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Kawa, the Java-based Scheme system

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5. Features of R5RS not implemented

Kawa implements all the required and optional features of R5RS, with the following exceptions.

The entire "numeric tower" is implemented. However, some transcendental function only work on reals. Integral function do not necessarily work on inexact (floating-point) integers. (The whole idea of "inexact integer" in R5RS seems rather pointless ...)

Also, call-with-current-continuation is only "upwards" (?). I.e. once a continuation has been exited, it cannot be invoked. These restricted continuations can be used to implement catch/throw (such as the examples in R4RS), but not co-routines or backtracking.

Kawa now does general tail-call elimination, but only if you use the flag --full-tail-calls. (Currently, the eval function itself is not fully tail-recursive, in violation of R5RS.) The --full-tail-calls flag is not on by default, partly because it is noticably slower (though I have not measured how much), and partly I think it is more useful for Kawa to be compilatible with standard Java calling conventions and tools. Code compiled with --full-tail-calls can call code compiled without it and vice versa.

Even without --full-tail-calls, if the compiler can prove that the procedure being called is the current function, then the tail call will be replaced by a jump. This means the procedure must be defined using a letrec, not a define (because the compiler does not know if someone might re-define a global definition), and there must be no assignments (using set!) to the procedure binding.

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