:! echo module A where {} >A.hs
:! echo module B where { import A } >B.hs

:load B

-- We're going to replace B.hs with an invalid module but without
-- changing its timestamp.  A :reload should *not* look at the
-- contents of the file, because the timestamp hasn't changed.
:! cp B.hs B.hs-copy
:! touch -r B.hs B.hs-copy
:! echo "*** INVALID ***" >B.hs
:! touch -r B.hs-copy B.hs

:reload

-- Put the original file back, now it should work
:! cp B.hs-copy B.hs
:reload