Homework 5: Symbolic Evaluation of Boolean Expressions

Due: Wednesday, Oct 14, 2020 at 11:59PM

200 pts.

Overview

Write a Racket function reduce that reduces boolean expressions (represented in Racket notation) to simplified form. For the purposes of this assignment, boolean expressions are Racket expressions constructed from:

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The course staff is providing functions parse and unparse in the file parse.rkt that rkt that convert boolean expressions in Racket notation to a simple inductively defined type called BoolExp and vice-versa. The coding of parse and unparse is not difficult, but it is tedious (like most parsing) so the course staff is providing this code rather than asking students to write it. The Racket primitive read: -> RacketExp is a procedure of no arguments that reads a Racket expression from the console. DrRacket pops up an input box to serve as the console when (read) is executed.

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• a boolean constant true and false ;
• a symbol S representing a boolean variable;
• (make-Not X) where X is a BoolExp;
• (make-And X Y) where X and Y are BoolExps BoolExps;
• (make-Or X Y) where X and Y are BoolExps BoolExps;
• (make-Implies X Y) where X and Y are BoolExps BoolExps; or
• (make-If X Y Z) where X , Y , and Z are BoolExps BoolExps.

A bool-RacketExp BoolRacketExp is either:

• a boolean constant true or false;
• a symbol S;
• (list 'not X) where X is a bool-RacketExp BoolRacketExp ;
• (list op X Y) where op is 'and , 'or , or 'implies where X and Y are bool-RacketExps BoolRacketExps;
• (list 'if X Y Z) where X, Y, and Z are bool-RacketExps BoolRacketExps.

The provided functions parse and unparse have the following signatures.

parse: bool-RacketExpBoolRacketExp -> BoolExp
unparse: BoolExp -> bool-RacketExpBoolRacketExp

The course staff is also providing a very simple test file\ for the eval and reduce functions and a file\ containing a sequence of raw input formulas (to be parsed by parse function in parse.ss\). A good solution to this problem will include much more comprehensive test data for all functions, including some much larger test cases for reduce. The normalize function is difficult to test on large data because the printed output for some important normalized trees (represented as DAGs (Directed Acyclic Graphs) in memory) is so large.

Given a parsed input of type BoolExp , the simplification process consists of following four phases:

• Conversion to if If form implemented by the function convert-to-if convertToIf.
• Normalization implemented by the function normalize.
• Symbolic evaluation implemented by the function eval.
• Conversion back to conventional boolean form implemented by the function convert-to-bool convertToBool.

A description of each of these phases follows. The reduce function has type boolBoolRacketExp -RacketExp -> bool-RacketExpBoolRacketExp.

Conversion to if form

A boolean expression (BoolExp) can be converted to if form (a boolean expression where the only constructor is make-If) by repeatedly applying the following rewrite rules in any order until no rule is applicable.

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In these rules, X and Y denote arbitrary BoolExpsBoolExps}. The conversion process always terminates (since each rewrite strictly reduces the number of logical connectives excluding {{make make-If) and  and yields a unique answer independent of the order in which the rewrites are performed. This property is called the Church-Rosser property, after the logicians (Alonzo Church and Barkley Rosser) who invented the concept.

Since the reduction rules for this phase are Church-Rosser, you can write the function convert-to-if using convertToIf using simple structural recursion. For each of the boolean operators And, Or, Not, Implies, and ifIf, reduce the component expressions first and then applying the matching reduction (except for if If for which there is no top-level reduction).

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(check-expect  (convert-to-ifconvertToIf (make-Or (make-And 'x 'y) 'z))    (make-If (make-If 'x 'y false) true 'z))
(check-expect  (convert-to-ifconvertToIf (make-Implies 'x (make-Not 'y))   (make-If 'x (make-If 'y false  true) true))

We suggest simply traversing the tree using the structural recursion template for type BoolExp and converting all structures (other than if If) to the corresponding if structures.

Write an inductive data definition and template for boolean formulas in if form, naming this type ifExp. (Note: make-If is  is the only constructor, other than variables and constants, for ifExp.

The provided function parse: input -> boolExpBoolExp takes a Racket expression and returns the corresponding boolExp BoolExp.

Normalization

An ifExp is normalized iff every sub-expression in test position is either a variable (symbol) or a constant (true or false). We call this type norm-ifExp NormIfExp .

For example, the ifExp (make-If (make-If X Y Z) U V)) is not a norm-ifExp NormIfExp because it has an If construction in test position. In contrast, the equivalent ifExp (make-If X (make-If Y U V) (make-If Z U V)) is normalized and hence is an norm-ifExp NormIfExp.

The normalization process, implemented by the function normalize: ifExp -> norm-ifExpNormIfExp eliminates all if constructions that appear in test positions inside if constructions. We perform this transformation by repeatedly applying the following rewrite rule (to any portion of the expression) until it is inapplicable:

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Hint: define a sub-function head-normalize that takes three norm-ifExps NormIfExps X, Y, and Z and constructs a norm-ifExp NormIfExp equivalent to (makeIf X Y Z). This help function processes X because the test position must be a variable or a constant, yet X can be an arbitrary norm-ifExp NormIfExp. In contrast, (head-normalize X Y Z) never even inspects Y and Z because they are already normalized and the normalizing transformations performed in head-normalize never place these expressions in test position.

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Before you start writing normalize, write the template corresponding to the inductive data definition of norm-ifExp NormIfExp.

Symbolic Evaluation

The symbolic evaluation process, implemented by the function eval: norm-if-form : NormIfExp environment -> norm-if-formNormIfExp, reduces a norm-if-form NormIfExp to simple form. In particular, it reduces all tautologies (expressions that are always true) to true and all contradictions (expressions that are always false) to false.

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(make-If  true  X  Y)	   =>	X
(make-If  false  X  Y)	   =>	Y
(make-If  X  true  false)  =>	X
(make-If  X  Y  Y) 	   =>	Y
(make-If  X  Y  Z)	   =>	(make-If  X  Y\[X <\- true\]  Z\[X <\- false\])

The notation M[X <- N] means M with all occurrences of the symbol X replaced by the expression N. It is very costly to actually perform these subtitutions on =norm-if-form= datasubstitutions on NormIfExp data. To avoid this computational expense, we simply maintain a list of bindings which are pairs consisting of symbols (variable names) and boolean values {true, false. The following data definition definition formally defines the binding type.

A binding is a pair (make-binding s v) where s is a symbol (a variable) and v is a boolean value (an element of { true, false }.

An environment is a (list-of binding) binding-list.

When the eval function encounters a variable (symbol), it looks up the symbol in the environment and replaces the symbol it's boolean value if it exists.

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We recommend applying the rules in the order shown from the top down until no more reductions are possible (using the constraint on the final rule). Note that the last rule should only be applied once to a given sub-expressionsubexpression.

Conversion to Boolean Form

The final phase converts an expression in (not necessarily reduced or normalized) If form to an equivalent expression constructed from variables and { true, false, And, Or, Not, Implies, If. This process eliminates every expression of the form

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where X , Y , and Z are arbitrary If forms. This set of rules is Church-Rosser, so the rules can safely be applied using simple structural recursion.

Points Dsitribution

• convert-to-if convertToIf (10%)
• normalize (20%)
• eval (20%)
• convert-to-bool convertToBool (10%)
• reduce (40%)