Type Mismatch trying to make a session modification function reusable

This compiles:

.exec( _.set( “foo”, “bar” ) )

This does not:

def foobar = ( Session : Session ) = _.set( “foo”, “bar” )

.exec( foobar )

It is telling me that foobar is of the wrong type. It says it expects one of ActionBuilder, Execs[], Iterable[Execs[]], or session.Expression[Session].

Exactly what am I doing wrong? Am I missing an import, maybe?

This is 3.2.1