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4.2 Hairy function representation

Non-fixed-arg functions are represented using Optional-Dispatch. An Optional-Dispatch has an entry-point function for each legal number of optionals, and one for when extra args are present. Each entry point function is a simple lambda. The entry point function for an optional is passed the arguments which were actually supplied; the entry point function is expected to default any remaining parameters and evaluate the actual function body.

If no supplied-p arg is present, then we can do this fairly easily by having each entry point supply its default and call the next entry point, with the last entry point containing the body. If there are supplied-p args, then entry point function is replaced with a function that calls the original entry function with T’s inserted at the position of all the supplied args with supplied-p parameters.

We want to be a bit clever about how we handle arguments declared special when doing optional defaulting, or we will emit really gross code for special optionals. If we bound the arg specially over the entire entry-point function, then the entry point function would be caused to be non-tail-recursive. What we can do is only bind the variable specially around the evaluation of the default, and then read the special and store the final value of the special into a lexical variable which we then pass as the argument. In the common case where the default is a constant, we don’t have to special-bind at all, since the computation of the default is not affected by and cannot affect any special bindings.

Keyword and rest args are both implemented using a LEXPR-like “more args” convention. The More-Entry takes two arguments in addition to the fixed and optional arguments: the argument context and count. (ARG <context> <n>) accesses the N’th additional argument. Keyword args are implemented directly using this mechanism. Rest args are created by calling %Listify-Rest-Args with the context and count.

The More-Entry parses the keyword arguments and passes the values to the main function as positional arguments. If a keyword default is not constant, then we pass a supplied-p parameter into the main entry and let it worry about defaulting the argument. Since the main entry accepts keywords in parsed form, we can parse keywords at compile time for calls to known functions. We keep around the original parsed lambda-list and related information so that people can figure out how to call the main entry.