[Haskell-beginners] best way to proceed with package install

Chaddaï Fouché chaddai.fouche at gmail.com
Tue Oct 30 10:33:45 CET 2012

On Tue, Oct 30, 2012 at 1:00 AM, Miguel Negrao
<miguel.negrao-lists at friendlyvirus.org> wrote:
> A 29/10/2012, às 23:10, Michael Litchard escreveu:
>> I am trying to install updated version of yesod-platform, and got this
>> output from "cabal install yesod-platform". Could someone please
>> advise on the most efficient way to proceed?
> I get this sort of errors all the time and never seem to understand how to fix them. Is there some place I can read on how to understand these errors, and understand what to do about them, instead of feeling totally at loss and coming every time to the list to ask for help ? Also, why can the reinstalls mess up things ?

There is some good explanations of that phenomenon around under the
name "Butterfly effect" like :

This is really painful when you start to have accumulated a number of
packages, and the more dependencies, the more likely you are to hit
Which is why most people now recommend using sandboxes for
installations, to allows you to have several different packages
database and not have to scratch your whole GHC installation once
you've damaged it (there are always alternative to this radical step,
but it takes more time than restarting from zero so nobody use them).
Basically, as soon as you hit a problem with a cabal installation that
would require a reinstall, you restart from a new sandbox. The
solutions to do that are virthualenv, capri or cabal-dev and the next
release of cabal itself should have a sandbox mode.

The proper solution would be to allow several version of the same
package version to cohabit but that's pretty complicated and not yet


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