Add support for cabal sandbox to syntastic. Write this in your .vimrc.

gistfile1.vim

function! s:get_cabal_sandbox()
  if filereadable('cabal.sandbox.config')
    let l:output = system('cat cabal.sandbox.config | grep local-repo')
    let l:dir = matchstr(substitute(l:output, '\n', ' ', 'g'), 'local-repo: \zs\S\+\ze\/packages')
    return '-s ' . l:dir
  else
    return ''
  endif
endfunction

let g:syntastic_haskell_ghc_mod_args = s:get_cabal_sandbox()