On this page:
1 Interpreter
1.1 Additional Preliminaries
1.2 Problem Description

Problem Set 09

Last updated: Fri, 20 Mar 2015 13:25:14 -0400

OUT: Monday 3/16/2015, 9pm EST
DUE: Monday 3/23/2015, 9pm EST

1 Interpreter
1.1 Additional Preliminaries

Save your solutions for this problem to a file named eval.rkt.
Run the following expression (you must have required extras.rkt) to check that your file is properly named and is in the proper directory:

(check-location "09" "eval.rkt")

Add these additional provides at the top of your file (below the requires), so that we can test your solution:
(provide eval)
(provide lambda?)
(provide errstr?)
(provide subst)
(provide expr->expr/no-var)
(provide expr=?)

1.2 Problem Description

Although you are having a great time programming with Racket, you’ve decided to create your own programming language, called PDPLang . Your task for this assignment is to implement an interpreter for this language.
Note: Though this assignment is self-contained, you may find it helpful by reviewing Chapter 24 of the textbook.
Specifically, design the following function:
; eval : Program -> ListOf<Result>
; Evaluates a PDPLang program to a list of Results.
; Specifically, evaluates the Exprs in p, in the context of the given Defs.
; WHERE: A function may be called before it is defined.
; WHERE: The produced results are in the same relative order as their
; originating Exprs in p.
(define (eval p) ...)
To help us test, please also implement and provide the following predicates:
; lambda? : Expr -> Boolean
; Returns true if e is a PDPLang lambda expression.
(define (lambda? e) ...)
; errstr? : Expr -> Boolean
; Returns true if e is a PDPLang ErrString expression.
(define (errstr? e) ...)
Assume that PDPLang programs (i.e., information) have already been parsed to the following data representations.
; A UniqueListOf<X> is a ListOf<X>
; WHERE: none of the elements of the list are equal? to each other
; A 2ListOf<X> is a (cons X (cons X ListOf<X>))
; Represents a list of 2 or more elements.
; A Program is a:
; - empty
; - (cons Expr Program)
; - (cons Def Program)
; Represents a PDPLang program consisting of function defs and expressions.
; WHERE: no two Defs have the same name
; An Expr is one of:
; - Number
; - Boolean
; - Var
; - ErrString
; - Lambda
; - (make-arith ArithOp 2ListOf<Expr>) ; an arithmetic expression
; - (make-bool BoolOp 2ListOf<Expr>)   ; a boolean expression
; - (make-cmp CmpOp 2ListOf<Expr>)     ; a comparison expression
; - (make-if-exp Expr Expr Expr) ; an if conditional
; - (make-call Expr ListOf<Expr>) ; a function call
; Represents a PDPLang expression.
(define-struct arith (op args))
(define-struct bool (op args))
(define-struct cmp (op args))
(define-struct if-exp (test branch1 branch2))
(define-struct call (fn args))
; A Var is a Symbol, representing PDPLang variable.
; An ErrString is a String, representing a PDPLang error message.
; A Lambda is a (make-lam UniqueListOf<Param> Expr)
; Represents a lambda expression in PDPLang
(define-struct lam (params body))
; A Param is a Var, representing a function parameter.
; An ArithOp is one of:
; - '+
; - '-
; - '*
; - '/
; Represents an arithmetic operation in PDPLang
; A BoolOp is one of:
; - 'and
; - 'or
; Represents a boolean operation in PDPLang
; A CmpOp is one of:
; - '=
; - '<
; - '>
; Represents a comparison operation in PDPLang
; A Def is a (make-def FnName UniqueListOf<Param> Expr)
; Represents the definition of a function with the specified parameters.
(define-struct def (name params body))
; A FnName is a Var, representing a function name.
; A Result is a:
; - Number
; - Boolean
; - ErrString
; - Lambda
Complete the Data Design by adding templates and data examples.
Here’s how to evaluate a PDPLang Program.
  • A PDPLang Program evaluates to a list of Results, where the list of Results are result of evaluating the Exprs in the program.

  • Exprs in a PDPLang Program are evaluated assuming that all the Defs in the program are already defined.

Here’s how to evaluate a PDPLang Expr.
  • An Expr that is already a Result evaluates to itself.

  • A Var, if it represents a defined Def, should evaluate to an equivalent lambda Expr. Otherwise, a Var should evaluate to an appropriate ErrStr.
    (define F (make-def 'f '(x) 'x))
    (check-pred lambda? (first (eval (list F 'f))) "result is lambda")
    (check-pred errstr? (first (eval (list F 'g))) "err: undefined var")
    (check-pred errstr? (first (eval (list 'x))) "err: undefined var")

  • Arithmetic and boolean operations evaluate to the obvious Result.

    UPDATE 2015-03-19: Boolean operations are not short circuiting.

  • An if expression:
    • first evaluates its test expression.

    • The result of evaluating an if expression is the result of evaluating the first branch if the test expression evaluates to true.

    • The result of evaluating an if expression is the result of evaluating the second branch if the test expression evaluates to false.

    • Otherwise, the result of evaluating the if expression should be an appropriate error message.

  • Evaluate a function call by:
    1. evaluating its function subexpression; if the result is not a Lambda, the result of the function call is an appropriate ErrString;

    2. evaluating its arguments;

    3. replacing each parameter reference in the function body with its corresponding argument and evaluating the resulting expressions; a function call evaluates to an appropriate ErrString if the number of arguments does not match the number of function parameters.

    4. To help you with function call evaluation, implement, provide and use the following function:
      ; subst : Result Var Expr -> Expr
      ; Replaces references to x in e with r.
      ; Does not replace x with r if x occurs in the body of a lambda
      ; that shadows x.
      ; WHERE: r has no unbound variables
        (check-equal? (subst 4 'x 'x) 4 "x matches")
        (check-equal? (subst 4 'y 'x) 'x "y doesnt match")
         (subst 4 'x (make-arith '+ '(x 5)))
         (make-arith '+ '(4 5))
         "subst in arith")
         (subst 4 'x (make-lam '(y) (make-arith '+ '(x y))))
         (make-lam '(y) (make-arith '+ '(4 y)))
         "subst in lambda")
         (subst 4 'x (make-lam '(x) (make-arith '+ '(x 5))))
         (make-lam '(x) (make-arith '+ '(x 5)))
         "dont subst shadowed vars in lambdas"))
      (define (subst r x e) ...)

    5. UPDATE 2015-03-20: Here are some function call examples:

      (define MK-ADD-DEF
         'mk-add '(n)
         (make-lam '(m) (make-arith '+ '(n m)))))
      (define ADD5-DEF
         'add5 '(x)
         (make-call (make-call 'mk-add '(5)) '(x))))
      ; add5-or-6 : adds 5 to y if y it's positive, else adds 6
      (define ADD5-OR-6-DEF
         'add5-or-6 '(y)
         (make-if-exp (make-cmp '> '(y 0))
                      (make-call 'add5 '(y))
                      (make-call (make-call 'mk-add '(6)) '(y)))))
         (make-call 'add5 '(10))
         (make-call 'add5-or-6 '(200))
         (make-call 'add5-or-6 '(-100))))
       (list 15 205 -94)
       "call-fn evaluates to a function")

  • Any expressions whose subexpressions evaluate to the wrong kind of Result should evaluate to an ErrString.
      (eval (list (make-arith '+ (list 2 true))
                  (make-bool 'and (list 2 true))
                  (make-if-exp 3 1 2))))
     "wrong kind of args")

UPDATE 2015-03-18: Finally, implement expr->expr/no-var and expr=? (this part of problem set is based on exercise 451 of the textbook).

You might also want to review the in-class exercise from Module07 that computes tree depths.
To help you even more, here are slides from a previous semester for somewhat-related problem:
; An ExprNoVar is one of:
; - Number
; - Boolean
; - StaticDist
; - ErrString
; - LamNoVar
; - (make-arith ArithOp 2ListOf<ExprNoVar>) ; an arithmetic expression
; - (make-bool BoolOp 2ListOf<ExprNoVar>)   ; a boolean expression
; - (make-cmp CmpOp 2ListOf<ExprNoVar>)     ; a comparison expression
; - (make-if-exp ExprNoVar ExprNoVar ExprNoVar) ; an if conditional
; - (make-call ExprNoVar ListOf<ExprNoVar>) ; a function call
; Represents an Expr without explicit variables.
; A StaticDist is a (list Depth Index)
; Represents a variable reference
; where depth is number of additional lambdas between this var ref and the
; lambda for which this variable is a parameter,
; and index is the (0-based) position of this variable in that lambda's
; parameter list.
; A Depth is a Natural
; An Index is a Natural
; A LamNoVar is a (make-lam/no-var ExprNoVar)
(define-struct lam/no-var (body))
; expr->expr/no-var : Expr -> ExprNoVar
; Replaces Var in e with StaticDist.
; WHERE: there are no unbound variables in e.
   (expr->expr/no-var (make-lam '(x) 'x))
   (make-lam/no-var '(0 0))
   "basic lambda")
   (expr->expr/no-var (make-lam '(x y) (make-lam '(z) (make-call 'x '(y z)))))
   (make-lam/no-var (make-lam/no-var (make-call '(1 0) '((1 1) (0 0)))))
   "nested lambdas"))
(define (expr->expr/no-var e) ...)
; expr=? : Expr Expr -> Boolean
; Returns true if e1 and e2 are structurally equivalent, up to some
; renaming of variable names.
   (make-lam '(x) 'x)
   (make-lam '(y) 'y)
   "equivalent basic lambdas")
    (make-lam '(x y) (make-call 'x '(y)))
    (make-lam '(y x) (make-call 'x '(y))))
   "not equivalent")
   (make-lam '(y) (make-lam '(x) (make-call 'y '(x))))
   (make-lam '(x) (make-lam '(y) (make-call 'x '(y))))
   "equivalent nested-lambdas"))
(define (expr=? e1 e2) ...)