Cabal spelunking considered tricky

Duncan Coutts duncan.coutts at
Sun Apr 20 17:48:14 EDT 2008

On Sun, 2008-04-20 at 13:59 -0700, Bryan O'Sullivan wrote:
> As I've been maintaining cabal-rpm to try to keep up with developments
> in Cabal, I've found myself wondering why it's not using many of the
> usual tricks to hide the plumbing, such as a custom monad transformer stack.

One such change I'd like to make is to lift Cabal off the IO monad.
There will always have to be IO somewhere down at the bottom but it'll
help lots of things to have a layer in between. In particular it'll help
us to regularise quite a lot of code. If you're in IO you can do
anything and so we often do. A more restricted action monad would help
us to better separate deciding what to do from actually doing it. It'd
also reduce the number of ways of doing the same thing. I'd also like to
separate IO actions that have mutative side effects from IO actions that
merely query the system environment. Another useful thing would be to
introspect IO actions to dynamically enforce policies, like never
writing files outside of ./dist/. Such policies don't have to be global:

actionWatcher checkFileWrites $ do ...

Another candidate to monadify is logging. At the moment we have to
remember to do logging consistently and we do it immediately to stderr.
Callers like IDEs would like to get logging output in a more convenient


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