Chicken (Scheme Implementation) - Design

Design

Like many Scheme compilers, Chicken uses standard C as an intermediate language. A Scheme program is translated into C by the Chicken compiler, and then a C compiler translates the C program into machine code for the target architecture, producing an executable program. The universal availability of C makes it ideal for this purpose.

Chicken's design was inspired by a 1994 paper by Henry Baker that outlined an innovative strategy for Scheme compilation into C. A scheme program is compiled into C functions. These C functions never reach the return statement; instead, they call a new continuation when complete. These continuations are C functions themselves and are passed on as extra arguments to other C functions. They are calculated by the compiler.

So far, this is the essence of continuation-passing style. Baker's novel idea is to use the C stack for the Scheme heap. Hence, normal C stack operations such as automatic variable creation, variable-sized array allocation, and so on can be used. When the stack fills up (that is, the stack pointer reaches the top of the stack), a garbage collection can be initiated. The design used is a copying garbage collector originally devised by C.J. Cheney, which copies all live continuations and other live objects to the heap. Despite this, the C code does not copy C stack frames, only Scheme objects, so it does not require knowledge of the C implementation.

In full, the Scheme heap consists of the C stack as the nursery together with the two heaps required by the generational garbage collector. This approach gives the speed of the C stack for many operations, and it allows the use of continuations as simple calls to C functions. Further, Baker's solution guarantees asymptotic tail recursive behavior, as required by the Scheme language standard. The implementation in the Chicken scheme compiler is even asymptotically safe for space.