Stop using system-ghc silently…or at all.

[bitemyapp]

This happens a fair bit, and not just in connection with ghc-mod although it's the most common culprit. While doing the training at LambdaConf we had a beginner get very confused because their system installed GHC was pre-FTP and the Stack provided GHC was 7.10.3. From there, they didn't understand ghci and stack ghci would be from substantially different versions of GHC.

I'd really like Stack to stop implicitly using system-installed versions of GHC without explicit direction from the user.

[mgsloan]

Agreed, I'm tired of asking whether or not the user is using a custom ghc install.

This inspired me to consider another source of reproduction difficulty is whether stack was built via stack or cabal - I've opened this ticket on that.

[sjakobi]

For context, @mgsloan's comment from #1901:

Some discussions of why the system ghc is preferred:

• https://groups.google.com/forum/#!topic/haskell-stack/-XaXiq5Wdo8
• Change default to not use system-ghc unless otherwise specified. #1129

I'm not sure if I buy it, I'm leaning towards making "system ghc" usage opt-in, but this would be a breaking change...

[bitemyapp]

I got bit by system-ghc being the default last week when I tried to switch to profiling libraries. Remembered I might accidentally be using system-ghc which was causing the dependency failures. Lost a half hour or more to that and I know Stack pretty well. Imagine it'd be a hard stop for a new person trying to profile their project.

[sjakobi]

Ok, so I'm totally in favor of switching to --no-system-ghc by default in one of the next few releases.

I don't have a good idea of the breakage that could result from changing the default behaviour, and currently use stack-provided compilers exclusively, so I'd like to do the switch with sufficient time before a release, so it can be beta-tested thoroughly.

This means that the switch can only happen once the store version-hash issue (#2296) has been resolved because I'd feel really bad about all the beta-testers wasting their time while their extra-deps compile…

@mgsloan: Any idea when #2296 might be resolved?

[sjakobi]

This FAQ entry will need to be edited once the switch is done:

https://docs.haskellstack.org/en/stable/faq/#i-already-have-ghc-installed-can-i-still-use-stack

[shanemikel]

First attempt at profiling Haskell today. I've been digging around for hours now, and I've finally got something to compile (with --no-system-ghc). Until this is fixed, would somebody mind adding a note to the docs section on profiling?

[sjakobi]

First attempt at profiling Haskell today. I've been digging around for hours now, and I've finally got something to compile (with --no-system-ghc).

@shanemikel: What were the symptoms of using the wrong GHC? Feel free to make a PR (targeting the stable branch) for that note!

[bitemyapp]

Just to add to this, I just got bitten by profiling <> system-ghc again