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<title>Language-Integrated Verification</title>
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<div id='toc' class='mobile-hidden'>
<ul class='chapter'>
<li><a href='01-intro.html'><b>1.</b>Follow Along at This URL</a></li>
<ul class='section'>
<li><a href='01-intro.html#whats-this'><b>1.1.</b> Whats this?</a></li>
<li><a href='01-intro.html#the-first-bug'><b>1.2.</b> The First <em>Bug</em></a></li>
<li><a href='01-intro.html#fast-forward-to-present-day'><b>1.3.</b> Fast forward to Present Day</a></li>
<li><a href='01-intro.html#fast-forward-to-present-day-1'><b>1.4.</b> Fast forward to Present Day</a></li>
<li><a href='01-intro.html#fast-forward-to-present-day-2'><b>1.5.</b> Fast forward to Present Day</a></li>
<li><a href='01-intro.html#programming-languages-research'><b>1.6.</b> Programming Languages Research</a></li>
<li><a href='01-intro.html#modern-languages'><b>1.7.</b> Modern Languages</a></li>
<li><a href='01-intro.html#modern-languages-1'><b>1.8.</b> Modern Languages</a></li>
<li><a href='01-intro.html#modern-languages-2'><b>1.9.</b> Modern Languages</a></li>
<li><a href='01-intro.html#modern-languages-3'><b>1.10.</b> Modern Languages?</a></li>
<li><a href='01-intro.html#well-typed-programs-can-go-very-wrong'><b>1.11.</b> Well-typed programs can go very wrong!</a></li>
<li><a href='01-intro.html#well-typed-programs-can-go-very-wrong-1'><b>1.12.</b> Well-typed programs can go very wrong!</a></li>
<li><a href='01-intro.html#well-typed-programs-can-go-very-wrong-2'><b>1.13.</b> Well-typed programs can go very wrong!</a></li>
<li><a href='01-intro.html#goal-language-integrated-verification'><b>1.14.</b> Goal: Language-Integrated Verification</a></li>
<li><a href='01-intro.html#goal-language-integrated-verification-1'><b>1.15.</b> Goal: Language-Integrated Verification</a></li>
<li><a href='01-intro.html#outline'><b>1.16.</b> Outline</a></li>
<li><a href='01-intro.html#outline-1'><b>1.17.</b> Outline</a></li>
<li><a href='01-intro.html#outline-2'><b>1.18.</b> Outline</a></li>
<li><a href='01-intro.html#outline-3'><b>1.19.</b> Outline</a></li>
<li><a href='01-intro.html#outline-4'><b>1.20.</b> Outline</a></li>
<li><a href='01-intro.html#outline-5'><b>1.21.</b> Outline</a></li>
</ul>
<li><a href='02-refinements.html'><b>2.</b>Simple Refinement Types</a></li>
<ul class='section'>
<li><a href='02-refinements.html#types'><b>2.1.</b> Types</a></li>
<li><a href='02-refinements.html#predicates'><b>2.2.</b> Predicates</a></li>
<li><a href='02-refinements.html#predicates-1'><b>2.3.</b> Predicates</a></li>
<li><a href='02-refinements.html#predicates-2'><b>2.4.</b> Predicates</a></li>
<li><a href='02-refinements.html#predicates-3'><b>2.5.</b> Predicates</a></li>
<li><a href='02-refinements.html#predicates-4'><b>2.6.</b> Predicates</a></li>
<li><a href='02-refinements.html#example-singletons'><b>2.7.</b> Example: Singletons</a></li>
<li><a href='02-refinements.html#example-natural-numbers'><b>2.8.</b> Example: Natural Numbers</a></li>
<li><a href='02-refinements.html#a-term-can-have-many-types'><b>2.9.</b> A Term Can Have <em>Many</em> Types</a></li>
<li><a href='02-refinements.html#predicate-subtyping-nuprl-pvs'><b>2.10.</b> 1. Predicate Subtyping <a href="http://pvs.csl.sri.com/papers/subtypes98/tse98.pdf">[NUPRL, PVS]</a></a></li>
<li><a href='02-refinements.html#predicate-subtyping-nuprl-pvs-1'><b>2.11.</b> 1. Predicate Subtyping <a href="http://pvs.csl.sri.com/papers/subtypes98/tse98.pdf">[NUPRL, PVS]</a></a></li>
<li><a href='02-refinements.html#predicate-subtyping-nuprl-pvs-2'><b>2.12.</b> 1. Predicate Subtyping <a href="http://pvs.csl.sri.com/papers/subtypes98/tse98.pdf">[NUPRL, PVS]</a></a></li>
<li><a href='02-refinements.html#example-natural-numbers-1'><b>2.13.</b> Example: Natural Numbers</a></li>
<li><a href='02-refinements.html#example-natural-numbers-2'><b>2.14.</b> Example: Natural Numbers</a></li>
<li><a href='02-refinements.html#typing-applications-function-calls'><b>2.15.</b> 2. Typing Applications (Function Calls)</a></li>
<li><a href='02-refinements.html#typing-applications-function-calls-1'><b>2.16.</b> 2. Typing Applications (Function Calls)</a></li>
<li><a href='02-refinements.html#typing-applications-function-calls-2'><b>2.17.</b> 2. Typing Applications (Function Calls)</a></li>
<li><a href='02-refinements.html#typing-applications-function-calls-3'><b>2.18.</b> 2. Typing Applications (Function Calls)</a></li>
<li><a href='02-refinements.html#recap-refinement-types-101'><b>2.19.</b> Recap: Refinement Types 101</a></li>
<li><a href='02-refinements.html#recap-refinement-types-101-1'><b>2.20.</b> Recap: Refinement Types 101</a></li>
<li><a href='02-refinements.html#recap-refinement-types-101-2'><b>2.21.</b> Recap: Refinement Types 101</a></li>
<li><a href='02-refinements.html#outline'><b>2.22.</b> Outline</a></li>
<li><a href='02-refinements.html#outline-1'><b>2.23.</b> Outline</a></li>
</ul>
<li><a href='03-examples.html'><b>3.</b>Refinement Types by Example</a></li>
<ul class='section'>
<li><a href='03-examples.html#refinement-types-by-example-1'><b>3.1.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#refinement-types-by-example-2'><b>3.2.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#refinement-types-by-example-3'><b>3.3.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#specifications-pre-conditions'><b>3.4.</b> Specifications: Pre-Conditions</a></li>
<li><a href='03-examples.html#specifications-post-conditions'><b>3.5.</b> Specifications: Post-Conditions</a></li>
<li><a href='03-examples.html#refinement-types-by-example-4'><b>3.6.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#verification-vector-sum'><b>3.7.</b> Verification: Vector Sum</a></li>
<li><a href='03-examples.html#verification-vector-sum-1'><b>3.8.</b> Verification: Vector Sum</a></li>
<li><a href='03-examples.html#verification-vector-sum-2'><b>3.9.</b> Verification: Vector Sum</a></li>
<li><a href='03-examples.html#refinement-types-by-example-5'><b>3.10.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#inference'><b>3.11.</b> Inference</a></li>
<li><a href='03-examples.html#inference-vector-sum'><b>3.12.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#inference-vector-sum-1'><b>3.13.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#inference-vector-sum-2'><b>3.14.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#inference-vector-sum-3'><b>3.15.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#inference-vector-sum-4'><b>3.16.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#inference-vector-sum-5'><b>3.17.</b> Inference: Vector Sum</a></li>
<li><a href='03-examples.html#refinement-types-by-example-6'><b>3.18.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#collections-higher-order-functions'><b>3.19.</b> Collections & Higher-Order Functions</a></li>
<li><a href='03-examples.html#collections-higher-order-functions-1'><b>3.20.</b> Collections & Higher-Order Functions</a></li>
<li><a href='03-examples.html#collections-higher-order-functions-2'><b>3.21.</b> Collections & Higher-Order Functions</a></li>
<li><a href='03-examples.html#collections-higher-order-functions-3'><b>3.22.</b> Collections & Higher-Order Functions</a></li>
<li><a href='03-examples.html#refinement-types-by-example-7'><b>3.23.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#example-list-average'><b>3.24.</b> Example: List <code>average</code></a></li>
<li><a href='03-examples.html#refinements-for-datatypes'><b>3.25.</b> Refinements for Datatypes</a></li>
<li><a href='03-examples.html#measures-yield-refined-constructors'><b>3.26.</b> Measures Yield Refined Constructors</a></li>
<li><a href='03-examples.html#example-map-over-lists'><b>3.27.</b> Example: <code>map</code> over Lists</a></li>
<li><a href='03-examples.html#refinements-for-datatypes-1'><b>3.28.</b> Refinements for Datatypes</a></li>
<li><a href='03-examples.html#refinements-for-datatypes-2'><b>3.29.</b> Refinements for Datatypes</a></li>
<li><a href='03-examples.html#refinements-for-datatypes-3'><b>3.30.</b> Refinements for Datatypes</a></li>
<li><a href='03-examples.html#refinement-types-by-example-8'><b>3.31.</b> Refinement Types by Example</a></li>
<li><a href='03-examples.html#outline'><b>3.32.</b> Outline</a></li>
<li><a href='03-examples.html#outline-1'><b>3.33.</b> Outline</a></li>
</ul>
<li><a href='04-abstracting.html'><b>4.</b>Invariants In Constructors</a></li>
<ul class='section'>
<li><a href='04-abstracting.html#invariants-in-constructors-1'><b>4.1.</b> Invariants In Constructors</a></li>
<li><a href='04-abstracting.html#invariants-in-constructors-2'><b>4.2.</b> Invariants In Constructors</a></li>
<li><a href='04-abstracting.html#invariants-in-constructors-3'><b>4.3.</b> Invariants In Constructors</a></li>
<li><a href='04-abstracting.html#abstracting-refinements'><b>4.4.</b> Abstracting Refinements</a></li>
<li><a href='04-abstracting.html#abstracting-refinements-1'><b>4.5.</b> Abstracting Refinements</a></li>
<li><a href='04-abstracting.html#abstracting-refinements-2'><b>4.6.</b> Abstracting Refinements</a></li>
<li><a href='04-abstracting.html#using-abstract-refinements'><b>4.7.</b> Using Abstract Refinements</a></li>
<li><a href='04-abstracting.html#recap'><b>4.8.</b> Recap</a></li>
<li><a href='04-abstracting.html#recap-1'><b>4.9.</b> Recap</a></li>
<li><a href='04-abstracting.html#bounded-refinements-induction-as-a-type'><b>4.10.</b> Bounded Refinements: Induction as a Type</a></li>
<li><a href='04-abstracting.html#bounded-refinements-induction-as-a-type-1'><b>4.11.</b> Bounded Refinements: Induction as a Type</a></li>
<li><a href='04-abstracting.html#bounded-refinements-induction-as-a-type-2'><b>4.12.</b> Bounded Refinements: Induction as a Type</a></li>
<li><a href='04-abstracting.html#bounded-refinements-induction-as-a-type-3'><b>4.13.</b> Bounded Refinements: Induction as a Type</a></li>
</ul>
<li><a href='04-termination.html'><b>5.</b>Avoiding Infinite Loops</a></li>
<ul class='section'>
<li><a href='04-termination.html#example-proving-termination-of-sum'><b>5.1.</b> Example: Proving Termination of <code>sum</code></a></li>
<li><a href='04-termination.html#proving-termination'><b>5.2.</b> Proving Termination</a></li>
<li><a href='04-termination.html#proving-termination-1'><b>5.3.</b> Proving Termination</a></li>
<li><a href='04-termination.html#proving-termination-2'><b>5.4.</b> Proving Termination</a></li>
<li><a href='04-termination.html#proving-termination-3'><b>5.5.</b> Proving Termination</a></li>
<li><a href='04-termination.html#example-proving-termination-of-sum-1'><b>5.6.</b> Example: Proving Termination of <code>sum</code></a></li>
<li><a href='04-termination.html#user-specified-termination-metrics'><b>5.7.</b> User Specified Termination Metrics</a></li>
<li><a href='04-termination.html#user-specified-termination-metrics-1'><b>5.8.</b> User Specified Termination Metrics</a></li>
<li><a href='04-termination.html#user-specified-termination-metrics-2'><b>5.9.</b> User Specified Termination Metrics</a></li>
<li><a href='04-termination.html#proving-termination-4'><b>5.10.</b> Proving Termination</a></li>
<li><a href='04-termination.html#lexicographic-termination'><b>5.11.</b> Lexicographic Termination</a></li>
<li><a href='04-termination.html#how-about-data-types'><b>5.12.</b> How About Data Types?</a></li>
<li><a href='04-termination.html#user-specified-metrics-on-data-types'><b>5.13.</b> User specified metrics on Data Types</a></li>
<li><a href='04-termination.html#termination-can-be-tricky'><b>5.14.</b> Termination Can be Tricky</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-1'><b>5.15.</b> Avoiding Infinite Loops</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-2'><b>5.16.</b> Avoiding Infinite Loops</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-3'><b>5.17.</b> Avoiding Infinite Loops</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-4'><b>5.18.</b> Avoiding Infinite Loops</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-is-easy-in-practice'><b>5.19.</b> Avoiding Infinite Loops is Easy (in Practice)</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-is-easy-in-practice-1'><b>5.20.</b> Avoiding Infinite Loops is Easy (in Practice)</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-is-easy-in-practice-2'><b>5.21.</b> Avoiding Infinite Loops is Easy (in Practice)</a></li>
<li><a href='04-termination.html#avoiding-infinite-loops-is-easy-in-practice-3'><b>5.22.</b> Avoiding Infinite Loops is Easy (in Practice)</a></li>
<li><a href='04-termination.html#outline'><b>5.23.</b> Outline</a></li>
<li><a href='04-termination.html#outline-1'><b>5.24.</b> Outline</a></li>
</ul>
<li><a href='05-reflection.html'><b>6.</b>Types as Theorems, Programs as Proofs</a></li>
<ul class='section'>
<li><a href='05-reflection.html#types-as-theorems'><b>6.1.</b> Types As Theorems</a></li>
<li><a href='05-reflection.html#types-as-theorems-example'><b>6.2.</b> Types as Theorems: Example</a></li>
<li><a href='05-reflection.html#programs-as-proofs-example'><b>6.3.</b> Programs As Proofs: Example</a></li>
<li><a href='05-reflection.html#types-as-theorems-1'><b>6.4.</b> Types As Theorems</a></li>
<li><a href='05-reflection.html#types-as-theorems-example-1'><b>6.5.</b> Types as Theorems: Example</a></li>
<li><a href='05-reflection.html#programs-as-proofs-example-1'><b>6.6.</b> Programs As Proofs: Example</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-1'><b>6.7.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#those-proofs-were-boring'><b>6.8.</b> Those Proofs were Boring</a></li>
<li><a href='05-reflection.html#those-proofs-were-boring-1'><b>6.9.</b> Those Proofs were Boring</a></li>
<li><a href='05-reflection.html#theorems-about-functions'><b>6.10.</b> Theorems about Functions</a></li>
<li><a href='05-reflection.html#refinement-reflection'><b>6.11.</b> Refinement Reflection</a></li>
<li><a href='05-reflection.html#reflect-function-into-output-type'><b>6.12.</b> Reflect Function into Output Type</a></li>
<li><a href='05-reflection.html#reflect-function-into-output-type-1'><b>6.13.</b> Reflect Function into Output Type</a></li>
<li><a href='05-reflection.html#reflection-at-result-type'><b>6.14.</b> Reflection at Result Type</a></li>
<li><a href='05-reflection.html#structuring-proofs-as-calculations'><b>6.15.</b> Structuring Proofs as Calculations</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-2'><b>6.16.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#reusing-proofs-functions-as-lemmas'><b>6.17.</b> Reusing Proofs: Functions as Lemmas</a></li>
<li><a href='05-reflection.html#reusing-proofs-functions-as-lemmas-1'><b>6.18.</b> Reusing Proofs: Functions as Lemmas</a></li>
<li><a href='05-reflection.html#reusing-proofs-functions-as-lemmas-2'><b>6.19.</b> Reusing Proofs: Functions as Lemmas</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-3'><b>6.20.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-4'><b>6.21.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#proof-by-logical-evaluation'><b>6.22.</b> Proof by Logical Evaluation</a></li>
<li><a href='05-reflection.html#proof-by-logical-evaluation-1'><b>6.23.</b> Proof by Logical Evaluation</a></li>
<li><a href='05-reflection.html#proof-by-logical-evaluation-2'><b>6.24.</b> Proof by Logical Evaluation</a></li>
<li><a href='05-reflection.html#proof-by-induction'><b>6.25.</b> Proof by Induction</a></li>
<li><a href='05-reflection.html#proof-by-induction-1'><b>6.26.</b> Proof by Induction</a></li>
<li><a href='05-reflection.html#proof-by-induction-2'><b>6.27.</b> Proof by Induction</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-5'><b>6.28.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-6'><b>6.29.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-7'><b>6.30.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#theorems-about-data'><b>6.31.</b> Theorems about Data</a></li>
<li><a href='05-reflection.html#theorems-about-data-associativity-of-append'><b>6.32.</b> Theorems about Data: Associativity of <code>append</code></a></li>
<li><a href='05-reflection.html#case-study-mapreduce'><b>6.33.</b> Case Study: MapReduce</a></li>
<li><a href='05-reflection.html#case-study-mapreduce-1'><b>6.34.</b> Case Study: MapReduce</a></li>
<li><a href='05-reflection.html#reduce-theorem'><b>6.35.</b> Reduce Theorem</a></li>
<li><a href='05-reflection.html#types-as-theorems-programs-as-proofs-8'><b>6.36.</b> Types as Theorems, Programs as Proofs</a></li>
<li><a href='05-reflection.html#outline'><b>6.37.</b> Outline</a></li>
<li><a href='05-reflection.html#outline-1'><b>6.38.</b> Outline</a></li>
</ul>
<li><a href='06-concl.html'><b>7.</b>LiquidHaskell</a></li>
<ul class='section'>
<li><a href='06-concl.html#liquidhaskell-1'><b>7.1.</b> LiquidHaskell</a></li>
<li><a href='06-concl.html#liquidhaskell-2'><b>7.2.</b> LiquidHaskell</a></li>
<li><a href='06-concl.html#evaluation'><b>7.3.</b> Evaluation</a></li>
<li><a href='06-concl.html#recap-refinement-types'><b>7.4.</b> Recap: Refinement Types</a></li>
<li><a href='06-concl.html#recap-refinement-types-1'><b>7.5.</b> Recap: Refinement Types</a></li>
<li><a href='06-concl.html#recap-refinement-types-2'><b>7.6.</b> Recap: Refinement Types</a></li>
<li><a href='06-concl.html#recap-refinement-types-3'><b>7.7.</b> Recap: Refinement Types</a></li>
<li><a href='06-concl.html#many-friends-many-directions'><b>7.8.</b> Many Friends, Many Directions</a></li>
<li><a href='06-concl.html#thank-you'><b>7.9.</b> Thank You!</a></li>
</ul>
</ul>
</div>
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<div id="program-0" class="programbox">{-@ LIQUID "--no-termination" @-}
{-@ LIQUID "--short-names" @-}
module Examples
( sum
, sum'
, sum''
, average
-- , insertSort
, length
)
where
import Prelude hiding (foldr, map, sum, length, (!))
data Vector a
range :: Int -> Int -> [Int]
at :: Vector a -> Int -> a
at = undefined
size :: Vector a -> Int
size = undefined</div>
</div>
</div>
<div id="slide-0" class="slide">
<h2 id="refinement-types-by-example">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p>Verification</p>
<p>Inference</p>
<p>Collections & HOFs</p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-1" class="slide">
<h2 id="refinement-types-by-example-1">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Specifications</strong></p>
<p>Verification</p>
<p>Inference</p>
<p>Collections & HOFs</p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-2" class="slide">
<h2 id="refinement-types-by-example-2">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<div class="mybreak">
<br>
</div>
<p><strong>Property: In-bounds Array Access</strong></p>
</div>
<div id="slide-3" class="slide">
<h2 id="refinement-types-by-example-3">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<div class="mybreak">
<br>
</div>
<p><strong>Property: In-bounds Array Access</strong></p>
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<div id="program-1" class="programbox">{-@ measure vlen :: Vector a -> Int @-}</div>
</div>
</div>
<div id="slide-4" class="slide">
<h2 id="specifications-pre-conditions">Specifications: Pre-Conditions</h2>
<div class="mybreak">
<br>
</div>
<p>What does a function <strong>require</strong> for correct execution?</p>
<div class="mybreak">
<br>
</div>
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<div id="program-2" class="programbox">{-@ at :: vec:Vector a -> {i:Nat| i < vlen vec} -> a @-}</div>
</div>
<div class="mybreak">
<br>
</div>
<p>Refinement on the function's <strong>input type</strong></p>
</div>
<div id="slide-5" class="slide">
<h2 id="specifications-post-conditions">Specifications: Post-Conditions</h2>
<div class="mybreak">
<br>
</div>
<p>What does a function <strong>ensure</strong> about its result?</p>
<div class="mybreak">
<br>
</div>
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<div id="program-3" class="programbox">{-@ size :: v:Vector a -> {n:Int | n == vlen v} @-}</div>
</div>
<div class="mybreak">
<br>
</div>
<p>Refinement on the function's <strong>output type</strong></p>
</div>
<div id="slide-6" class="slide">
<h2 id="refinement-types-by-example-4">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p><strong>Verification</strong></p>
<p>Inference</p>
<p>Collections & HOFs</p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-7" class="slide">
<h2 id="verification-vector-sum">Verification: Vector Sum</h2>
<div class="mybreak">
<br>
</div>
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<div id="program-4" class="programbox">sum :: Vector Int -> Int
sum v = loop 0
where
{-@ loop :: Nat -> Int @-}
loop i
| i <= size v = at v i + loop (i + 1)
| otherwise = 0</div>
</div>
<div class="mybreak">
<br>
</div>
<p>Oops! What gives?</p>
</div>
<div id="slide-8" class="slide">
<h2 id="verification-vector-sum-1">Verification: Vector Sum</h2>
<p><img src="img/sum-code-numbers.png" height=150px></p>
</div>
<div id="slide-9" class="slide">
<h2 id="verification-vector-sum-2">Verification: Vector Sum</h2>
<p><img src="img/sum-code-numbers.png" height=150px></p>
<p><strong>Verification Conditions</strong></p>
<p><span class="math display">\[\begin{array}{lll}
\True
& \Rightarrow v = 0
& \Rightarrow 0 \leq v
& \mbox{(A)} \\
0 \leq i \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i + 1
& \Rightarrow 0 \leq v
& \mbox{(B)} \\
0 \leq i \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i
& \Rightarrow 0 \leq v < \mathit{vlen}\ v
& \mbox{(C)} \\
\end{array}\]</span></p>
</div>
<div id="slide-10" class="slide">
<h2 id="refinement-types-by-example-5">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p>Verification</p>
<p><strong>Inference</strong></p>
<p>Collections & HOFs</p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-11" class="slide">
<h2 id="inference">Inference</h2>
<p><br></p>
<pre><code>The more interesting your types get,
the less fun it is to write them down.
-- Benjamin Pierce</code></pre>
</div>
<div id="slide-12" class="slide">
<h2 id="inference-vector-sum">Inference: Vector Sum</h2>
<div class="mybreak">
<br>
</div>
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<div id="program-5" class="programbox">sum' :: Vector Int -> Int
sum' v = loop 0
where
{-@ loop :: _ -> _ @-}
loop i
| i < size v = at v i + loop (i + 1)
| otherwise = 0</div>
</div>
</div>
<div id="slide-13" class="slide">
<h2 id="inference-vector-sum-1">Inference: Vector Sum</h2>
<p><br></p>
<p>Not magic, just <strong>Abstract Interpretation</strong></p>
</div>
<div id="slide-14" class="slide">
<h2 id="inference-vector-sum-2">Inference: Vector Sum</h2>
<p><br></p>
<p>Not magic, just Abstract Interpretation</p>
<p>Represent <strong>unknown refinements</strong> with <span class="math inline">\(\kvar{}{\cdot}\)</span> variables ...</p>
<p>... Solve resulting <strong>Horn Constraints</strong></p>
<p><br></p>
<p><a href="http://dl.acm.org/citation.cfm?id=1375602">[PLDI 2008]</a></p>
</div>
<div id="slide-15" class="slide">
<h2 id="inference-vector-sum-3">Inference: Vector Sum</h2>
<p><img src="img/sum-code-infer.png" height=150px></p>
</div>
<div id="slide-16" class="slide">
<h2 id="inference-vector-sum-4">Inference: Vector Sum</h2>
<p><img src="img/sum-code-infer.png" height=150px></p>
<p><strong>Horn Constraints</strong></p>
<p><span class="math display">\[\begin{array}{lll}
\True
& \Rightarrow v = 0
& \Rightarrow \kvar{}{v}
& \mbox{(A)} \\
\kvar{}{i} \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i + 1
& \Rightarrow \kvar{}{v}
& \mbox{(B)} \\
\kvar{}{i} \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i
& \Rightarrow 0 \leq v < \mathit{vlen}\ v
& \mbox{(C)} \\
\end{array}\]</span></p>
</div>
<div id="slide-17" class="slide">
<h2 id="inference-vector-sum-5">Inference: Vector Sum</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Horn Constraints</strong></p>
<p><span class="math display">\[\begin{array}{lll}
\True
& \Rightarrow v = 0
& \Rightarrow \kvar{}{v}
& \mbox{(A)} \\
\kvar{}{i} \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i + 1
& \Rightarrow \kvar{}{v}
& \mbox{(B)} \\
\kvar{}{i} \wedge n = \mathit{vlen}\ v \wedge i < n
& \Rightarrow v = i
& \Rightarrow 0 \leq v < \mathit{vlen}\ v
& \mbox{(C)} \\
\end{array}\]</span></p>
<div class="mybreak">
<br>
</div>
<p><strong>Synthesized Solution</strong></p>
<p><span class="math display">\[\kvar{}{v} = 0 \leq v\]</span></p>
</div>
<div id="slide-18" class="slide">
<h2 id="refinement-types-by-example-6">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p>Verification</p>
<p>Inference</p>
<p><strong>Collections & HOFs</strong></p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-19" class="slide">
<h2 id="collections-higher-order-functions">Collections & Higher-Order Functions</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Composition >> Recursion!</strong></p>
</div>
<div id="slide-20" class="slide">
<h2 id="collections-higher-order-functions-1">Collections & Higher-Order Functions</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Generic Sequences</strong></p>
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<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-6" class="programbox">range lo hi
| lo < hi = lo : range (lo + 1) hi
| otherwise = []</div>
</div>
<div class="mybreak">
<br>
</div>
<p>(What's a good type for <code>range</code>?)</p>
</div>
<div id="slide-21" class="slide">
<h2 id="collections-higher-order-functions-2">Collections & Higher-Order Functions</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Fold over Sequences</strong></p>
<div id="program-pane-7" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-7" class="programbox">foldr :: (a -> b -> b) -> b -> [a] -> b
foldr f b [] = b
foldr f b (x:xs) = f x (foldr f b xs)</div>
</div>
</div>
<div id="slide-22" class="slide">
<h2 id="collections-higher-order-functions-3">Collections & Higher-Order Functions</h2>
<div class="mybreak">
<br>
</div>
<p><strong>"Wholemeal" Vector Sum</strong></p>
<div id="program-pane-8" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-8" class="programbox">sum'' :: Vector Int -> Int
sum'' v = foldr add 0 is
where
add = \i n -> n + at v i
is = range 0 (size v)</div>
</div>
<p>Types make refinement inference <em>"just work"</em> ...</p>
</div>
<div id="slide-23" class="slide">
<h2 id="refinement-types-by-example-7">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p>Verification</p>
<p>Inference</p>
<p>Collections & HOFs</p>
<p><strong>Refinements for Datatypes</strong></p>
</div>
<div id="slide-24" class="slide">
<h2 id="example-list-average">Example: List <code>average</code></h2>
<div id="program-pane-9" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-9" class="programbox">{-@ average :: [Int] -> Int @-}
average xs = total `div` n
where
total = foldr (+) 0 xs
n = length xs
length :: [a] -> Int
length [] = 0
length (_:xs) = 1 + length xs</div>
</div>
<p>Yikes! <code>average</code> requires <strong>non-empty</strong> lists!</p>
</div>
<div id="slide-25" class="slide">
<h2 id="refinements-for-datatypes">Refinements for Datatypes</h2>
<p><strong>Lift</strong> (some) functions into specification logic:</p>
<div id="program-pane-10" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-10" class="programbox">{-@ measure length @-}</div>
</div>
<p>which lets us define a type alias</p>
<div id="program-pane-11" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-11" class="programbox">{-@ type ListNE a = {v:[a] | 0 < length v} @-}</div>
</div>
<div class="mybreak">
<br>
</div>
<p>Now lets go back and <em>fix</em> <code>average</code> ...</p>
</div>
<div id="slide-26" class="slide">
<h2 id="measures-yield-refined-constructors">Measures Yield Refined Constructors</h2>
<p><strong>Lift</strong> (some) functions into specification logic:</p>
<pre class="spec"><code>data [a] where
[] :: {v:[a] | length v = 0}
(:) :: a
-> t:[a]
-> {v:[a] | length v = 1 + length t}
</code></pre>
<p>Where <code>length</code> is <strong>uninterpreted</strong> in refinement Logic</p>
</div>
<div id="slide-27" class="slide">
<h2 id="example-map-over-lists">Example: <code>map</code> over Lists</h2>
<p>What's the problem here? (Lets fix it!)</p>
<div id="program-pane-12" class="welleditor" style="background:#fff; position:relative">
<!-- Verify Source -->
<button class="btn btn-xs btn-link actbutton" type="button" style="z-index:1"
ng-show="isUnknown" ng-click="verifySource()">
<span class="glyphicon glyphicon-play"></span>
</button>
<div id="program-12" class="programbox">{-@ hwAverage :: ListNE (a, Int) -> Int @-}
hwAverage nxs = average (map snd nxs)
{-@ map :: (a -> b) -> [a] -> [b] @-}
map f [] = []
map f (x:xs) = f x : map f xs</div>
</div>
</div>
<div id="slide-28" class="slide">
<h2 id="refinements-for-datatypes-1">Refinements for Datatypes</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Measures</strong></p>
<p>Specify properties as <em>functions over datatypes</em></p>
</div>
<div id="slide-29" class="slide">
<h2 id="refinements-for-datatypes-2">Refinements for Datatypes</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Measures</strong></p>
<p>Specify properties as <em>functions over datatypes</em></p>
<div class="mybreak">
<br>
</div>
<p><strong>Refined Constructors</strong></p>
<p>Instantiate constraints at <em>fold</em> (<code>C ...</code>) & <em>unfold</em> (<code>case-of</code>)</p>
</div>
<div id="slide-30" class="slide">
<h2 id="refinements-for-datatypes-3">Refinements for Datatypes</h2>
<div class="mybreak">
<br>
</div>
<p><strong>Measures</strong></p>
<p>Specify properties as functions over datatypes</p>
<div class="mybreak">
<br>
</div>
<p><strong>Refined Constructors</strong></p>
<p>Instantiate constraints at <em>fold</em> (<code>C ...</code>) & <em>unfold</em> (<code>case-of</code>)</p>
<div class="mybreak">
<br>
</div>
<p><strong>Automate verification of data types</strong></p>
</div>
<div id="slide-31" class="slide">
<h2 id="refinement-types-by-example-8">Refinement Types by Example</h2>
<div class="mybreak">
<br>
</div>
<p>Specifications</p>
<p>Verification</p>
<p>Inference</p>
<p>Collections & HOFs</p>
<p>Refinements for Datatypes</p>
</div>
<div id="slide-32" class="slide">
<h2 id="outline">Outline</h2>
<p><br></p>
<p><a href="01-intro.html">Motivation</a></p>
<p><a href="02-refinements.html">Refinements 101</a></p>
<p><a href="03-examples.html">Refinements by Example</a></p>
</div>
<div id="slide-33" class="slide">
<h2 id="outline-1">Outline</h2>
<p><br></p>
<p><a href="01-intro.html">Motivation</a></p>
<p><a href="02-refinements.html">Refinements 101</a></p>
<p><a href="03-examples.html">Refinements by Example</a></p>
<p><a href="04-termination.html">How to Avoid Infinite Loops</a></p>
</div>
</div>
</div>
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