(guile.info.gz) Creating a Procedure

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 14.2.3 Creating and Using a New Procedure
 -----------------------------------------
 
 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) EXPRESSION ...)
 
 is a newly created procedure that takes two arguments: `name' and
 `address'.  The behaviour of the new procedure is determined by the
 sequence of EXPRESSIONs in the "body" of the procedure definition.
 (Typically, these EXPRESSIONs 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 procedure 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.
 

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(guile.info.gz) Simple Invocation

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