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\chapter{Control Structures\label{CHPTCONTROL}}

This chapter describes {\ChezScheme} extensions to the set of standard

control structures.

See Chapter~\ref{TSPL:CHPTCONTROL} of {\TSPLFOUR} or the Revised$^6$ Report

on Scheme for a description of standard control structures.

\section{Conditionals}

%----------------------------------------------------------------------------

\entryheader

\formdef{exclusive-cond}{\categorysyntax}{(exclusive-cond \var{clause_1} \var{clause_2} \dots)}

\returns see below

\listlibraries

\endentryheader

\scheme{exclusive-cond} is a version of \scheme{cond}

(Section~\ref{TSPL:SECTCONDITIONALS} of {TSPLFOUR}) that differs

from \scheme{cond} in that the tests embedded within the clauses

are assumed to be exclusive in the sense that if one of the tests

is true, the others are not.

This allows the implementation to reorder clauses when profiling

information is available at expansion time (Section~\ref{SECTMISCPROFILE}).

The \scheme{(\var{test})} form of clause is not supported.

The order chosen when profiling information is available is based

on the relative numbers of times the RHS of each clause is executed,

and \scheme{(\var{test})} has no RHS.

\scheme{(\var{test} => values)} is equivalent, abeit less concise.

%----------------------------------------------------------------------------

\entryheader

\formdef{case}{\categorysyntax}{(case \var{expr_0} \var{clause_1} \var{clause_2} \dots)}

\returns see below

\listlibraries

\endentryheader

\noindent

Each clause but the last must take one of the forms:

\schemedisplay

((\var{key} \dots) \var{expr_1} \var{expr_2} \dots)

(\var{key} \var{expr_1} \var{expr_2} \dots)

\endschemedisplay

\noindent

where each \var{key} is a datum distinct from the other keys.

The last clause may be in the above form or it may be an

\index{\scheme{else}}\scheme{else} clause of the form

\schemedisplay

(else \var{expr_1} \var{expr_2} \dots)

\endschemedisplay

\var{expr_0} is evaluated and the result is compared

(using \scheme{equal?}) against the keys of each clause in order.

If a clause containing a matching key is found, the

expressions \scheme{\var{expr_1} \var{expr_2} \dots} are evaluated in sequence

and the values of the last expression are returned.

If none of the clauses contains a matching key and an \scheme{else} clause

is present, the expressions \scheme{\var{expr_1} \var{expr_2} \dots} of the

\scheme{else} clause are evaluated in sequence and the values of the last

expression are returned.

If none of the clauses contains a matching key and no \scheme{else} clause

is present, the value or values are unspecified.

The Revised$^6$ Report version of \scheme{case} does not support singleton

keys (the second of the first two clause forms above) and uses

\scheme{eqv?} rather than \scheme{equal?} as the comparison procedure.

Both versions are defined in terms of \scheme{exclusive-cond} so that

if profiling information is available at expansion time, the clauses will

be reordered to put those that are most frequently executed first.

\schemedisplay

(let ([ls '(ii iv)])

(case (car ls)

[i 1]

[ii 2]

[iii 3]

[(iiii iv) 3]

[else 'out-of-range])) ;=> 2

(define p

(lambda (x)

(case x

[("abc" "def") 'one]

[((a b c)) 'two]

[else #f])))

(p (string #\d #\e #\f)) ;=> one

(p '(a b c)) ;=> two

\endschemedisplay

%----------------------------------------------------------------------------

\noskipentryheader

\formdef{record-case}{\categorysyntax}{(record-case \var{expr} \var{clause_1} \var{clause_2} \dots)}

\returns see explanation

\listlibraries

\endnoskipentryheader

\noindent

\scheme{record-case} is a restricted form of \scheme{case} that supports the

destructuring of \index{records}\emph{records}, or \index{tagged lists}\emph{tagged lists}.

A record has as its first element a tag that determines what ``type''

of record it is; the remaining elements are the fields of the record.

Each clause but the last must take the form

\schemedisplay

((\var{key} \dots) \var{formals} \var{body_1} \var{body_2} \dots)

\endschemedisplay

\noindent

where each \var{key} is a datum distinct from the other keys.

The last clause may be in the above form or it may be an

\index{\scheme{else}}\scheme{else} clause of the form

\schemedisplay

(else \var{body_1} \var{body_2} \dots)

\endschemedisplay

\var{expr} must evaluate to a pair.

\var{expr} is evaluated and the car of its value is compared

(using \scheme{eqv?}) against the keys of each clause in order.

If a clause containing a matching key is found, the variables in

\var{formals} are bound to the remaining elements

of the list and the expressions

\scheme{\var{body_1} \var{body_2} \dots} are evaluated in sequence.

The value of the last expression is returned.

The effect is identical to the application of

\schemedisplay

(lambda \var{formals} \var{body_1} \var{body_2} \dots)

\endschemedisplay

\noindent

to the cdr of the list.

If none of the clauses contains a matching key and an \scheme{else} clause

is present, the expressions \scheme{\var{body_1} \var{body_2} \dots} of the

\scheme{else} clause are evaluated in sequence and the value of the last

expression is returned.

If none of the clauses contains a matching key and no \scheme{else} clause

is present, the value is unspecified.

\schemedisplay

(define calc

(lambda (x)

(record-case x

[(add) (x y) (+ x y)]

[(sub) (x y) (- x y)]

[(mul) (x y) (* x y)]

[(div) (x y) (/ x y)]

[else (assertion-violationf 'calc "invalid expression ~s" x)])))

(calc '(add 3 4)) ;=> 7

(calc '(div 3 4)) ;=> 3/4

\endschemedisplay

\section{Mapping and Folding}

%----------------------------------------------------------------------------

\noskipentryheader

\formdef{ormap}{\categoryprocedure}{(ormap \var{procedure} \var{list_1} \var{list_2} \dots)}

\returns see explanation

\listlibraries

\endnoskipentryheader

\noindent

\scheme{ormap} is identical to the Revised$^6$ Report \scheme{exists}.

%----------------------------------------------------------------------------

\entryheader

\formdef{andmap}{\categoryprocedure}{(andmap \var{procedure} \var{list_1} \var{list_2} \dots)}

\returns see explanation

\listlibraries

\endentryheader

\noindent

\scheme{andmap} is identical to the Revised$^6$ Report \scheme{for-all}.

\section{Continuations}

{\ChezScheme} supports one-shot continuations as well as the standard

multi-shot continuations obtainable via \scheme{call/cc}.

One-shot continuations are continuations that may be invoked at most

once, whether explicitly or implicitly.

They are obtained with \scheme{call/1cc}.

%----------------------------------------------------------------------------

\entryheader

\formdef{call/1cc}{\categoryprocedure}{(call/1cc \var{procedure})}

\returns see below

\listlibraries

\endentryheader

\noindent

\scheme{call/1cc} obtains its continuation and passes it to \var{procedure},

which should accept one argument.

The continuation itself is represented by a procedure.

This procedure normally takes one argument but may take an arbitrary

number of arguments depending upon whether the context of the call

to \scheme{call/1cc}

expects multiple return values or not.

When this procedure is applied to a value or values, it returns the values

to the continuation of the \scheme{call/1cc} application.

The continuation obtained by \scheme{call/1cc} is a

\index{one-shot continuations}``one-shot continuation.''

A one-shot continuation should not be returned to multiple times, either

by invoking the continuation or returning normally from \var{procedure} more

than once.

A one-shot continuation is ``promoted'' into a normal (multishot)

continuation, however, if it is

still active when a

normal continuation is obtained by \scheme{call/cc}.

After a one-shot continuation is promoted into a multishot continuation,

it behaves exactly as if it had been obtained via \scheme{call/cc}.

This allows \scheme{call/cc} and \scheme{call/1cc} to be used together

transparently in many applications.

One-shot continuations may be more efficient for some applications than

multishot continuations.

See the paper ``Representing control in the presence of one-shot

continuations''~\cite{Bruggeman:oneshots} for more information about

one-shot continuations, including how they are implemented in

{\ChezScheme}.

The following examples highlight the similarities and differences

between one-shot and normal continuations.

\schemedisplay

(define prod

; compute the product of the elements of ls, bugging out

; with no multiplications if a zero element is found

(lambda (ls)

(lambda (k)

(if (null? ls)

1

(if (= (car ls) 0)

(k 0)

(* (car ls) ((prod (cdr ls)) k)))))))

(call/cc (prod '(1 2 3 4))) ;=> 24

(call/1cc (prod '(1 2 3 4))) ;=> 24

(call/cc (prod '(1 2 3 4 0))) ;=> 0

(call/1cc (prod '(1 2 3 4 0))) ;=> 0

(let ([k (call/cc (lambda (x) x))])

(k (lambda (x) 0))) ;=> 0

(let ([k (call/1cc (lambda (x) x))])

(k (lambda (x) 0))) ;=> \var{exception}

\endschemedisplay

%----------------------------------------------------------------------------

\entryheader\label{dynamic-wind}

\formdef{dynamic-wind}{\categoryprocedure}{(dynamic-wind \var{in} \var{body} \var{out})}

\formdef{dynamic-wind}{\categoryprocedure}{(dynamic-wind \var{critical?} \var{in} \var{body} \var{out})}

\returns values resulting from the application of \var{body}

\listlibraries

\endentryheader

The first form is identical to the Revised$^6$ Report \scheme{dynamic-wind}.

When the optional \var{critical?} argument is present and non-false,

the \var{in} thunk is invoked in a critical section along with the code

that records that the body has been entered, and the \var{out} thunk is

invoked in a critical section section along with the code that records

that the body has been exited.

Extreme caution must be taken with this form of \scheme{dynamic-wind},

since an error or long-running computation can leave interrupts

and automatic garbage collection disabled.

\section{Engines\label{SECTENGINES}}

\index{engines}Engines are a high-level process abstraction supporting

\index{timed preemption}\emph{timed preemption}~\cite{Dybvig:engines,Haynes:abstracting}.

Engines may be used to simulate \index{multiprocessing}multiprocessing, implement operating

system kernels, and perform \index{nondeterministic computations}nondeterministic computations.

%----------------------------------------------------------------------------

\entryheader

\formdef{make-engine}{\categoryprocedure}{(make-engine \var{thunk})}

\returns an engine

\listlibraries

\endentryheader

An engine is created by passing a thunk (no argument procedure)

to \scheme{make-engine}.

The body of the thunk is the computation to be performed by the engine.

An engine itself is a procedure of three arguments:

\begin{description}

\item[\var{ticks}:]

\index{ticks@\var{ticks}|see{engines}}a positive integer that specifies

the amount of \emph{fuel} to be given

to the engine.

An engine executes until this fuel runs out or until its computation

finishes.

\item[\var{complete}:]

\index{complete@\var{complete}|see{engines}}a procedure of one or more

arguments that

specifies what to do if the computation finishes.

Its arguments are the amount of fuel left over and the

values produced by the computation.

\item[\var{expire}:]

\index{expire@\var{expire}|see{engines}}a procedure of one argument that

specifies what to do if the fuel runs

out before the computation finishes.

Its argument is a new engine capable of continuing the computation

from the point of interruption.

\end{description}

When an engine is applied to its arguments, it sets up a timer

to fire in \var{ticks} time units.

(See \scheme{set-timer} on page~\pageref{desc:set-timer}.)

If the engine computation completes before the timer expires, the

system invokes \var{complete}, passing

it the number of \var{ticks} left over and

the values produced by the computation.

If, on the other hand, the timer goes off before the engine computation

completes, the system creates a new engine from the continuation of

the interrupted computation and passes this engine to \var{expire}.

\var{complete} and \var{expire} are invoked in the continuation

of the engine invocation.

An implementation of engines is given

in Section~\ref{TSPL:SECTEXENGINES}.

of {\TSPLFOUR}.

Do not use the timer interrupt (see \index{\scheme{set-timer}}\scheme{set-timer}) and \index{engines}engines

at the same time, since engines are implemented in terms of the timer.

The following example creates an engine from a trivial computation,

3, and gives the engine 10 ticks.

\schemedisplay

(define eng

(make-engine

(lambda () 3)))

(eng 10

(lambda (ticks value) value)

(lambda (x) x)) ;=> 3

\endschemedisplay

It is often useful to pass \scheme{list} as the \var{complete}

procedure to an engine, causing an engine that completes to return a

list whose first element is the ticks remaining and whose remaining elements

are the values returned by the computation.

\schemedisplay

(define eng

(make-engine

(lambda () 3)))

(eng 10

list

(lambda (x) x)) ;=> (9 3)

\endschemedisplay

\noindent

In the example above, the value is 3 and there are 9 ticks left over,

i.e., it takes one unit of fuel to evaluate 3.

(The fuel amounts given here are for illustration only.

Your mileage may vary.)

Typically, the engine computation does not finish in one try.

\index{\scheme{fibonacci}}The following example displays the use of an engine to

compute the 10th Fibonacci number in steps.

\schemedisplay

(define fibonacci

(lambda (n)

(let fib ([i n])

(cond

[(= i 0) 0]

[(= i 1) 1]

[else (+ (fib (- i 1))

(fib (- i 2)))]))))

(define eng

(make-engine

(lambda ()

(fibonacci 10))))

(eng 50

list

(lambda (new-eng)

(set! eng new-eng)

"expired")) ;=> "expired"

(eng 50

list

(lambda (new-eng)

(set! eng new-eng)

"expired")) ;=> "expired"

(eng 50

list

(lambda (new-eng)

(set! eng new-eng)

"expired")) ;=> "expired"

(eng 50

list

(lambda (new-eng)

(set! eng new-eng)

"expired")) ;=> (21 55)

\endschemedisplay

\noindent

Each time the engine's fuel runs out, the \var{expire} procedure assigns

\scheme{eng} to the new engine.

The entire computation requires four blocks of 50 ticks to complete; of the

last 50 it uses all but 21.

Thus, the total amount of fuel used is 179 ticks.

This leads to the following procedure, \scheme{mileage}, which ``times'' a

computation using engines:

\schemedisplay

(define mileage

(lambda (thunk)

(let loop ([eng (make-engine thunk)] [total-ticks 0])

(eng 50

(lambda (ticks . values)

(+ total-ticks (- 50 ticks)))

(lambda (new-eng)

(loop new-eng

(+ total-ticks 50)))))))

(mileage (lambda () (fibonacci 10))) ;=> 179

\endschemedisplay

\noindent

The choice of 50 for the number of ticks to use each time is

arbitrary, of course.

It might make more sense to pass a much larger number, say 10000,

in order to reduce the number of times the computation is interrupted.

The next procedure is similar to \scheme{mileage}, but it returns a list

of engines, one for each tick it takes to complete the computation.

Each of the engines in the list represents a ``snapshot'' of the

computation, analogous to a single frame of a moving picture.

\scheme{snapshot} might be useful for ``single stepping'' a computation.

\schemedisplay

(define snapshot

(lambda (thunk)

(let again ([eng (make-engine thunk)])

(cons eng

(eng 1 (lambda (t . v) '()) again)))))

\endschemedisplay

\noindent

The recursion embedded in this procedure is rather strange.

The complete procedure performs the base case, returning the empty

list, and the expire procedure performs the recursion.

The next procedure, \index{\scheme{round-robin}}\scheme{round-robin}, could be the basis for a simple

time-sharing \index{operating system}operating system.

\scheme{round-robin} maintains a queue of processes (a list of engines),

cycling through the queue in a \emph{round-robin} fashion, allowing each

process to run for a set amount of time.

\scheme{round-robin} returns a list of the values returned by the engine

computations in the order that the computations complete.

Each computation is assumed to produce exactly one value.

\schemedisplay

(define round-robin

(lambda (engs)

(if (null? engs)

'()

((car engs)

1

(lambda (ticks value)

(cons value (round-robin (cdr engs))))

(lambda (eng)

(round-robin

(append (cdr engs) (list eng))))))))

\endschemedisplay

\noindent

Since the amount of fuel supplied each time, one tick, is constant,

the effect of \scheme{round-robin} is to return a list of the values sorted

from the quickest to complete to the slowest to complete.

Thus, when we call \scheme{round-robin} on a list of engines, each computing

one of the Fibonacci numbers, the output list is sorted with the earlier

Fibonacci numbers first, regardless of the order of the input list.

\schemedisplay

(round-robin

(map (lambda (x)

(make-engine

(lambda ()

(fibonacci x))))

'(4 5 2 8 3 7 6 2))) ;=> (1 1 2 3 5 8 13 21)

\endschemedisplay

More interesting things can happen if the amount of fuel varies

each time through the loop.

\index{nondeterministic computations}In this case, the computation would

be nondeterministic, i.e., the results would vary from call to call.

The following syntactic form, \index{\scheme{por} (parallel-or)}\scheme{por} (parallel-or), returns the

first of its expressions to complete with a true value.

\scheme{por} is implemented with the procedure \scheme{first-true}, which is

similar to \scheme{round-robin} but quits when any of the engines

completes with a true value.

If all of the engines complete, but none with a true value,

\scheme{first-true} (and hence \scheme{por}) returns \scheme{#f}.

Also, although \scheme{first-true} passes a fixed amount of fuel to each

engine, it chooses the next engine to run at random, and is thus

nondeterministic.

\schemedisplay

(define-syntax por

(syntax-rules ()

[(_ x ...)

(first-true

(list (make-engine (lambda () x)) ...))]))

(define first-true

(let ([pick

(lambda (ls)

(list-ref ls (random (length ls))))])

(lambda (engs)

(if (null? engs)

#f

(let ([eng (pick engs)])

(eng 1

(lambda (ticks value)

(or value

(first-true

(remq eng engs))))

(lambda (new-eng)

(first-true

(cons new-eng

(remq eng engs))))))))))

\endschemedisplay

\noindent

The list of engines is maintained with \scheme{pick}, which randomly

chooses an element of the list, and \scheme{remq}, which removes the

chosen engine from the list.

Since \scheme{por} is nondeterministic, subsequent uses with the same

expressions may not return the same values.

\schemedisplay

(por 1 2 3) ;=> 2

(por 1 2 3) ;=> 3

(por 1 2 3) ;=> 2

(por 1 2 3) ;=> 1

\endschemedisplay

\noindent

Furthermore, even if one of the expressions is an infinite loop,

\scheme{por} still finishes as long as one of the other expressions

completes and returns a true value.

\schemedisplay

(por (let loop () (loop)) 2) ;=> 2

\endschemedisplay

\noindent

With \scheme{engine-return} and \scheme{engine-block}, it is possible to

terminate an engine explicitly.

\scheme{engine-return} causes the engine to complete, as if the

computation had finished.

Its arguments are passed to the \var{complete} procedure along with the

number of ticks remaining.

It is essentially a nonlocal exit from the engine.

Similarly, \scheme{engine-block} causes the engine to expire, as if the

timer had run out.

A new engine is made from the continuation of the call to \scheme{engine-block}

and passed to the \var{expire} procedure.

%----------------------------------------------------------------------------

\entryheader

\formdef{engine-block}{\categoryprocedure}{(engine-block)}

\returns does not return

\listlibraries

\endentryheader

\noindent

This causes a running engine to stop, create a new engine capable

of continuing the computation, and pass the new engine to the original

engine's third argument

(the expire procedure).

Any remaining fuel is forfeited.

\schemedisplay

(define eng

(make-engine

(lambda ()

(engine-block)

"completed")))

(eng 100

(lambda (ticks value) value)

(lambda (x)

(set! eng x)

"expired")) ;=> "expired"

(eng 100

(lambda (ticks value) value)

(lambda (x)

(set! eng x)

"expired")) ;=> "completed"

\endschemedisplay

%----------------------------------------------------------------------------

\entryheader

\formdef{engine-return}{\categoryprocedure}{(engine-return \var{obj} \dots)}

\returns does not return

\listlibraries

\endentryheader

\noindent

This causes a running engine to stop and pass control to the

engine's \var{complete} argument.

The first argument passed to the complete procedure is the amount of

fuel remaining, as usual, and

the remaining arguments are the objects \scheme{\var{obj} \dots}

passed to \scheme{engine-return}.

\schemedisplay

(define eng

(make-engine

(lambda ()

(reverse (engine-return 'a 'b 'c)))))

(eng 100

(lambda (ticks . values) values)

(lambda (new-eng) "expired")) ;=> (a b c)

\endschemedisplay