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Dafny is a programming language that is designed to support the development of correct software. It includes built-in specification constructs and automated proof features, which allow developers to formally verify the correctness of their code. Dafny's syntax is similar to C# or Java, making it relatively easy for developers familiar with those languages to learn. The key feature of Dafny is its ability to automatically prove properties about programs, such as ensuring that a function meets its specified behavior or proving that an algorithm doesn't have any bugs. This makes it particularly useful in critical systems where correctness is paramount. In addition, Dafny supports imperative programming paradigms including mutable variables and loops along with functional programming concepts like inductive datatypes and recursion. It also has support for object-oriented design through classes and interfaces. Dafny was developed by Microsoft Research as part of their efforts towards improving software reliability.

Ask the right questions secure the right Dafny talent among an increasingly shrinking pool of talent.

First 20 minutes

General Dafny app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in Dafny application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

What is the purpose of loop invariants in Dafny?

Loop invariants in Dafny are used to help the verifier reason about the loop's behavior. They are conditions that are true before and after each iteration of the loop.

How would you define a method in Dafny?

In Dafny, a method is defined using the 'method' keyword followed by the method name, parameters enclosed in parentheses, and the method body enclosed in curly braces.

What is the role of the 'ensures' clause in Dafny?

The 'ensures' clause in Dafny is used to specify the postconditions of a method. It describes the state of variables after the method's execution.

How would you use the 'requires' clause in Dafny?

The 'requires' clause in Dafny is used to specify the preconditions of a method. It describes the conditions that must be true before the method is called.

What are ghost variables in Dafny and how would you use them?

Ghost variables in Dafny are variables that are used for specification and verification purposes only. They do not affect the runtime behavior of the program.

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What you’re looking for early on

Has the candidate demonstrated a strong understanding of Dafny?

Does the candidate have experience working on complex projects using Dafny?

Has the candidate shown problem-solving abilities, especially in the context of Dafny?

Does the candidate have a good understanding of Dafny's syntax and semantics?

Next 20 minutes

Specific Dafny development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

How would you specify that a method modifies a certain variable in Dafny?

In Dafny, you can specify that a method modifies a certain variable using the 'modifies' clause followed by the name of the variable.

What is the difference between 'assert' and 'assume' in Dafny?

In Dafny, 'assert' is used to state a fact that the verifier should prove, while 'assume' is used to state a fact that the verifier should assume is true without proving it.

How would you use arrays in Dafny?

In Dafny, arrays are used similarly to other programming languages. You can declare an array, assign values to its elements, and access its elements using indices.

What is the purpose of 'ghost' methods in Dafny?

Ghost methods in Dafny are methods that are used for specification and verification purposes only. They do not affect the runtime behavior of the program.

What are the differences between a 'lemma' and a 'method' in Dafny?

In Dafny, a 'lemma' is a fact that can be proved and used in other proofs, while a 'method' is a procedure that can be called to perform some action. Lemmas cannot modify the state of the program, while methods can.

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At this point, a skilled Dafny engineer should demonstrate strong problem-solving abilities, proficiency in Dafny programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with Dafny.

What does the following Dafny code do?
method Main() { var x := 5; var y := 7; print x + y; }
This code declares two integer variables x and y, assigns them the values 5 and 7 respectively, and then prints the sum of x and y, which is 12.
What is the purpose of the following Dafny code?
method Sort(a: array) modifies a { for i := 0 to a.Length { for j := i + 1 to a.Length { if a[i] > a[j] { var temp := a[i]; a[i] := a[j]; a[j] := temp; } } } }
This code sorts an array of integers in ascending order using the Bubble Sort algorithm.
What will be the output of the following Dafny code?
method Main() { var a := new int[5]; a[0] := 1; a[1] := 2; a[2] := 3; a[3] := 4; a[4] := 5; print a[2]; }
This code will output '3'. It creates an array of 5 integers, assigns values to each element, and then prints the third element of the array.
What does the following Dafny code do?
class Counter { var value: int; method Increment() { value := value + 1; } method GetValue(): int { return value; } }
This Dafny code defines a class named 'Counter' with a field 'value' and two methods. The 'Increment' method increases the value by 1 each time it is called, and the 'GetValue' method returns the current value.

Wrap-up questions

Final candidate for Dafny Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle Dafny application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

How would you use the 'forall' keyword in Dafny?

In Dafny, the 'forall' keyword is used to specify a universal quantification. It is used to state that a certain condition holds for all elements in a set.

What is the difference between 'exists' and 'forall' in Dafny?

In Dafny, 'exists' is used to specify an existential quantification, stating that there exists an element in a set that satisfies a certain condition. 'forall', on the other hand, is used to specify a universal quantification, stating that all elements in a set satisfy a certain condition.

How would you use inductive datatypes in Dafny?

In Dafny, inductive datatypes are used to define types that are built up from a finite number of cases. You can define an inductive datatype using the 'datatype' keyword, and you can create instances of the datatype using its constructors.

Dafny application related

Product Perfect's Dafny development capabilities

Beyond hiring for your Dafny engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in Dafny projects, and can engage in multiple capacities.

Dafny Consultive assessments

Dafny Application Conversion Assessment Dafny Application Integration Assessment Dafny Data & Database Integration Assessment

Custom Dafny development projects

Custom Dafny App Conversion Dafny Custom App Development Process App UI Design for Dafny Facelifts

Hiring guide for Dafny Engineers

Dafny Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the Dafny engineer at this point.

Dafny Consultive assessments

Custom Dafny development projects