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5.11.4 Iteration mechanisms

Scheme has only few iteration mechanisms, mainly because iteration in Scheme programs is normally expressed using recursion. Nevertheless, R5RS defines a construct for programming loops, calling do. In addition, Guile has an explicit looping syntax called while.

— syntax: do ((variable init [step]) ...) (test [expr ...]) body ...

Bind variables and evaluate body until test is true. The return value is the last expr after test, if given. A simple example will illustrate the basic form, 

          (do ((i 1 (1+ i)))
              ((> i 4))
            (display i))
          -| 1234

Or with two variables and a final return value, 

          (do ((i 1 (1+ i))
               (p 3 (* 3 p)))
              ((> i 4)
               p)
            (format #t "3**~s is ~s\n" i p))
          -|
          3**1 is 3
          3**2 is 9
          3**3 is 27
          3**4 is 81
          =>
          789

The variable bindings are established like a let, in that the expressions are all evaluated and then all bindings made. When iterating, the optional step expressions are evaluated with the previous bindings in scope, then new bindings all made.

The test expression is a termination condition. Looping stops when the test is true. It's evaluated before running the body each time, so if it's true the first time then body is not run at all.

The optional exprs after the test are evaluated at the end of looping, with the final variable bindings available. The last expr gives the return value, or if there are no exprs the return value is unspecified.

Each iteration establishes bindings to fresh locations for the variables, like a new let for each iteration. This is done for variables without step expressions too. The following illustrates this, showing how a new i is captured by the lambda in each iteration (see The Concept of Closure). 

          (define lst '())
          (do ((i 1 (1+ i)))
              ((> i 4))
            (set! lst (cons (lambda () i) lst)))
          (map (lambda (proc) (proc)) lst)
          =>
          (4 3 2 1)
— syntax: while cond body ...

Run a loop executing the body forms while cond is true. cond is tested at the start of each iteration, so if it's #f the first time then body is not executed at all. The return value is unspecified.

Within while, two extra bindings are provided, they can be used from both cond and body.

— Scheme Procedure: break

Break out of the while form.

— Scheme Procedure: continue

Abandon the current iteration, go back to the start and test cond again, etc.

Each while form gets its own break and continue procedures, operating on that while. This means when loops are nested the outer break can be used to escape all the way out. For example, 

          (while (test1)
            (let ((outer-break break))
              (while (test2)
                (if (something)
                  (outer-break #f))
                ...)))

Note that each break and continue procedure can only be used within the dynamic extent of its while. Outside the while their behaviour is unspecified.

Another very common way of expressing iteration in Scheme programs is the use of the so-called named let.

Named let is a variant of let which creates a procedure and calls it in one step. Because of the newly created procedure, named let is more powerful than do–it can be used for iteration, but also for arbitrary recursion.

— syntax: let variable bindings body

For the definition of bindings see the documentation about let (see Local Bindings).

Named let works as follows:

The next example implements a loop which iterates (by recursion) 1000 times. 

          (let lp ((x 1000))
            (if (positive? x)
                (lp (- x 1))
                x))
          =>
          0