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Implementations may provide an interactive session called a REPL
(Read-Eval-Print Loop), where import declarations, expressions and
definitions can be entered and evaluated one at a time. For convenience
and ease of use, the global Scheme environment in a REPL must not be
empty, but must start out with at least the bindings provided by the base
library. This library includes the core syntax of Scheme and generally
useful procedures that manipulate data. For example, the variable
abs is bound to a procedure of one argument that computes the
absolute value of a number, and the variable + is bound to a
procedure that computes sums. The full list of (scheme base)
bindings can be found in Standard Libraries.
Implementations may provide an initial REPL environment which behaves as if all possible variables are bound to locations, most of which contain unspecified values. Top level REPL definitions in such an implementation are truly equivalent to assignments, unless the identifier is defined as a syntax keyword.
An implementation may provide a mode of operation in which the REPL reads its input from a file. Such a file is not, in general, the same as a program, because it can contain import declarations in places other than the beginning.