Specifying function arguments using key-value pairs

Since v129 in the processes branch, two new primitives call/keywords and apply/keywords have been added to let function argument be specified in an order that's different from the declaration order. These primitives do not change the evaluation complexity of functions at all, but simply expose a function's declared arguments as "keywords".

(call/keywords f 'key1 val1 'key2 val2 ...)

Evaluates the given function by explicitly binding each named argument to the given value. Can also be used on macros, in which case the value positions are used without being evaluated. This is the counterpart of the normal position based evaluation.

(apply/keywords f alist)

If you store a set of argument bindings in an alist, you can apply a function to those bindings using apply/keywords. This is the counterpart of the position-based apply.

Any arguments that don't feature in the supplied key-value pairs take on their current value in the environment. This lets you specify common arguments using just a let binding or a global define. Such common arguments are therefore said to be implemented via a limited form of dynamic scoping.