Simple Refinement Types

Simple Refinement Types

Refinement Types = Types + Predicates

Example: Integers equal to 0

{-@ type Zero = {v:Int | v = 0} @-} {-@ zero :: Zero @-} zero = 0

Refinement types via special comments {-@ ... @-}

Example: Natural Numbers

{-@ type Nat = {v:Int | 0 <= v} @-} {-@ nats :: [Nat] @-} nats = [0, 1, 2, 3]

Exercise: Positive Integers

{-@ type Pos = {v:Int | 0 <= v} @-} {-@ poss :: [Pos] @-} poss = [0, 1, 2, 3]

Q: First, can you fix Pos so poss is rejected?

Q: Next, can you modify poss so it is accepted?

Refinement Type Checking

A Term Can Have Many Types

What is the type of 0 ?

{-@ zero  :: Zero @-}
zero      = 0

{-@ zero' :: Nat  @-}
zero'     = zero


Predicate Subtyping [NUPRL, PVS]

In environment $$\Gamma$$ the type $$t_1$$ is a subtype of the type $$t_2$$

$\boxed{\Gamma \vdash t_1 \preceq t_2}$

Environment $$\Gamma$$ is a sequence of binders

$\Gamma \doteq \overline{\bindx{x_i}{P_i}}$

Predicate Subtyping [NUPRL, PVS]

$\boxed{\Gamma \vdash t_1 \preceq t_2}$

$\begin{array}{rll} {\mathbf{If\ VC\ is\ Valid}} & \bigwedge_i P_i \Rightarrow Q \Rightarrow R & (\mbox{By SMT}) \\ & & \\ {\mathbf{Then}} & \overline{\bindx{x_i}{P_i}} \vdash \reft{v}{b}{Q} \preceq \reft{v}{b}{R} & \\ \end{array}$

Example: Natural Numbers

$\begin{array}{rcrccll} \mathbf{VC\ is\ Valid:} & \True & \Rightarrow & v = 0 & \Rightarrow & 0 \leq v & \mbox{(by SMT)} \\ & & & & & & \\ \mathbf{So:} & \emptyset & \vdash & \Zero & \preceq & \Nat & \\ \end{array}$

And so, we can type:

{-@ zero' :: Nat @-} zero' = zero -- zero :: Zero <: Nat

Example: Natural Numbers

$\begin{array}{rcrccll} \mathbf{VC\ is\ Valid:} & x = 3 & \Rightarrow & v = x + 1 & \Rightarrow & 0 \leq v & \mbox{(by SMT)} \\ & & & & & \\ \mathbf{So:} & x = 3 & \vdash & \{v:Int\ |\ v = x + 1\} & \preceq & \Nat & \\ \end{array}$

And so, we can type:

{-@ four :: Nat @-} four = x + 1 -- x = 3 |- {v = x + 1} <: Nat where x = 3

SMT Automates Subtyping

Eliminates boring proofs ... makes verification practical.

Contracts: Function Types

Pre-Conditions

{-@ impossible :: {v:_ | false} -> a @-} impossible msg = error msg

No value satisfies false $$\Rightarrow$$ no valid inputs for impossible

Program type-checks $$\Rightarrow$$ impossible never called at run-time

Exercise: Pre-Conditions

Let's write a safe division function

{-@ type NonZero = {v:Int | v /= 0} @-} {-@ safeDiv :: Int -> Int -> Int @-} safeDiv _ 0 = impossible "divide-by-zero" safeDiv x n = x div n

Q: Yikes! Can you fix the type of safeDiv to banish the error?

Precondition Checked at Call-Site

avg2 x y = safeDiv (x + y) 2

Precondition

{-@ safeDiv :: n:Int -> d:NonZero -> Int @-}


Verifies As

$\inferrule{}{(v = 2) \Rightarrow (v \not = 0)} {\bindx{x}{\Int}, \bindx{y}{\Int} \vdash \reftx{v}{v = 2} \preceq \reftx{v}{v \not = 0}}$

Precondition Checked at Call-Site

avg :: [Int] -> Int avg xs = safeDiv total n where total = sum xs n = length xs -- returns a Nat

Rejected as n can be any Nat

$0 \leq n \Rightarrow (v = n) \not \Rightarrow (v \not = 0)$

How to talk about list length in logic?

Recap

Refinement Types Types + Predicates

Specify Properties

Via Refined Input- and Output- Types

Verify Properties

Via SMT based Predicate Subtyping

{-@ avg :: {v:[a]| 0 < length v } -> Pos @-}