How do I append dynamic to the Haskell options in vim's configuration files

[patenteng]

Haskell requires the -dynamic option to compile on my system. So I want to append -dynamic to g:ale_haskell_ghc_options. I tried simply adding

au FileType haskell let g:ale_haskell_ghc_options = g:ale_haskell_ghc_options . '-dynamic'

in .vim/after/plugin, but it complains that g:ale_haskell_ghc_options is not defined. If I do

au FileType haskell let g:ale_haskell_ghc_options = '-dynamic'

it overrides the default options -fno-code and -v0.

I can add the above options, but I want my g:ale_haskell_ghc_options to change to <defaults> -dynamic when ALE is updated. Any suggestions?