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Abstract:

We consider "context free " specification of visual syntax-- with the aid of visual (i.e., 2-dimensional and user-defined) "lexicals". We argue that context-free visual syntax specification can go a long way towards specification of visual languages in the presence of certain intelligent tools. We focus on the input and output behavior of visual languages in demonstrating this point. The usual notion of term construction in a editor is extended by allowing sharing of sub-terms. We demonstrate the utility of this for constructing input terms (programs) of visual languages. We also allow such sharing of sub-terms in equations editors which are used to specify language semantics. This sharing not only allows the building of appropriate (visual) terms but also provides information on the desired view of an output term. We introduce a technique called "Share-Where " which is used to preserve layout information and demonstrate its use as well as its limitations. We then describe an extension to our formalism which allows dynamic input and output-- interaction during term execution. We demonstrate this with a full example for the FSA language to animate FSA term evaluation.

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