Cabal and simultaneous installations of the same package

Simon Peyton Jones simonpj at
Thu May 7 20:34:05 UTC 2015

Dear Cabal developers
I guess everyone is busy, but I feel a bit stuck on knowing how to make progress on this thread.

From: Simon Peyton Jones
Sent: 20 April 2015 09:12
To: Simon Peyton Jones; cabal-devel at
Cc: haskell-infrastructure at; Haskell Libraries; ghc-devs at
Subject: RE: Cabal and simultaneous installations of the same package


We started this thread (below) a month ago, but it has once more run out of steam.  The last contribution was from Vishal Agrawal

I am already planning to do a GSoC project based on it with a slightly larger aim. You can find my work in progress proposal at Also I have written a patch to make cabal non-destructive at Can you review the proposal and give me suggestions.

I don't feel qualified to drive this process, but I do think it's important to complete it.  (I might be wrong about this too... please say so if so.) Nor do I understand why it's difficult to tie up the bow; the underlying infrastructure work is done.

Duncan especially: how can we make progress?  Do you think it's important to make progress, or are other things in cabal-land more important?  My reason for thinking that it's important is that it appears to be the root cause of many people's difficulties with Haskell and Cabal.  It might not be a panacea for all ills; but it might be a cheap remedy for a significant proportion of ills.  And that would be a Good Thing.



|  -----Original Message-----

|  From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Simon

|  Peyton Jones

|  Sent: 23 March 2015 08:46

|  To: cabal-devel at<mailto:cabal-devel at>

|  Cc: haskell-platform at<mailto:haskell-platform at>; haskell-

|  infrastructure at<mailto:infrastructure at>; Haskell Libraries; ghc-

|  devs at<mailto:devs at>

|  Subject: Cabal and simultaneous installations of the same package


|  Dear Cabal developers


|  You'll probably have seen the thread about the Haskell Platform.


|  Among other things, this point arose:


|  |  Another thing we should fix is the (now false) impression that HP

|  | gets in  the way of installing other packages and versions due to

|  cabal hell.


|  People mean different things by "cabal hell", but the inability to

|     simultaneously install multiple versions of the same package,

|     compiled against different dependencies is certainly one of them,

|  and I think it is the one that Yitzchak is referring to here.


|  But some time now GHC has allowed multiple versions of the same package

|  (compiled against different dependencies) to be installed

|  simultaneously.  So all we need to do is to fix Cabal to allow it too,

|  and thereby kill of a huge class of cabal-hell problems at one blow.


|  But time has passed and it hasn't happened. Is this because I'm

|  misunderstanding?  Or because it is harder than I think?  Or because

|  there are much bigger problems?  Or because there is insufficient

|  effort available?  Or what?


|  Unless I'm way off beam, this "multiple installations of the same

|  package" thing has been a huge pain forever, and the solution is within

|  our grasp.  What's stopping us grasping it?


|  Simon


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|  ghc-devs at<mailto:ghc-devs at>

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