Tail Recursion

Tail recursion is recursion in which the results of recursive calls are returned immediately, rather than being used in some other way. These recursive calls are called tail calls. Tail calls are interesting because they can be compiled to a jump instruction rather than a call instruction, and the current stack frame can be re-used instead of pushing a new frame. In other words, tail-recursive functions are actually loops.

A common way to make a recursive function faster is to rewrite it in accumulator-passing style. Instead of using the call stack to remember what is to be done with the result of a recursive call, an additional argument called an accumulator is used to collect this information. For example, an accumulator for a tail-recursive function that reverses a list contains the already-seen list entries, in reverse order.

In Lean, only self-tail-calls are optimized into loops. In other words, two functions that each end with a tail call to the other will not be optimized.

Reference Counting and In-Place Updates

Rather than using a tracing garbage collector, as is done in Java, C#, and most JavaScript implementations, Lean uses reference counting for memory management. This means that each value in memory contains a field that tracks how many other values refer to it, and the run-time system maintains these counts as references appear or disappear. Reference counting is also used in Python, PHP, and Swift.

When asked to allocate a fresh object, Lean's run-time system is able to recycle existing objects whose reference counts are falling to zero. Additionally, array operations such as Array.set and Array.swap will mutate an array if its reference count is one, rather than allocating a modified copy. If Array.swap holds the only reference to an array, then no other part of the program can tell that it was mutated rather than copied.

Writing efficient code in Lean requires the use of tail recursion and being careful to ensure that large arrays are used uniquely. While tail calls can be identified by inspecting the function's definition, understanding whether a value is referred to uniquely may require reading the whole program. The debugging helper dbgTraceIfShared can be used at key locations in the program to check that a value is not shared.

Proving Programs Correct

Rewriting a program in accumulator-passing style, or making other transformations that make it run faster, can also make it more difficult to understand. It can be useful to keep the original version of the program that is more clearly correct, and then use it as an executable specification for the optimized version. While techniques such as unit testing work just as well in Lean as in any other language, Lean also enables the use of mathematical proofs that completely ensure that both versions of the function return the same result for all possible inputs.

Typically, proving that two functions are equal is done using function extensionality (the funext tactic), which is the principle that two functions are equal if they return the same values for every input. If the functions are recursive, then induction is usually a good way to prove that their outputs are the same. Usually, the recursive definition of the function will make recursive calls on one particular argument; this argument is a good choice for induction. In some cases, the induction hypothesis is not strong enough. Fixing this problem usually requires thought about how to construct a more general version of the theorem statement that provides induction hypotheses that are strong enough. In particular, to prove that a function is equivalent to an accumulator-passing version, a theorem statement that relates arbitrary initial accumulator values to the final result of the original function is needed.

Safe Array Indices

The type Fin n represents natural numbers that are strictly less than n. Fin is short for "finite". As with subtypes, a Fin n is a structure that contains a Nat and a proof that this Nat is less than n. There are no values of type Fin 0.

If arr is an Array α, then Fin arr.size always contains a number that is a suitable index into arr. Many of the built-in array operators, such as Array.swap, take Fin values as arguments rather than separated proof objects.

Lean provides instances of most of the useful numeric type classes for Fin. The OfNat instances for Fin perform modular arithmetic rather than failing at compile time if the number provided is larger than the Fin can accept.

Provisional Proofs

Sometimes, it can be useful to pretend that a statement is proved without actually doing the work of proving it. This can be useful when making sure that a proof of a statement would be suitable for some task, such as a rewrite in another proof, determining that an array access is safe, or showing that a recursive call is made on a smaller value than the original argument. It's very frustrating to spend time proving something, only to discover that some other proof would have been more useful.

The sorry tactic causes Lean to provisionally accept a statement as if it were a real proof. It can be seen as analogous to a stub method that throws a NotImplementedException in C#. Any proof that relies on sorry includes a warning in Lean.

Be careful! The sorry tactic can prove any statement, even false statements. Proving that 3 < 2 can cause an out-of-bounds array access to persist to runtime, unexpectedly crashing a program. Using sorry is convenient during development, but keeping it in the code is dangerous.

Proving Termination

When a recursive function does not use structural recursion, Lean cannot automatically determine that it terminates. In these situations, the function could just be marked partial. However, it is also possible to provide a proof that the function terminates.

Partial functions have a key downside: they can't be unfolded during type checking or in proofs. This means that Lean's value as an interactive theorem prover can't be applied to them. Additionally, showing that a function that is expected to terminate actually always does terminate removes one more potential source of bugs.

The termination_by clause that's allowed at the end of a function can be used to specify the reason why a recursive function terminates. The clause maps the function's arguments to an expression that is expected to be smaller for each recursive call. Some examples of expressions that might decrease are the difference between a growing index into an array and the array's size, the length of a list that's cut in half at each recursive call, or a pair of lists, exactly one of which shrinks on each recursive call.

Lean contains proof automation that can automatically determine that some expressions shrink with each call, but many interesting programs will require manual proofs. These proofs can be provided with have, a version of let that's intended for locally providing proofs rather than values.

A good way to write recursive functions is to begin by declaring them partial and debugging them with testing until they return the right answers. Then, partial can be removed and replaced with a termination_by clause. Lean will place error highlights on each recursive call for which a proof is needed that contains the statement that needs to be proved. Each of these statements can be placed in a have, with the proof being sorry. If Lean accepts the program and it still passes its tests, the final step is to actually prove the theorems that enable Lean to accept it. This approach can prevent wasting time on proving that a buggy program terminates.