# [plt-scheme] Re: Expressing closures in PLT Redex

 From: David Van Horn (dvanhorn at cs.brandeis.edu) Date: Fri Jul 21 13:45:44 EDT 2006 Previous message: [plt-scheme] Expressing closures in PLT Redex Next message: [plt-scheme] errata HTDP 23.1 Messages sorted by: [date] [thread] [subject] [author]

```Arjun Guha wrote:
> Say I have a language where closures are expressed as
> environment-abstraction pairs:

Hello Arjun,

This doesn't address your question directly, but may shed some light on
the issues.

Below is a PLT Redex formulation of the applicative order reduction
strategy for the "lambda rho hat" calculus, an extension of Curien's
calculus of closures, lambda rho, that allows for one-step reduction to
be expressed.

The reference for this work is Section 2.2 and 2.4 of A Concrete
Framework for Environment Machines, Magorzata Biernacka and Olivier Danvy.

http://www.brics.dk/RS/06/3/

(Note that this uses de Bruijn indices-- variables are represented by
natural numbers and a substitution (an environment) is a list of values.
A variable's number gives the index into the list for value to which
it is bound.  Environment extension is just cons.)

This is my first PLT Redex program, so caveat emptor, etc.

David

(define lang
(language

;; Evaluation contexts
(e hole
(e c)
(v e))

;; Substitutions
(s (v ...))

;; Terms
(t number
(t t)
(lambda t))

;; Closures
(c (t s)
(c c))

;; Values
(v ((lambda t) s))))

(define reductions
(list
(reduction/context/name
"Beta" lang e
(((lambda t_0) s_0) v_0)
(term (t_0 (v_0 . s_0))))

(reduction/context/name
"App" lang e
((t_0 t_1) s_0)
(term ((t_0 s_0) (t_1 s_0))))

(reduction/context/name
"Var" lang e
(number_0 s_0)
(term-let ((c_0 (list-ref (term s_0) (term number_0))))
(term c_0)))))

```

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