How do I determine my GHC version?

ghc --version did the trick on Ubuntu, thanks Karolis!

Edit: works on mac too (macOS Sierra 10.12.6)

To find which version my stack is using, I ran

$ stack ghc -- --version

and got

$ stack ghc -- --version
The Glorious Glasgow Haskell Compilation System, version 8.0.2

Depending on your install method and architecture, some commands may differ. Some people may have several versions installed. Here is a selection of information that may be useful:

stack version

❯ stack --version
Version 2.1.3, Git revision 0fa51b9925decd937e4a993ad90cb686f88fa282 (7739 commits) x86_64 hpack-0.31.2

ghc version (as noted by @mherzl)

❯ stack ghc -- --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5

standard ghc version command

❯ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.8.1

cabal version

❯ cabal --version
cabal-install version 3.0.0.0
compiled using version 3.0.0.0 of the Cabal library

GHC's repl

❯ ghci --version
The Glorious Glasgow Haskell Compilation System, version 8.8.1

If using llvm with -fllvm flag

❯ llvm-gcc -v
Apple clang version 11.0.0 (clang-1100.0.33.16)
Target: x86_64-apple-darwin19.2.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin