(repl [eval [write [read]]])
This procedure invokes an implementation-dependent read-eval-print loop (REPL), which reads a datum representing an expression or definition using read, evaluates it using eval, and prints the results if any using write. It then loops until an eof-object is read.
By default, eval is repl-eval, write is repl-write, and read is repl-read. The arguments are given in the order of most likely replacement, not the order of use. Allowing these defaults to be individually overridden means that languages written in Scheme can make use of the host implementation's REPL smarts, whatever they may be. The write argument should respect current-output-port and the read argument should respect current-input-port if it makes sense to do so.
(repl-read)
This procedure is implementation-dependent, but has the general effect of reading a datum from the current input port, possibly with some additional features.
(repl-write obj)
This procedure is implementation-dependent, but has the general effect of writing a datum to the current output port, possibly with some additional features.
(repl-eval obj)
This procedure is implementation dependent, but is similar to (lambda (x) (eval x (interaction-environment)) with the interaction environment augmented with the variables listed below. In addition, obj may be a definition, which extends the interaction environment. There may be other implementation-dependent features.
The following variables are known to repl-eval; their values are local to a specific invocation of repl. They are equivalent to the Common Lisp variables with the same names but without the leading - character; this is necessary because Scheme is a Lisp-1, so + - * / are already bound.
--
The expression or definition currently being evaluated by repl-eval.
-+
The expression or definition last evaluated by repl-eval.
-++
The previous value of -+.
-+++
The previous value of -++.
-*
The result of the expression or definition most recently evaluated by repl-eval. If a definition was last evaluated, the value of -* is implementation-dependent. If the expression returned zero values, the value of -* is #f; if the expression returned more than one value, the value of -* is the first value returned.
-**
The previous value of -*.
-***
The previous value of -**.
-/
A list of the results of the expression most recently evaluated by repl-eval. If a definition was last evaluated, the value of -/ is implementation-dependent.
-//
The previous value of -/.
-///
The previous value of -//.