perform a sequence of state-changing ``error triples''
Major Section:  PROGRAMMING

(er-progn (check-good-foo-p (f-get-global 'my-foo state) state)
          (value (* (f-get-global 'my-foo state)
                    (f-get-global 'bar state))))

This sequencing primitive is only useful when programming with state, something that very few users will probably want to do. See state.

Er-progn is used much the way that progn is used in Common Lisp, except that it expects each form within it to evaluate to an ``error triple'' of the form (mv erp val state). The first such form, if any, that evaluates to such a triple where erp is not nil yields the error triple returned by the er-progn. If there is no such form, then the last form returns the value of the er-progn form.

We intend to write more about this topic, especially if there are requests to do so.