6.12.1 Top Level Variable Definitions

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 with 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 define and 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 another module.

Scheme Syntax: define name value

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 define are scm_define and 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 Function: scm_define (sym, value)

C Function: scm_c_define (const char *name, value)

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_define and 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 longer visible.

Attention: Scheme definitions inside local binding constructs (see section Local Variable Bindings) act differently (see section 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 reload-module, the reload REPL command, the load procedure, or even just pasting code into a REPL.) If you are one of these people, you will find that sometimes you there are some variables that you don’t want to redefine all the time. For these, use define-once.

Scheme Syntax: define-once name value

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, defvar.

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