A Capsule Summary of F*

F* is a dependently type programming language that aims to play several roles:

  • A general purpose programming language, which encourages higher-order functional programming with effects, in the tradition of the ML family of languages.

  • A compiler, which translates F* programs to OCaml or F#, and even C or Wasm, for execution.

  • A proof assistant, in which to state and prove properties of programs.

  • A program verification engine, leveraging SMT solvers to partially automate proofs of programs.

  • A metaprogramming system, supporting the programmatic construction of F* programs and proof automation procedures.

To achieve these goals, the design of F* revolves around a few key elements, described below. Not all of this may make sense to you—that’s okay, you’ll learn about it as we go.

  • A core language of total functions with full dependent types, including an extensional form of type conversion, indexed inductive types, and pattern matching, recursive functions with semantic termination checking, dependent refinement types and subtyping, and polymorphism over a predicative hierarchy of universes.

  • A system of user-defined indexed effects, for modeling, encapsulating, and statically reasoning about various forms of computational effects, including a primitive notion of general recursion and divergence, as well as an open system of user-defined effects, with examples including state, exceptions, concurrency, algebraic effects, and several others.

  • A built-in encoding of a classical fragment of F*’s logic into the first order logic of an SMT solver, allowing many proofs to be automatically discharged.

  • A reflection within F* of the syntax and proof state of F*, enabling Meta-F* programs to manipulate F* syntax and proof goals and for users to build proofs interactively with tactics.

DSLs Embedded in F*

In practice, rather than a single language, the F* ecosystem is also a collection of domain-specific languages (DSLs). A common use of F* is to embed within it programming languages at different levels of abstraction or for specific programming tasks, and for the embedded language to be engineered with domain-specific reasoning, proof automation, and compilation backends. Some examples include:

  • Low*, an shallowly embedded DSL for sequential programming against a C-like memory model including explicit memory management on the stack and heap; a Hoare logic for partial correctness based on implicit dynamic frames; and a custom backend (Karamel) to compile Low* programs to C for further compilation by off-the-shelf C compilers.

  • EverParse, a shallow embedding of a DSL (layered on top of the Low* DSL) of parser and serializer combinators, for low-level binary formats.

  • Vale, a deeply embedded DSL for structured programming in a user-defined assembly language, with a Hoare logic for total correctness, and a printer to emit verified programs in a assembly syntax compatible with various standard assemblers.

  • Steel, a shallow embedding of concurrency as an effect in F*, with an extensible concurrent separation logic for partial correctness as a core program logic, and proof automation built using a combination of Meta-F* tactics, higher-order unification, and SMT.

To get a taste of F*, let’s dive right in with some examples. At this stage, we don’t expect you to understand these examples in detail, though it should give you a flavor of what is possible with F*.

F* is a dependently typed language

Dependently typed programming enables one to more precisely capture properties and invariants of a program using types. Here’s a classic example: the type vec a n represents an n-dimensional vector of a-typed elements; or, more simply, a list of n values each of type a. Like other dependently typed languages, F* supports inductively defined definitions of types.

type vec (a:Type) : nat -> Type =
  | Nil : vec a 0
  | Cons : #n:nat -> hd:a -> tl:vec a n -> vec a (n + 1)

Operations on a vectors can be given types that describe their behavior in terms of vector lengths.

For example, here’s a recursive function append to concatenate two vectors. Its type shows that the resulting vector has a length that is the sum of the lengths of the input vectors.

let rec append #a #n #m (v1:vec a n) (v2:vec a m)
  : vec a (n + m)
  = match v1 with
    | Nil -> v2
    | Cons hd tl -> Cons hd (append tl v2)

Of course, once a function like append is defined, it can be used to define other operations and its type helps in proving further properties. For example, it’s easy to show that reversing a vector does not change its length.

let rec reverse #a #n (v:vec a n)
  : vec a n
  = match v with
    | Nil -> Nil
    | Cons hd tl -> append (reverse tl) (Cons hd Nil)

Finally, to get an element from a vector, one can program a selector whose type also includes a refinement type to specify that the index i is less than the length of the vector.

let rec get #a #n (i:nat{i < n}) (v:vec a n)
  : a
  = let Cons hd tl = v in
    if i = 0 then hd
    else get (i - 1) tl

While examples like this can be programmed in other dependently typed languages, they can often be tedious, due to various technical restrictions. F* provides a core logic with a more flexible notion of equality to make programming and proving easier. For now, a takeaway is that dependently typed programming patterns that are quite technical in other languages are often fairly natural in F*. You’ll learn more about this in a later chapter.

F* supports user-defined effectful programming

While functional programming is at the heart of the language, F* is about more than just pure functions. In fact, F* is a Turing complete language. That this is even worth mentioning may come as a surprise to readers with a background in general-purpose programming languages like C# or Scala, but not all dependently typed languages are Turing complete, since nontermination can break soundness. However, F* supports general recursive functions and non-termination in a safe manner, without compromising soundness.

Beyond nontermination, F* supports a system of user-defined computational effects which can be used to model a variety of programming idioms, including things like mutable state, exceptions, concurrency, IO, etc.

Here below is some code in an F* dialect called Low* which provides a sequential, imperative C-like programming model with mutable memory. The function malloc_copy_free allocates an array dest, copies the contents of an array of bytes src into a dest, deallocates src and returns dest.

let malloc_copy_free (len:uint32 { 0ul < len })
                     (src:lbuffer len uint8)
  : ST (lbuffer len uint8)
       (requires fun h -> 
         live h src /\
         freeable src)
       (ensures fun h0 dest h1 -> 
         live h1 dest /\
         (forall (j:uint32). j < len ==> get h0 src j == get h1 dest j))
  = let dest = malloc 0uy len in
    memcpy len 0ul src dest;
    free src;

It’ll take us until much later to explain this code in full detail, but here are two main points to take away:

  • The type signature of the procedure claims that under specific constraints on a caller, malloc_copy_free is safe to execute (e.g., it does not read outside the bounds of allocated memory) and that it is correct (i.e., that it successfully copies src to dest without modifying any other memory)

  • Given the implementation of a procedure, F* actually builds a mathematical proof that it is safe and correct with respect to its signature.

While other program verifiers offer features similar to what we’ve used here, a notable thing about F* is that the semantics of programs with side effects (like reading and writing memory) is entirely encoded within F*’s logic using a system of user-defined effects.

Whereas malloc_copy_free is programmed in Low* and specified using a particular kind of Floyd-Hoare logic, there’s nothing really special about it in F*.

Here, for example, is a concurrent program in another user-defined F* dialect called Steel. It increments two heap-allocated references in parallel and is specified for safety and correctness in concurrent separation logic, a different kind of Floyd-Hoare logic than the one we used for malloc_copy_free.

let par_incr (#v0 #v1:erased int) (r0 r1:ref int)
  : SteelT _ (pts_to r0 v0 `star` pts_to r1 v1)
             (fun _ -> pts_to r0 (v0 + 1) `star` pts_to r1 (v1 + 1))
  = par (incr r0) (incr r1)

As an F* user, you can choose a programming model and a suite of program proof abstractions to match your needs. You’ll learn more about this in the section on user-defined effects.

F* proofs use SMT solving, symbolic computation and tactics

Stating a theorem or lemma in F* amounts to declaring a type signature and a doing a proof corresponds to providing an implementation of that signature. Proving theorems can take a fair bit of work by a human and F* seeks to reduce that burden, using a variety of techniques.

SMT Solving

Proving even a simple program often involves proving dozens or hundreds of small facts, e.g., proving that bounded arithmetic doesn’t overflow, or that ill-defined operations like divisions by zero never occur. All these little proofs can quickly overwhelm a user.

The main workhorse for proofs in F* is an automated theorem prover, known as a Satisfiability Modulo Theories, or SMT, solver. The F* toolchain integrates the Z3 SMT Solver.

By default, the F* typechecker collects all the facts that must be proven in a program and encodes them to the SMT solver, an engine that is capable of solving problems in various combinations of mathematical logics—F* encodes problems to Z3 in a combination of first-order logic, with uninterpreted functions and integer arithmetic.

Z3 is remarkably effective at solving the kinds of problems that F* generates for it. The result is that some F* programs enjoy a high level of automation, e.g., in memcpy, we specified a pre- and postcondition and a loop invariant, and the system took care of all the remaining proofs.

You’ll learn more about how to use leverage Z3 to prove theorems in F* in this chapter.

That said, Z3 cannot solve all problems that F* feeds to it. As such, F* offers several other mechanisms with varying levels of user control.

Symbolic computation

SMT solvers are great at proofs that involve equational rewriting, but many proofs can be done simply by computation. In fact, proofs by computation are a distinctive feature of many dependently typed languages and F* is no exception.

As a very simple example, consider proving that pow2 12 == 4096, where pow2 is the recursive function shown below.

let rec pow2 (n:nat) : nat =
  if n = 0 then 1
  else 2 * pow2 (n - 1)

let proof_by_normalization ()
  : Lemma (pow2 12 == 4096)
  = normalize_term_spec (pow2 12)

An easy way to convince F* of this fact is to ask it (using normalize_term_spec) to simply compute the result of pow2 12 on an interpreter that’s part of the F* toolchain, which it can do instantly, rather than relying on an SMT solvers expensive equational machinery to encode the reduction of a recursive function.

This reduction machinery (called the normalizer) is capable not only of fully computing terms like pow2 12 to a result, but it can also partially reduce symbolic F* terms, as shown in the proof below.

let partially_reduce_fold_right f more
  : (fold_right f ([1;2;3]@more) 0 == f 1 (f 2 (f 3 (fold_right f more 0))))
  =  _ by (T.trefl())

The proof invokes the F* normalizer from a tactic called T.trefl, another F* feature that we’ll review quickly, next.

Tactics and Metaprogramming

Finally, for complete control over a proof, F* includes a powerful tactic and metaprogramming system.

Here’s a simple example of an interactive proof of a simple fact about propositions using F* tactics.

let a_very_explicit_tactic_proof (a b : prop) : (a ==> b ==> b /\ a)
  = _ by
       (let ha = implies_intro () in
        let hb = implies_intro () in
        split ();
        hyp hb;
        hyp ha;
        qed ())

This style of proof is similar to what you might find in systems like Coq or Lean. An F* tactic is just an F* program that can manipulate F* proof states. In this case, to prove the theorem a ==> b ==> (b /\ a), we apply commands to transform the proof state by applying the rules of propositional logic, building a proof of the theorem.

Tactics are an instance of a more general metaprogramming system in F*, which allows an F* program to generate other F* programs.

F* programs compile to OCaml and F#, C and Wasm

Of course, you’ll want a way to actually execute the programs you write. For this, F* provides several ways to compile a program to other languages for execution, including support to compile programs to OCaml, F#, C and Wasm.

As such, a common way to use F* is to develop critical components of larger software systems in it, use its proof-oriented facilities to obtain assurances about those components, and then to integrate those formally proven components into a larger system by compiling the F* program to C, OCaml, or F# and linking the pieces together.

In this case, using a tool called KaRaMeL, a compiler used with F*, we can produce the following C code for memcpy.

uint8_t *MemCpy_malloc_copy_free(uint32_t len, uint8_t *src)
  KRML_CHECK_SIZE(sizeof (uint8_t), len);
  uint8_t *dest = KRML_HOST_CALLOC(len, sizeof (uint8_t));
  MemCpy_memcpy(len, (uint32_t)0U, src, dest);
  return dest;

Notice that the code we get contains no additional runtime checks: the detailed requires and ensures clauses are all gone and what’s left is just a plain C code. Later we’ll see how to actually write loops, so that you’re not left with recursive functions in C. The point is that all the proof and specification effort is done before the program is compiled, imposing no runtime overhead at all.

To F*, or not to F*?

We’ve quickly seen a bit of what F* has to offer—that may have been bit overwhelming, if you’re new to program proofs. So, you may be wondering now about whether it’s worth learning F* or not. Here are some things to consider.

If you like programming and want to get better at it, no matter what your level is, learning about program proofs will help. Proving a program, or even just writing down a specification for it, forces you to think about aspects of your program that you may never have considered before. There are many excellent resources available to learn about program proofs, using a variety of other tools, including some of the following:

All of these are excellent resources and each tool has unique offerings. This book about F* offers a few unique things too. We discuss a few pros and cons, next.

Dependent Types and Extensionality

F*’s dependent types are similar in expressiveness to Coq, Lean, Agda, or Idris, i.e., the expressive power allows formalizing nearly all kinds of mathematics. What sets F* apart from these other languages (and more like Nuprl) is its extensional notion of type equality, making many programming patterns significantly smoother in F* (cf. the vector example). However, this design also makes typechecking in F* undecidable. The practical consequences of this are that F* typechecker can time-out and refuse to accept your program. Other dependently typed languages have decidable typechecking, though they can, in principle, take arbitrarily long to decide whether or not your program is type correct.

A Variety of Proof Automation Tools

F*’s use of an SMT solver for proof automation is unique among languages with dependent types, though in return, one needs to also trust the combination of F* and Z3 to believe in the validity of an F* proof. Isabelle/HOL provides similar SMT-assisted automation (in its Sledgehammer tool), for the weaker logic provided by HOL, though Sledgehammer’s design ensures that the SMT solver need not be trusted. F*’s use of SMT is also similar to what program verifiers like Dafny and Liquid Haskell offer. However, unlike their SMT-only proof strategies, F*, like Coq and Lean, also provides symbolic reduction, tactics, and metaprogramming. That said, F*’s tactic and metaprogramming engines are less mature than other systems where tactics are the primary way of conducting proofs.

A Focus on Programming

Other dependently typed languages shine in their usage in formalizing mathematics—Lean’s mathlib and Coq’s Mathematical Components are two great examples. In comparison, to date, relatively little pure mathematics has been formalized in F*. Rather, F*, with its focus on effectful programming and compilation to mainstream languages like C, has been used to it produce industrial-grade high-assurance software, deployed in settings like the Windows and Linux kernels, among many others.

Maturity and Community

Isabelle/HOL and Coq are mature tools that have been developed and maintained for many decades, have strong user communities in academia, and many sources of documentation. Lean’s community is growing fast and also has excellent tools and documentation. F* is less mature, its design has been the subject of several research papers, making it somewhat more experimental. The F* community is also smaller, its documentation is more sparse, and F* users are usually in relatively close proximity to the F* development team. However, F* developments also have a good and growing track record of industrial adoption.

A Bit of F* History

F* is an open source project at GitHub by researchers at a number of institutions, including Microsoft Research, MSR-Inria, Inria, Rosario, and Carnegie-Mellon.

The name The F in F* is a homage to System F ( which was the base calculus of an early version of F*. We’ve moved beyond it for some years now, however. The F part of the name is also derived from several prior languages that many authors of F* worked on, including Fable, F7, F9, F5, FX, and even F#.

The “*” was meant as a kind of fixpoint operator, and F* was meant to be a sort of fixpoint of all those languages. The first version of F* also had affine types and part of the intention then was to use affine types to encode separation logic—so the “*” was also meant to evoke the separation logic “*”. But, the early affine versions of F* never really did have separation logic. It took until almost a decade later to have a separation logic embedded in F* (see Steel), though without relying on affine types.