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We have seen quite a few primitive Scheme procedures,
eg, cons, string->list, and the like. Users
can create their own procedures using the special form
lambda. For example, the following defines a
procedure that adds 2 to its argument:
(lambda (x) (+ x 2))
The first subform, (x), is the list of parameters.
The remaining subform(s) constitute the procedure's
body. This procedure can be called on an argument,
just like a primitive procedure:
((lambda (x) (+ x 2)) 5)
=> 7
If we wanted to call this same procedure many times, we
could create a replica using lambda each time, but
we can do better. We can use a variable to hold the
procedure value:
(define add2
(lambda (x) (+ x 2)))
We can then use the variable add2 each time we need
a procedure for adding 2 to its argument:
(add2 4) => 6
(add2 9) => 11
3.1.1 Procedure parameters
The parameters of a
lambda-procedure are specified
by its first subform (the form immediately following
the head, the symbol
lambda).
add2
is a single-argument -- or
unary -- procedure, and so its parameter
list is the singleton list (
x). The symbol
x
acts as a variable holding the procedure's argument.
Each occurrence of
x in the procedure's body refers
to the procedure's argument. The variable
x is
said to be
local to the procedure's body.
We can use 2-element lists for 2-argument procedures,
and in general, n-element lists for n-argument procedures. The following is a 2-argument
procedure that calculates the area of a rectangle. Its
two arguments are the length and breadth of the
rectangle.
(define area
(lambda (length breadth)
(* length breadth)))
Notice that area multiplies its arguments, and so
does the primitive procedure *. We could have
simply said:
(define area *)
3.1.2 Variable number of arguments
Some procedures can be called at different times with
different numbers of arguments. To do this, the
lambda parameter list is replaced by a single
symbol. This symbol acts as a variable that is bound
to the list of the arguments that the procedure is
called on.
In general, the lambda parameter list can be a list
of the form (x ...), a symbol, or a dotted pair of
the form (x ... . z). In the dotted-pair case, all
the variables before the dot are bound to the
corresponding arguments in the procedure call, with the
single variable after the dot picking up all all the
remaining arguments as one list.
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