The stack eval command

stack eval CODE [--[no-]ghc-package-path] [--[no-]stack-exe]
           [--package PACKAGE] [--rts-options RTSFLAG] [--cwd DIR]

GHC has an expression-evaluation mode, set by passing the GHC option -e <expression>. Commanding stack eval <code> is equivalent to commanding:

stack exec ghc -- -e <code>

For further information, see the stack exec command documentation.