HW05

Homework 5: Symbolic Evaluation of Boolean Expressions

Due: 11:59pm, Thursday, Oct 13, 2022
200 points

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 (in input format to be parsed) are Racket expressions constructed from:

...

The reduce function take a BoolRacketExp as input, parses it, simplifies it (as described above), and unparses the simplified result. Hence, reduce maps BoolRacketExp to BoolRacketExp.
A detailed description of each of the four phases of the simplication process follows.

Conversion to IfForm

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.
(make-Not X) =>(make-If X false true)
(make-And X Y)=>(make-If X Y false)
(make-Or X Y)=>(make-If X true Y)
(make-Implies X Y)=>(make-If X Y true)
In these rules, X and Y denote arbitrary BoolExps}. The conversion process always terminates (since each rewrite strictly reduces the number of logical connectives excluding make-If 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 convertToIf using simple structural recursion. For each of the boolean operators And, Or, Not, Implies, and If, reduce the component expressions first and then applying the matching reduction (except for If for which there is no top-level reduction).
The following examples illustrate the conversion process:
(check-expect (convertToIf (make-Or (make-And 'x 'y) 'z)) (make-If (make-If 'x 'y false) true 'z))
(check-expect (convertToIf (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) 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 the only constructor, other than variables and constants, for ifExp.
The provided function parse: input -> BoolExp takes a Racket expression and returns the corresponding 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 NormIfExp .
For example, the ifExp (make-If (make-If X Y Z) U V)) is not a 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 NormIfExp.
The normalization process, implemented by the function normalize: ifExp -> NormIfExp 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:
(make-If (make-If X Y Z) U V) => (make-If X (make-If Y U V) (make-If Z U V)).
This transformation always terminates and yields a unique answer independent of the order in which rewrites are performed. The proof of this fact is left as an optional exercise.
In the normalize function, it is critically important not to duplicate any work, so the order in which reductions are made really matters. Do NOT apply the normalization rule above unless U and V are already normalized, because the rule duplicates both U and V. If you reduce the consequent and the alternative (U and V in the left hand side of the rule above) before reducing the test, normalize runs in linear time (in the number of nodes in the input); if done in the wrong order it runs in exponential time in the worst case. (And some of our test cases will exhibit this worst case behavior.)
Hint: define a sub-function head-normalize that takes three NormIfExps X, Y, and Z and constructs a 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 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.
The following examples illustrate how the normalize and head-normalize functions behave:
(check-expect (head-normalize 'x 'y 'z) (make-If 'x 'y 'z))
(check-expect (head-normalize true 'y 'z) (make-If true 'y 'z))
(check-expect (head-normalize false 'y 'z) (make-If false 'y 'z))
(check-expect (head-normalize (make-If 'x 'y 'z) 'u 'v) (make-If 'x (make-If 'y 'u 'v) (make-If 'z 'u 'v)))
(check-expect (head-normalize (make-If 'x (make-If 'yt 'yc 'ya) (make-If 'zt 'zc 'za)) 'u 'v)
(make-If 'x (make-If 'yt (make-If 'yc 'u 'v) (make-If 'ya 'u 'v)) (make-If 'zt (make-If 'zc 'u 'v) (make-If 'za 'u 'v))))
(check-expect (normalize true) true)
(check-expect (normalize false) false)
(check-expect (normalize 'x) 'x)
(check-expect (normalize (make-If 'x 'y 'z)) (make-If 'x 'y 'z))
(check-expect (normalize (make-If (make-If 'x 'y 'z) 'u 'v)) (make-If 'x (make-If 'y 'u 'v) (make-If 'z 'u 'v)))
Once a large formula has been normalized, do not try to print it unless you know that the formula is small! The printed form can be exponentially larger than the internal representation (because the internal representation can share subtrees).
Before you start writing normalize, write the template corresponding to the inductive data definition of NormIfExp.

Symbolic Evaluation

The symbolic evaluation process, implemented by the function eval: NormIfExp environment -> NormIfExp, reduces a 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.
Symbolic evaluation applies the following rewrite rules to an expression until none is applicable (with one exception discussed below):
(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 substitutions 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 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.
These rewrite rules do not have the Church-Rosser property. The last two rewrite rules are the spoilers; the relative order in which they are applied can affect the result in some cases. However, the rewrite rules do have the Church-Rosser property on expressions which are tautologies or contradictions.
If the final rule is applied only when X actually occurs in either Y or Z, then the symbolic evaluation process is guaranteed to terminate. In this case, every rule either reduces the size of the expression or the number of variable occurrences in it.
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 subexpression.
Note that all of the transformation rules map NormIfExp to NormIfExp when viewed as transformations on entire NormIfExp formulas.

Conversion to Boolean Form

The final phase, performed by the function convertToBool, converts an expression in normalized If form (an element of the type NormalizedIfExp) to an equivalent expression constructed from variables and { true, false, And, Or, Not, Implies, If. This process eliminates every expression of the form
(make-If X Y Z)
where one of the arguments {X, Y, Z is a constant { true, false }.
Use the following set of reduction rules to perform this conversion
(make-If X false true) => (make-Not X)
(make-If X Y false) => (make-And X Y)
(make-If X true Y) => (make-Or X Y)
(make-If X Y true) => (make-Implies X Y)
where X , Y , and Z are arbitrary IfForms. This set of rules is Church-Rosser, so the rules can safely be applied using simple structural recursion.

Grading

The 200 points of credit possible on this assignment will be partitioned among the five functions you must write

• convertToIf (10%)
• normalize (20%)
• eval (20%)
• convertToBool (10%)
• reduce (40%)

...