1)   ~~>   >= 1 && < 2.1  

Although this syntax is closer to mathematical notation, I prefer the
former for it is more explicit and more similar to common conventions
used in other languages/package systems (an less scary scary to
non-math-aficiados).

I'd like include this into Cabal-1.2 which is scheduled with the next
ghc release, so it can fearlessly be used when getting packages ready
for GC 6.8.1.

Please comment/vote.

Thanks,

/ Thomas