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### 2.7.16 Interfaces to External Code

A few other combinators have been incorporated into the virtual machine as alternatives to the style of interactive applications described in Output From Interactive Applications. These make it possible to interface with external libraries and applications either by a simple function call, or by executing a run-time generated transducer as described previously. In either case, there is no need for any particular command line options to specify interactive invocation, nor for the application to be designed that way from the outset. Existing virtual code applications may therefore be enhanced to make use of these features without radical changes.

To account for these additional capabilities, it is not entirely adequate to continue defining the virtual machine semantics in terms of a mathematical function, but it is done nevertheless due to the lack of any appealing alternative. Although most library functions are in fact functions in the sense that their outputs are determined by their arguments, they defy a concise specification within the present mathematical framework, especially insofar as they may involve finite precision floating point numbers. More problematically, the effect of interaction with a shell is neither well defined nor deterministic. The descriptions that follow presuppose a computational procedure associated with the following definitions but leave its exact nature unspecified.

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