Scheme Programming/Local Scope
This section defines local scope and variable scoping in general.
Variable scope concepts edit
Scheme uses lexical scoping, which means that the variable scope can be immediately seen from the program text and that the variable scope is defined at compile time.
Scheme is block structured. A block here means a stretch of code lines that have start and end points. Block defines one scope and a scope is where a variable is defined (or bound). Blocks can be nested. There are outer and inner blocks.
Top level scope is a special scope, since by definition is has no outer scope. Top level scope is also called global scope. Any other scope is nested, and all of them are affected by the outer scope or scopes. Local scope is the same as the current scope.
Top level scope edit
Top level scope is the outmost scope (block) in the program's scopes.
We define here a variable in the top level scope and evaluate its value:
> (define x 1) > x 1
Formally we can say that variable
x is bound to
value 1. In general, top level scope contains global bindings which
can be procedures or data values.
Local scope edit
New scope (i.e. inner scope to the current) can be created with
> (let ((y 2)) > y) 2
The value of a
let expression is the last evaluated
expression within the body of
let. In this case we get the value
y, which is 2.
y exists only in the inner scope:
> y ;; Error: unbound variable: y
However, variables from the top level are visible to all inner scopes:
> (define x 1) > (let ((y 2)) > (+ x y)) 3
It is possible to use the same variable name in the inner scope. Even when the variable in the inner scope has the same name, as in the outer scope, they are different bindings and they are stored into different locations. Scheme uses variable bindings from the innermost scope where the variable identifier is found.
Thus we can shadow variables in the outer scope:
> (define x 1) > (let ((x 7)) > x) 7 > x 1
x exists only within the
expression, but the outer
x exists before and after the
There are no limits in the nesting depth of blocks:
> (define x 1) > (let ((x 7)) > (let ((x 13)) > x)) 13 > x 1
Note that, the code above is only demonstrating nesting of scopes and it does not make much sense in general.
So far we have referred variables from normal expressions that
produce data values. However, we can refer variables from outer scope
also from procedures. The lexical scoping rules apply the same way to
procedures as for other expressions. In fact
let can be
expressed as syntax extension of
lambda, so actually we
have referenced variables from procedures all the time.
When a procedure refers to a variable that is not bound within the procedure itself (i.e. bound in the outer scope), a closure is created. The external variables become part of the closure. This connection is so strong that even if the scope of a variable is outlived, it still exists in the closure. In that sense closures break the normal block scope of a variable. This happens when a closure is the passed value from an inner scope to the outer scope.
Here is a very simple example of a closure:
> (define x 0) > (define inc-x (lambda () (set! x (+ x 1)) x)) > (inc-x) 1 > (inc-x) 2
x by 1 each time it is
evaluated. The procedure does not take any
inc-x refers to the external variable
x. Also, it might be worth noting that it refers to
+ as well.
+ is actually a variable in
Scheme that just happens to be bound to a procedure that adds numbers together.
Closures can be used in many ways. Higher order functions take other functions as arguments and apply them, and an argument can be a closure. Closures can also be used to emulate objects. They maintain state in an external variable, and update the state in each application.
Here is an example, where closure is used along with high-order
> (define x 1) > (map (lambda (num) (+ num x)) '(10 11 12)) (11 12 13)
map takes two arguments: a procedure and a list. The
procedure is applied to each element in the list and a new list
is created from results of the procedure application.
There are two scopes involved in this example: the top level scope and
the inner scope created by
exists in the local scope of
x is hence part of the closure created by
map is part of a normal
expression, and it does not start a new scope.