Iteration control clauses allow direction of loop iteration.
The loop keywords for and as
designate iteration control clauses.
Iteration control clauses differ with respect to the specification of
termination tests and to the initialization and stepping_1
of loop variables. Iteration clauses by themselves
do not cause the Loop Facility to return values, but they
can be used in conjunction with value-accumulation clauses to
return values.
All variables are initialized in the loop prologue. A variable binding has lexical scope unless it is proclaimed special; thus, by default, the variable can be accessed only by forms that lie textually within the loop. Stepping assignments are made in the loop body before any other forms are evaluated in the body.
The variable argument in iteration control clauses can be a destructuring list. A destructuring list is a tree whose non-nil atoms are variable names. See Destructuring.
The iteration control clauses for, as, and repeat
must precede any other loop clauses, except
initially, with, and named,
since they establish variable bindings.
When iteration control clauses are
used in a loop,
the corresponding
termination tests in the loop body are evaluated
before any other loop body code is executed.
If multiple iteration clauses are used to control iteration, variable
initialization and stepping_1 occur sequentially by default.
The and construct can be used to connect two or more
iteration clauses when sequential binding and
stepping_1 are not necessary.
The iteration behavior of clauses joined by and
is analogous to the behavior of the macro do with
respect to do*.
The for and as clauses iterate by using one or more local
loop variables that are initialized to some value and that
can be modified or stepped_1 after each iteration.
For these clauses, iteration terminates when a local
variable reaches some supplied value or when some other loop clause
terminates iteration.
At each iteration, variables can be
stepped_1 by an increment or a decrement
or can be assigned a new value by the evaluation of a form).
Destructuring can be used to assign
values to variables during iteration.
The for and as keywords are synonyms; they can be used
interchangeably. There are seven syntactic formats for these constructs.
In each syntactic format, the type of
var can be supplied by the optional type-spec
argument. If var is a destructuring list, the type
supplied by the type-spec argument must appropriately match
the elements of the list.
By convention, for introduces new iterations and as
introduces iterations that depend on a previous iteration specification.