Scheme has lots of standard procedures, and Guile provides all of these via predefined top level variables. All of these standard procedures are documented in the later chapters of this reference manual.
Before very long, though, you will want to create new procedures that
encapsulate aspects of your own applications’ functionality. To do
this, you can use the famous lambda
syntax.
For example, the value of the following Scheme expression
(lambda (name address) body ...)
is a newly created procedure that takes two arguments: name
and
address
. The behavior of the new procedure is determined by the
sequence of expressions and definitions in the body of the
procedure definition. (Typically, body would use the arguments in
some way, or else there wouldn’t be any point in giving them to the
procedure.) When invoked, the new procedure returns a value that is the
value of the last expression in the body.
To make things more concrete, let’s suppose that the two arguments are both strings, and that the purpose of this procedure is to form a combined string that includes these arguments. Then the full lambda expression might look like this:
(lambda (name address) (string-append "Name=" name ":Address=" address))
We noted in the previous subsection that the procedure part of a procedure invocation expression can be any Scheme expression whose value is a procedure. But that’s exactly what a lambda expression is! So we can use a lambda expression directly in a procedure invocation, like this:
((lambda (name address) (string-append "Name=" name ":Address=" address)) "FSF" "Cambridge")
This is a valid procedure invocation expression, and its result is the string:
"Name=FSF:Address=Cambridge"
It is more common, though, to store the procedure value in a variable —
(define make-combined-string (lambda (name address) (string-append "Name=" name ":Address=" address)))
— and then to use the variable name in the procedure invocation:
(make-combined-string "FSF" "Cambridge")
Which has exactly the same result.
It’s important to note that procedures created using lambda
have
exactly the same status as the standard built in Scheme procedures, and
can be invoked, passed around, and stored in variables in exactly the
same ways.