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2 Language

The Logic Evaluator reads a sequence of input lines; it continues with parsing and executing after every line read (i.e., in interactive mode, when the user has terminated a line with the Enter key). However, when executing as an applet embedded into an HTML page, the evaluator only runs while the focus is on the evaluator's input field such that this field is gray; if this field is not gray, execution is suspended. Applets may be reinitialized by pressing the keys Shift+Reload (Netscape Communicator) respectively Refresh (Microsoft Internet Explorer).

All responses of the evaluator are prefixed by the token `>':

fun f(x) = x;
> function f/1.

Any text starting with the token `//' is ignored until the end of a line is encountered:

fun f(x) = x; // this is the identity function
> function f/1.

Apart from that, the separation of input into lines has no significance:

fun f(x) = // this is the identity function
  x;
> function f/1.

We describe the grammar of the evaluator's language by an extended version of BNF where "{ Phrase }" denotes zero or more repetitions of Phrase and "[ Phrase ]" denotes zero or one occurrence of Phrase. Do not confuse the meta-symbols `{', `}', `[', `]' with the language symbols `{', `}', `[', `]' (which are typeset in a different font).