A grammar for effects (~session types)

[pygy]

It would be nice to be able to define a grammar for effects using a PEG/CFG-like DSL.

You'd have to track divergence (mark off unbounded loops perhaps except where users mark it as manually vetted).

[effect HttpResponse
  [fn status [code]]
  [fn header [name value]]
  [fn body [content[[
  [constrain
    [sequence
      status
      [zeroPlus header]
      [maybe body]]]]

It may also be nice to be able to prohibit some effects in portions of the code, with a compile-time check (shadowing handlers).