Logic & SMT

Syntax

A logical predicate is, informally speaking, a Boolean valued term drawn from a restricted subset of Haskell. In particular, the expressions are drawn from the following grammar comprising constants, expressions and predicates.

A Constant² c is simply one of the numeric values:

    c := 0, 1, 2, ...

A Variable v is one of x, y, z, etc., these will refer to (the values of) binders in our source programs.

    v := x, y, z, ...

An Expression e is one of the following forms; that is, an expression is built up as linear arithmetic expressions over variables and constants and uninterpreted function applications.

    e := v                      -- variable
       | c                      -- constant
       | (e + e)                -- addition
       | (e - e)                -- subtraction
       | (c * e)                -- multiplication by constant
       | (v e1 e2 ... en)       -- uninterpreted function application
       | (if p then e else e)   -- if-then-else

Examples of Expressions include the following:

• x + y - z
• 2 * x
• 1 + size x

A Relation is one of the usual (arithmetic) comparison operators:

    r := ==               -- equality
       | /=               -- disequality
       | >=               -- greater than or equal
       | <=               -- less than or equal
       | >                -- greater than
       | <                -- less than

A Predicate is either an atomic predicate, obtained by comparing two expressions, or, an application of a predicate function to a list of arguments, or the Boolean combination of the above predicates with the operators && (and), || (or), ==> (implies ³), <=> (if and only if ⁴), and not.

    p := (e r e)                -- binary relation
       | (v e1 e2 ... en)       -- predicate (or alias) application
       | (p && p)               -- and
       | (p || p)               -- or
       | (p => p) | (p ==> p)   -- implies
       | (p <=> p)              -- iff
       | (not p)                -- negation
       | true | True
       | false | False

Examples of Predicates include the following:

• x + y <= 3
• null x
• x < 10 ==> y < 10 ==> x + y < 20
• 0 < x + y <=> 0 < y + x

Semantics

The syntax of predicates tells us what they look like, that is, what we can write down as valid predicates. Next, let us turn our attention to what a predicate means. Intuitively, a predicate is just a Boolean valued Haskell function with &&, ||, not being the usual operators and ==> and <=> being two special operators.

The Implication operator ==> is equivalent to the following Haskell function. (For now, ignore the signature: it just says the output is a Bool that is equal to the logical implication between the inputs p and q.)

The If-and-only-if operator <=> is equivalent to the Haskell function:⁵

An Environment is a mapping from variables to their Haskell types. For example, let G be an environment defined as

    x :: Int
    y :: Int
    z :: Int

which maps each variable x, y and z to the type Int.

An Assignment under an environment, is a mapping from variables to values of the type specified in the environment. For example,

    x := 1
    y := 2
    z := 3

is an assignment under G that maps x, y and z to the Int values 1, 2 and 3 respectively.

A Predicate Evaluates to either True or False under a given assignment. For example, the predicate

    x + y > 10

evaluates to False given the above assignment but evaluates to True under the assignment

    x := 10
    y := 10
    z := 20

A Predicate is Satisfiable in an environment if there exists an assignment (in that environment) that makes the predicate evaluate to True. For example, in G the predicate

    x + y == z

is satisfiable, as the above assignment makes the predicate evaluate to True.

A Predicate is Valid in an environment if every assignment in that environment makes the predicate evaluate to True. For example, the predicate

    x < 10 || x == 10 || x > 10

is valid under G as no matter what value we assign to x, the above predicate will evaluate to True.

Verification Conditions

LiquidHaskell works without actually executing your programs. Instead, it checks that your program meets the given specifications in roughly two steps.

1. First, LH combines the code and types down to a set of Verification Conditions (VC) which are predicates that are valid only if your program satisfies a given property. ⁶

2. Next, LH queries an SMT solver to determine whether these VCs are valid. If so, it says your program is safe and otherwise it rejects your program.

The SMT Solver decides whether a predicate (VC) is valid without enumerating and evaluating all assignments. Indeed, it is impossible to do so as there are usually infinitely many assignments once the predicates refer to integers or lists and so on. Instead, the SMT solver uses a variety of sophisticated symbolic algorithms to deduce whether a predicate is valid or not. This process is the result of decades of work in mathematical logic and decision procedures; the Ph.D thesis of Greg Nelson is an excellent place to learn more about these beautiful algorithms.

We Restrict the Logic to ensure that all our VC queries fall within the decidable fragment. This makes LiquidHaskell extremely automatic – there is no explicit manipulation of proofs, just the specification of properties via types and of course, the implementation via Haskell code! This automation comes at a price: all our refinements must belong to the logic above. Fortunately, with a bit of creativity, we can say a lot in this logic. ⁷

Examples: Propositions

Finally, let’s conclude this quick overview with some examples of predicates, in order to build up our own intuition about logic and validity. Each of the below is a predicate from our refinement logic. However, we write them as raw Haskell expressions that you may be more familiar with right now, and so that we can start to use LiquidHaskell to determine whether a predicate is indeed valid or not.

Let `TRUE` be a refined type for Bool valued expressions that always evaluate to True. Similarly, we can define FALSE for Bool valued expressions that always evaluate to False:⁸

Thus, a valid predicate is one that has the type TRUE. The simplest example of a valid predicate is just True:

of course, False is not valid

We can get more interesting predicates if we use variables. For example, the following is valid predicate says that a Bool variable is either True or False.

Of course, a variable cannot be both True and False, and so the below predicate is valid:

The next few examples illustrate the ==> operator. You should read p ==> q as if p is true then q must also be true. Thus, the below predicates are valid as if both a and b are true, then well, a is true, and b is true.

The following predicates are valid because they encode modus ponens: if you know that a implies b and you know that a is true, then it must be the case that b is also true:

Recall that p <=> q (read p if and only if q) evaluates to True exactly when p and q evaluate to the same values (True or False). It is used to encode equalities between predicates. For example, we can write down De Morgan’s laws as the valid predicates:

Examples: Arithmetic

Next, let’s look at some predicates involving arithmetic. The simplest ones don’t have any variables, for example:

Again, a predicate that evaluates to False is not valid:

SMT Solvers determine Validity without enumerating assignments. For example, consider the predicate:

It is trivially valid; as via the usual laws of arithmetic, it is equivalent to 0 < 1 which is True independent of the value of x. The SMT solver is able to determine this validity without enumerating the infinitely many possible values for x. This kind of validity checking lies at the heart of LiquidHaskell.

operators, as shown in the following examples:

Examples: Uninterpreted Function

We say that function symbols are uninterpreted in the refinement logic, because the SMT solver does not “know” how functions are defined. Instead, the only thing that the solver knows is the axiom of congruence which states that any function f, returns equal outputs when invoked on equal inputs.

We Test the Axiom of Congruence by checking that the following predicate is valid:

Again, remember we are not evaluating the code above; indeed we cannot evaluate the code above because we have no definition of f. Still, the predicate is valid as the congruence axiom holds for any possible interpretation of f.

Here is a fun example; can you figure out why this predicate is indeed valid? (The SMT solver can…)

To get a taste of why uninterpreted functions will prove useful, let’s write a function to compute the size of a list:

We can now verify that the following predicates are valid:

Note that to determine that the above is valid, the SMT solver does not need to know the meaning or interpretation of size – merely that it is a function. When we need some information about the definition, of size we will put it inside the predicate. For example, in order to prove that the following is valid:

LiquidHaskell actually asks the SMT solver to prove the validity of a VC predicate which states that sizes are non-negative and that since ys equals x:xs, the size of ys is one more than xs. ⁹

Recap

This chapter describes exactly what we, for the purposes of this book, mean by the term logical predicate.

1. We defined a grammar – a restricted subset of Haskell corresponding to Bool valued expressions.
2. The restricted grammar lets us use SMT solvers to decide whether a predicate is valid that is, evaluates to True for all values of the variables.
3. Crucially, the SMT solver determines validity without enumerating and evaluating the predicates (which would take forever!) but instead by using clever symbolic algorithms.

Next, let’s see how we can use logical predicates to specify and verify properties of real programs.