25.1 Top Level Variable Definitions

On the top level of a program (i.e. when not inside the body of a procedure definition or a let, let* or letrec 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, 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.

Attention: Scheme definitions inside local binding constructs (see Local Bindings) act differently (see Internal Definitions).