cabal's does not exist error message

I've seen many people confused by this error message, and just ran into it myself, so decided a blog post to explain was in order.

As I was hacking on stack this morning, I pushed and got a build failure from Travis with the following content at the end:

Failed to install asn1-encoding-0.9.0
Last 10 lines of the build log ( /home/travis/.cabal/logs/asn1-encoding-0.9.0.log ):
cabal: /home/travis/.cabal/logs/asn1-encoding-0.9.0.log: does not exist

What the error message means is: I tried to load up the log file containing the output of the build, but the file does not exist. It's possible that there are multiple reasons for this, but I know of only one: when cabal is unable to download the necessary package from Hackage, usually because Hackage is having a temporary blip.

As a user: if you see this message, just try running your command again. If it's a temporary Hackage outage, the second attempt may succeed. Another option (which I'd strongly recommend) is to switch to a more reliable package index; see the blog post on FP Complete's S3 mirror for more details. I've been pushing for a while to make this the default remote-repo used by cabal-install, but unfortunately that hasn't happened. (For the record: stack does use the S3 mirror by default.)

For cabal: I see three steps worth taking:

1. Fix this confusing error message to say something like "Download of http://... failed". I thought there was a cabal issue opened about this already, which is why I didn't open up a new one right now. If there isn't one, it's worth opening.
2. Automatically retry a failing download. I'm not convinced this is a good thing to do, but it's a possible mitigation. (We don't do that right now in stack, though I've been toying with the idea. Feedback appreciated.)
3. Switch to a more reliable mirror for the default remote-repo.

Fixing this instability at the Hackage level would of course also be a good move.