#lang racket ; CSC324 — 2022W — Assignment 1 — Eve — Design and Testing
; • Eve: Eager By-Value Environmental Evaluator for an Extended Lambda Calculus
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; Task: understand the syntax and semantics of the language LCE described below,
; then create a good test suite for an evaluator eve of LCE which you will then
; implement in A1.eve.rkt
(require “A1.eve.rkt”
; We’ll refer to the language being interpreted in this part as “LCE”.
; · Terms in LCE
; LCE has the same terms as LCA, with the same representation.
; · Values, Closures, and Environments in LCE
; A closure pairs a (not necessarily closed) λ term with an environment that
; contains (at least) bindings for the open variables in the λ term.
; An environment pairs a local binding of a variable and value with a parent
; environment.
; For concision, closures and environments will be kept in a hash table to be
; referenced by symbolic indices of the form “λn” and “En”.
; So a closure is represented by a two-element list containing a λ term and an
; environment index.
; And an environment is represented by a two-element list containing a
; two-element list of variable and value along with an environment index.
; A value is then one of:
; closure index
; literal term
; · Semantics of LCE
; Evaluation now takes both a term and the index of an environment that contains
; (at least) bindings for the open variables in the term.
; Evaluation of a value is still just the value itself.
; Evaluation of a variable is its most local value starting from the environment
; and continuing up the parent chain.
; Evaluation Assumption: the variable has a binding in the chain.
; Evaluation of function call is still eager and by value, but with argument
; passed by environment.
; More precisely, evaluation of (
; 1. Evaluate
; Evaluation Assumption: the evaluation is not an infinite recursion
; and the result is the index of a closure with λ term (λ (
; and environment E.
; 2. Evaluate
; Evaluation Assumption: the evaluation is not an infinite recursion.
; 3. Produce the evaluation of in a new environment extending E with
; a local binding of
; Evaluation of a λ term is the index of a new closure containing the term and
; environment index.
; · Design and Testing
; Create a good test suite for eve, with a comment above each test case giving
; a rationale for its inclusion.
; Also include test cases for any significant helper functions you create
; (e.g. a recursive function is likely to be significant) to aid debugging, and
; export those helper functions from A1.eve.rkt so they can be referenced here.
; Illustrating indexer.
(define s (indexer ‘p))
; A result of indexer produces successive indices starting at 0.
(check-equal? (s) ‘p0)
(check-equal? (s) ‘p1)
(check-equal? (s) ‘p2)
; Warning: the result is not referentially transparent!
(check-not-equal? (s) (s))
; Illustrative example for the form of an environment-closure table.
; The test also illustrates the literal notation for (immutable) hash tables.
; In A1.eve.rkt you update a mutable hash table, but it’s converted to immutable
; just before return (an immutable hash table and a mutable hash table are
; always considered unequal).
#;(check-equal? (eve ‘(((λ (x) (λ (y) 1)) 20) 300))
‘(1 #hash((E1 . ((x 20) E0))
(E2 . ((y 300) E1))
(λ0 . ((λ (x) (λ (y) 1)) E0))
(λ1 . ((λ (y) 1) E1)))))
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