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Mon Sep 14 11:27:48 CEST 2009

Delimited Continuations and Staging

Avoiding monads using delimited continuations[1][2].

In the latter, ``scope extrusion'' (the possibility of bringing
variables outside of their scope using assignments) is illustrated
using the following MetaOCaml example:

      let r = ref .<1>. in
          .<fun y -> .~(r := .<y>.; .<()>.)>.; !r

      => .<y_1>.

Inside the escape .~( ) a code value is assigned to the reference r
and referenced outside of the quotation .< >. to be returned as the
value of the whole expression.  This code value is ill-formed: the
variable y_1 (y renamed) is no longer bound.

Arbitrary shift/reset will give similar problems: variables can be
transported outside of their scope.  The paper suggests an approach
where shift/reset is still used, but escapes are limited up to the
binding site.

So the thing to figure out is how this translates to scheme: is it
worth limiting control effects (since we have no typing, but do want
to have correct scoping).



[1] http://lambda-the-ultimate.org/node/3112
[2] http://okmij.org/ftp/Computation/staging/circle-shift.pdf



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