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Wed Jan 27 07:38:13 CET 2010

Closure conversion

Simple lambda lifting turns first class functions into first order
functions by adding extra parameters at the definition side, and
providing extra parameters at the call site.

  (lambda (x y)
    (let ((f (lambda (a b)
               (+ (* a x) (* b y)))))
      (f 1 2)))
=>

  (define f (lambda (a b x y)
              (+ (* a x) (* b y))))
  (lambda (a b x y)
    (f 1 2 x y))

This works only for downward closures; not for escaping closures.

Closure conversion turns each function into a first order function by
adding an extra argument: a vector of free variables.  Function
objects are then represented as function body + free variable vector.

This is similar to "Display Closures" idea by Dybvig.  However, there
seems to be some difference as the latter needs assignment conversion.
Or maybe [1] does too?  It looks like it.  It is not mentioned how to
create the vector of free variable values in the first place.


[1] http://www.iro.umontreal.ca/~boucherd/mslug/meetings/20041020/90-min-scc/90-min-scc.pdf



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