At the top level of a program (i.e., not nested within any other expression), a definition of the form
(define a value)
defines a variable called
a and sets it to the value value.
If the variable already exists in the current module, because it has
already been created by a previous
define expression with the
same name, its value is simply changed to the new value. In this
case, then, the above form is completely equivalent to
(set! a value)
This equivalence means that
define can be used interchangeably
set! to change the value of variables at the top level of
the REPL or a Scheme source file. It is useful during interactive
development when reloading a Scheme file that you have modified, because
it allows the
define expressions in that file to work as expected
both the first time that the file is loaded and on subsequent occasions.
Note, though, that
set! are not always
equivalent. For example, a
set! is not allowed if the named
variable does not already exist, and the two expressions can behave
differently in the case where there are imported variables visible from
Create a top level variable named name with value value.
If the named variable already exists, just change its value. The return
value of a
define expression is unspecified.
The C API equivalents of
scm_c_define, which differ from each other in whether the
variable name is specified as a
SCM symbol or as a
null-terminated C string.
C equivalents of
define, with variable name specified either by
sym, a symbol, or by name, a null-terminated C string. Both
variants return the new or preexisting variable object.
define (when it occurs at top level),
scm_c_define all create or set the value of a variable in the top
level environment of the current module. If there was not already a
variable with the specified name belonging to the current module, but a
similarly named variable from another module was visible through having
been imported, the newly created variable in the current module will
shadow the imported variable, such that the imported variable is no
Attention: Scheme definitions inside local binding constructs (see Local Variable Bindings) act differently (see Internal definitions).
Many people end up in a development style of adding and changing
definitions at runtime, building out their program without restarting
it. (You can do this using
load procedure, or even just pasting code into a
REPL.) If you are one of these people, you will find that sometimes
there are some variables that you don’t want to redefine all the
time. For these, use
Create a top level variable named name with value value, but only if name is not already bound in the current module.
Old Lispers probably know
define-once under its Lisp name,