Compiler version mismatch after stack init / stack build

[simonmar]

It looks like stack init picked up that I had ghc-8.0.1 installed, but then stack build got confused:

~/code/Haxl > ~/.local/bin/stack init
Looking for .cabal or package.yaml files to use to init the project.
Using cabal packages:
- haxl.cabal
- example/facebook/haxl-facebook.cabal
- example/sql/haxl-example.cabal

Selecting the best among 9 snapshots...

Populated index cache.    
* Matches lts-7.2

Selected resolver: lts-7.2
Initialising configuration using resolver: lts-7.2
Total number of user packages considered: 3
Writing configuration to file: stack.yaml
All done.
~/code/Haxl > ~/.local/bin/stack build
Compiler version mismatched, found ghc-7.10.3 (x86_64), but expected minor version match with ghc-8.0.1 (x86_64) (based on resolver setting in /home/smarlow/code/Haxl/stack.yaml).
Try running "stack setup" to install the correct GHC into /home/smarlow/.stack/programs/x86_64-linux/

[tolysz]

Looks like there is no global ghc matching ghc-8.0 prefix.
The easiest is to
~/.local/bin/stack build --install-ghc
Which will install ghc at the compatible version if is missing. It was decided that installing ghc without asking was not desired

[sjakobi]

It looks like stack init picked up that I had ghc-8.0.1 installed

No, AFAIK, stack init does not check which compilers are locally available.

[sjakobi]

It sounds like you have both ghc-7.10.3 and ghc-8.0.1 installed, but the ghc executable with version 7.10.3 comes first on your PATH.

To make stack pick up your system-installed ghc-8.0.1, I think there's currently no better way than to modify your PATH so ghc-8.0.1 comes first.

We want to improve stack's handling of multiple system ghc installations but I can't find the relevant issue right now.

[simonmar]

Oh, this is my confusion then. stack init just chose the most recent valid install plan, but it's my responsibility to ensure that I've installed a matching ghc and put it first on the PATH. When #2433 is done this will be easier, so I'll just wait for that.

Thanks for the quick response!