enso/docs/semantics/tail-call-optimization.md

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developer-doc	Managed Resources	semantics	resources finalization cleanup	10

Tail Call Optimization

Tail call optimization is a powerful technique for optimizing functional programs. It allows transforming recursive functions of certain shapes into loops, removing unnecessary intermediate function calls and saving stack space. This document outlines the usage and semantics of tail call optimization in Enso.

• Tail Calls
• Usage

Tail Calls

A Tail Call is a function call occurring as the last statement in a function body, i.e. an expression whose value is guaranteed not to be depended upon by the function itself. For example,

sum_1 n = if n == 0 then 0 else 1 + sum_1 n-1

sum_2 n acc = if n == 0 then acc else @Tail_Call sum_2 n-1 acc+n

In the code snippet above, only the sum_2 function is tail recursive. The result of calling sum_2 recursively is not depended upon by sum_2 itself or the definition of if_then_else method on booleans. On the other hand, sum_1 needs to know the value of its recursive call in order to perform the addition operation. It is advised that functions that can be expressed with tail-calls are implemented that way. Using tail call optimization, will lead to sum_2 being orders of magnitude faster than sum_1. Moreover, for n = 100000000, sum_1 will allocate a hundred million stack frames (over a gigabyte, likely resulting in a stack overflow error), while sum_2 is an allocation-free loop.

Usage

Enso does not currently perform automatic tail call detection and defers the optimization decisions to the user. To mark a function call as a tail call, the @Tail_Call annotation must be used. Note that if the annotation is placed incorrectly, it may either be reported as a warning by the compiler, or silently ignored if such analysis is impossible to perform due to the compiler's limited static analysis capabilities. However, it is guaranteed that a wrongly placed @Tail_Call annotation will not lead to incorrect runtime behavior.

If the @Tail_Call annotation is not placed, the call will be treated as a standard on-stack function call.

For example, the following code reverses a list in a tail recursive fashion:

reverse list =
    go list result = case list of
        Nil -> result
        Cons head tail -> @Tail_Call go tail (Cons head result)
    result = go list Nil
    result

Note the placement of @Tail_Call in the recursive branch of go. It is placed correctly, marking the last operation in a function, and therefore go will be interpreted as a loop rather than a chain of function calls.

Debugging Tail Calls

The way go is wrapped in the example above is recommended for most uses. Using the assignment and return of a variable, rather than a direct call, guarantees that calls to reverse won't themselves be removed from the call stack and therefore greatly aids debugging.