Why does Stack pick ghc 7.10 even with an lts for ghc8?

Question

I created a new project and, as you can see, it even downloaded a brand new build plan for lts-7.4 , which as you can see uses ghc-8.0.1.

But when running stack ghci , it built the project with GHC 7.10, and dropped me in a 7.10 ghci shell.

dario@curie /tmp> stack new foo
Downloading template "new-template" to create project "foo" in foo/ ...
Looking for .cabal or package.yaml files to use to init the project.
Using cabal packages:
- foo/foo.cabal

Selecting the best among 9 snapshots...

Downloaded lts-7.4 build plan.    
* Matches lts-7.4

Selected resolver: lts-7.4
Initialising configuration using resolver: lts-7.4
Total number of user packages considered: 1
Writing configuration to file: foo/stack.yaml
All done.
dario@curie /tmp> cd foo
dario@curie /t/foo> stack ghci
foo-0.1.0.0: configure
Configuring foo-0.1.0.0...
foo-0.1.0.0: build
Preprocessing library foo-0.1.0.0...
[1 of 1] Compiling Lib              ( src/Lib.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/Lib.o )
In-place registering foo-0.1.0.0...
Preprocessing executable 'foo-exe' for foo-0.1.0.0...
[1 of 1] Compiling Main             ( app/Main.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/foo-exe/foo-exe-tmp/Main.o )
Linking .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/foo-exe/foo-exe ...
foo-0.1.0.0: copy/register
Installing library in
/tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/lib/x86_64-linux-ghc-7.10.3/foo-0.1.0.0-6bylsnNRJPuHxByS3dKqs5
Installing executable(s) in
/tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/bin
Registering foo-0.1.0.0...
The following GHC options are incompatible with GHCi and have not been passed to it: -threaded
Using main module: 1. Package `foo' component exe:foo-exe with main-is file: /tmp/foo/app/Main.hs
Configuring GHCi with the following packages: foo
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Lib              ( /tmp/foo/src/Lib.hs, interpreted )
Ok, modules loaded: Lib.
[2 of 2] Compiling Main             ( /tmp/foo/app/Main.hs, interpreted )
Ok, modules loaded: Lib, Main.
> 

I always relied on the assumption that for every stackage snapshot, stack would use only one version of GHC, am I wrong?

I don't have system-ghc: true in my config, and the faq doesn't mention many other alternatives

This is the output of stack path :

dario@curie /t/foo> stack path
stack-root: /home/dario/.stack
project-root: /tmp/foo
config-location: /tmp/foo/stack.yaml
bin-path:     /home/dario/.stack/snapshots/x86_64-linux/lts-7.4/7.10.3/bin:/home/dario/.stack/programs/x86_64-linux/ghc-8.0.1/bin:/home/dario/Applications/bin:/home/dario/.rbenv/shims:/home/dario/.cabal/bin:/home/dario/.nix-profile/bin:/home/dario/.nix-profile/sbin:/home/dario/.sdkman/candidates/grails/current/bi    n:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/lib/jvm/java-8-oracle/bin:/usr/lib/jvm/java-8-oracle/db/bin:/usr/lib/jvm/java-8-oracle/jre/bin:/home/dario/.local/bin:/home/dario/.nix-profile/bin
programs: /home/dario/.stack/programs/x86_64-linux
compiler-exe: /usr/bin/ghc
compiler-bin: /usr/bin
local-bin: /home/dario/.local/bin
extra-include-dirs: 
extra-library-dirs: 
snapshot-pkg-db: /home/dario/.stack/snapshots/x86_64-linux/lts-7.4/7.10.3/pkgdb
local-pkg-db: /tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/pkgdb
global-pkg-db: /var/lib/ghc/package.conf.d
ghc-package-path: /tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/pkgdb:/home/dario/.stack/snapshots/x86_64-linux/lts-7.4/7.10.3/pkgdb:/var/lib/ghc/package.conf.d
snapshot-install-root: /home/dario/.stack/snapshots/x86_64-linux/lts-7.4/7.10.3
local-install-root: /tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3
snapshot-doc-root: /home/dario/.stack/snapshots/x86_64-linux/lts-7.4/7.10.3/doc
local-doc-root: /tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/doc
dist-dir: .stack-work/dist/x86_64-linux/Cabal-1.22.5.0
local-hpc-root: /tmp/foo/.stack-work/install/x86_64-linux/lts-7.4/7.10.3/hpc
local-bin-path: /home/dario/.local/bin
ghc-paths: /home/dario/.stack/programs/x86_64-linux
dario@curie /t/foo> echo $PATH
/home/dario/Applications/bin /home/dario/.rbenv/shims /home/dario/.cabal/bin /home/dario/.nix-profile/bin /home/dario/.nix-profile/sbin /home/dario/.sdkman/candidates/grails/current/bin /usr/local/sbin /usr/local/bin /usr/sbin /usr/bin /sbin /bin /usr/games /usr/local/games /snap/bin     /usr/lib/jvm/java-8-oracle/bin /usr/lib/jvm/java-8-oracle/db/bin /usr/lib/jvm/java-8-oracle/jre/bin /home/dario/.local/bin /home/dario/.nix-profile/bin

I rely on the lts determining the version of ghc in use also for CI, so it's a bit worrying if my assumption doesn't hold (anymore?)

(I'm running stack Version 1.2.0 x86_64 hpack-0.14.0 )

This is the output of stack setup

dario@curie /t/foo> stack setup
stack will use a sandboxed GHC it installed
For more information on paths, see 'stack path' and 'stack exec env'
To use this GHC and packages outside of a project, consider using:
stack ghc, stack ghci, stack runghc, or stack exec

Answer 1

I realized that this was due to the faulty way in which I "worked around" this issue:

https://github.com/commercialhaskell/stack/issues/2712

I might open it as another bug report, but it's definitely less important/interesting than fixing #2712