[Haskell-cafe] Cabal dependencies

[Yuras Shumovich]

On Sat, 2012-10-06 at 18:25 +0200, José Lopes wrote:
> OK.
> 
> But, wouldn't it be possible for xmobar to use mtl-2.0.1.0 and for 
> parsec to use mtl-2.1.1, while xmobar would use this parsec version?
> In this case, I am assuming that mtl-2.0.1.0 and mtl-2.1.1 are 
> considered two different libraries.

Usually it leads to "strange" compilation errors.

E.g.
Package A:
  data AA = AA String
  func0 :: Int -> AA
  func0 n = AA $ replicate n "A"
  func1 :: AA -> Int
  func1 (AA str) = length str

Package B:
  import A
  func2 :: AA -> Int
  func2 aa = func1 + 1

Package C:

  import A
  import B
  func3 :: Int -> Int
  func3 n = func2 $ func0 n

If C and B are compiled with different versions of C,
then func3 will not compile. Compiler will say that
AA returned by func0 doesn't match AA expected by func2

More real examples:
http://stackoverflow.com/questions/11068272/acid-state-monadstate-instance-for-update
http://stackoverflow.com/questions/12576817/couldnt-match-expected-type-with-actual-type-error-when-using-codec-bmp/12577025#12577025


> 
> Thanks,
> José