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[bpt/guile.git] / doc / ref / scheme-control.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@page
@node Control Mechanisms
@chapter Controlling the Flow of Program Execution

@menu
* begin::                       Evaluating a sequence of expressions.
* if cond case::                Simple conditional evaluation.
* and or::                      Conditional evaluation of a sequence.
* while do::                    Iteration mechanisms.
* Continuations::               Continuations.
* Multiple Values::             Returning and accepting multiple values.
* Exceptions::                  Throwing and catching exceptions.
* Error Reporting::             Procedures for signaling errors.
* Dynamic Wind::                Guarding against non-local entrance/exit.
* Frames::                      Another way to handle non-localness
* Handling Errors::             How to handle errors in C code.
@end menu

Scheme has a more general view of program flow than C, both locally and
non-locally.

Controlling the local flow of control involves things like gotos, loops,
calling functions and returning from them.  Non-local control flow
refers to situations where the program jumps across one or more levels
of function activations without using the normal call or return
operations.

The primitive means of C for local control flow is the @code{goto}
statement, together with @code{if}.  Loops done with @code{for},
@code{while} or @code{do} could in principle be rewritten with just
@code{goto} and @code{if}.  In Scheme, the primitive means for local
control flow is the @emph{function call} (together with @code{if}).
Thus, the repetition of some computation in a loop is ultimately
implemented by a function that calls itself, that is, by recursion.

This approach is theoretically very powerful since it is easier to
reason formally about recursion than about gotos.  In C, using
recursion exclusively would not be practical, tho, since it would eat
up the stack very quickly.  In Scheme, however, it is practical:
function calls that appear in a @dfn{tail position} do not use any
additional stack space.

A function call is in a tail position when it is the last thing the
calling function does.  The value returned by the called function is
immediately returned from the calling function.  In the following
example, the call to @code{bar-1} is in a tail position, while the
call to @code{bar-2} is not.  (The call to @code{1-} in @code{foo-2}
is in a tail position, tho.)

@lisp
(define (foo-1 x)
  (bar-1 (1- x)))

(define (foo-2 x)
  (1- (bar-2 x)))
@end lisp

Thus, when you take care to recurse only in tail positions, the
recursion will only use constant stack space and will be as good as a
loop constructed from gotos.

Scheme offers a few syntactic abstractions (@code{do} and @dfn{named}
@code{let}) that make writing loops slightly easier.

But only Scheme functions can call other functions in a tail position:
C functions can not.  This matters when you have, say, two functions
that call each other recursively to form a common loop.  The following
(unrealistic) example shows how one might go about determing whether a
non-negative integer @var{n} is even or odd.

@lisp
(define (my-even? n)
  (cond ((zero? n) #t)
        (else (my-odd? (1- n)))))

(define (my-odd? n)
  (cond ((zero? n) #f)
        (else (my-even? (1- n)))))
@end lisp

Because the calls to @code{my-even?} and @code{my-odd?} are in tail
positions, these two procedures can be applied to arbitrary large
integers without overflowing the stack.  (They will still take a lot
of time, of course.)

However, when one or both of the two procedures would be rewritten in
C, it could no longer call its companion in a tail position (since C
does not have this concept).  You might need to take this
consideration into account when deciding which parts of your program
to write in Scheme and which in C.

In addition to calling functions and returning from them, a Scheme
program can also exit non-locally from a function so that the control
flow returns directly to an outer level.  This means that some functions
might not return at all.

Even more, it is not only possible to jump to some outer level of
control, a Scheme program can also jump back into the middle of a
function that has already exited.  This might cause some functions to
return more than once.

In general, these non-local jumps are done by invoking
@dfn{continuations} that have previously been captured using
@code{call-with-current-continuation}.  Guile also offers a slightly
restricted set of functions, @code{catch} and @code{throw}, that can
only be used for non-local exits.  This restriction makes them more
efficient.  Error reporting (with the function @code{error}) is
implemented by invoking @code{throw}, for example.  The functions
@code{catch} and @code{throw} belong to the topic of @dfn{exceptions}.

Since Scheme functions can call C functions and vice versa, C code can
experience the more general control flow of Scheme as well.  It is
possible that a C function will not return at all, or will return more
than once.  While C does offer @code{setjmp} and @code{longjmp} for
non-local exits, it is still an unusual thing for C code.  In
contrast, non-local exits are very common in Scheme, mostly to report
errors.

You need to be prepared for the non-local jumps in the control flow
whenever you use a function from @code{libguile}: it is best to assume
that any @code{libguile} function might signal an error or run a pending
signal handler (which in turn can do arbitrary things).

It is often necessary to take cleanup actions when the control leaves a
function non-locally.  Also, when the control returns non-locally, some
setup actions might be called for.  For example, the Scheme function
@code{with-output-to-port} needs to modify the global state so that
@code{current-output-port} returns the port passed to
@code{with-output-to-port}.  The global output port needs to be reset to
its previous value when @code{with-output-to-port} returns normally or
when it is exited non-locally.  Likewise, the port needs to be set again
when control enters non-locally.

Scheme code can use the @code{dynamic-wind} function to arrange for
the setting and resetting of the global state.  C code could use the
corresponding @code{scm_internal_dynamic_wind} function, but it might
prefer to use the @dfn{frames} concept that is more natural for C
code.


@node begin
@section Evaluating a Sequence of Expressions

@cindex begin
@cindex sequencing
@cindex expression sequencing

The @code{begin} syntax is used for grouping several expressions
together so that they are treated as if they were one expression.
This is particularly important when syntactic expressions are used
which only allow one expression, but the programmer wants to use more
than one expression in that place.  As an example, consider the
conditional expression below:

@lisp
(if (> x 0)
    (begin (display "greater") (newline)))
@end lisp

If the two calls to @code{display} and @code{newline} were not embedded
in a @code{begin}-statement, the call to @code{newline} would get
misinterpreted as the else-branch of the @code{if}-expression.

@deffn syntax begin expr1 expr2 @dots{}
The expression(s) are evaluated in left-to-right order and the value
of the last expression is returned as the value of the
@code{begin}-expression.  This expression type is used when the
expressions before the last one are evaluated for their side effects.

Guile also allows the expression @code{(begin)}, a @code{begin} with no
sub-expressions.  Such an expression returns the `unspecified' value.
@end deffn

@node if cond case
@section Simple Conditional Evaluation

@cindex conditional evaluation
@cindex if
@cindex case
@cindex cond

Guile provides three syntactic constructs for conditional evaluation.
@code{if} is the normal if-then-else expression (with an optional else
branch), @code{cond} is a conditional expression with multiple branches
and @code{case} branches if an expression has one of a set of constant
values.

@deffn syntax if test consequent [alternate]
All arguments may be arbitrary expressions.  First, @var{test} is
evaluated.  If it returns a true value, the expression @var{consequent}
is evaluated and @var{alternate} is ignored.  If @var{test} evaluates to
@code{#f}, @var{alternate} is evaluated instead.  The value of the
evaluated branch (@var{consequent} or @var{alternate}) is returned as
the value of the @code{if} expression.

When @var{alternate} is omitted and the @var{test} evaluates to
@code{#f}, the value of the expression is not specified.
@end deffn

@deffn syntax cond clause1 clause2 @dots{}
Each @code{cond}-clause must look like this:

@lisp
(@var{test} @var{expression} @dots{})
@end lisp

where @var{test} and @var{expression} are arbitrary expression, or like
this

@lisp
(@var{test} => @var{expression})
@end lisp

where @var{expression} must evaluate to a procedure.

The @var{test}s of the clauses are evaluated in order and as soon as one
of them evaluates to a true values, the corresponding @var{expression}s
are evaluated in order and the last value is returned as the value of
the @code{cond}-expression.  For the @code{=>} clause type,
@var{expression} is evaluated and the resulting procedure is applied to
the value of @var{test}.  The result of this procedure application is
then the result of the @code{cond}-expression.

The @var{test} of the last @var{clause} may be the symbol @code{else}.
Then, if none of the preceding @var{test}s is true, the
@var{expression}s following the @code{else} are evaluated to produce the
result of the @code{cond}-expression.
@end deffn

@deffn syntax case key clause1 clause2 @dots{}
@var{key} may be any expression, the @var{clause}s must have the form

@lisp
((@var{datum1} @dots{}) @var{expr1} @var{expr2} @dots{})
@end lisp

and the last @var{clause} may have the form

@lisp
(else @var{expr1} @var{expr2} @dots{})
@end lisp

All @var{datum}s must be distinct.  First, @var{key} is evaluated.  The
the result of this evaluation is compared against all @var{datum}s using
@code{eqv?}.  When this comparison succeeds, the expression(s) following
the @var{datum} are evaluated from left to right, returning the value of
the last expression as the result of the @code{case} expression.

If the @var{key} matches no @var{datum} and there is an
@code{else}-clause, the expressions following the @code{else} are
evaluated.  If there is no such clause, the result of the expression is
unspecified.
@end deffn


@node and or
@section Conditional Evaluation of a Sequence of Expressions

@code{and} and @code{or} evaluate all their arguments in order, similar
to @code{begin}, but evaluation stops as soon as one of the expressions
evaluates to false or true, respectively.

@deffn syntax and expr @dots{}
Evaluate the @var{expr}s from left to right and stop evaluation as soon
as one expression evaluates to @code{#f}; the remaining expressions are
not evaluated.  The value of the last evaluated expression is returned.
If no expression evaluates to @code{#f}, the value of the last
expression is returned.

If used without expressions, @code{#t} is returned.
@end deffn

@deffn syntax or expr @dots{}
Evaluate the @var{expr}s from left to right and stop evaluation as soon
as one expression evaluates to a true value (that is, a value different
from @code{#f}); the remaining expressions are not evaluated.  The value
of the last evaluated expression is returned.  If all expressions
evaluate to @code{#f}, @code{#f} is returned.

If used without expressions, @code{#f} is returned.
@end deffn


@node while do
@section Iteration mechanisms

@cindex iteration
@cindex looping
@cindex named let

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 @code{do}.  In
addition, Guile has an explicit looping syntax called @code{while}.

@deffn syntax do ((variable1 init1 step1) @dots{}) (test expr @dots{}) command @dots{}
The @var{init} expressions are evaluated and the @var{variables} are
bound to their values. Then looping starts with testing the @var{test}
expression.  If @var{test} evaluates to a true value, the @var{expr}
following the @var{test} are evaluated and the value of the last
@var{expr} is returned as the value of the @code{do} expression.  If
@var{test} evaluates to false, the @var{command}s are evaluated in
order, the @var{step}s are evaluated and stored into the @var{variables}
and the next iteration starts.

Any of the @var{step} expressions may be omitted, so that the
corresponding variable is not changed during looping.
@end deffn

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

Within @code{while}, two extra bindings are provided, they can be used
from both @var{cond} and @var{body}.

@deffn {Scheme Procedure} break
Break out of the @code{while} form.
@end deffn

@deffn {Scheme Procedure} continue
Abandon the current iteration, go back to the start and test
@var{cond} again, etc.
@end deffn

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

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

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

@cindex named let
Another very common way of expressing iteration in Scheme programs is
the use of the so-called @dfn{named let}.

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

@deffn syntax let variable bindings body
For the definition of @var{bindings} see the documentation about
@code{let} (@pxref{Local Bindings}).

Named @code{let} works as follows:

@itemize @bullet
@item
A new procedure which accepts as many arguments as are in @var{bindings}
is created and bound locally (using @code{let}) to @var{variable}.  The
new procedure's formal argument names are the name of the
@var{variables}.

@item
The @var{body} expressions are inserted into the newly created procedure.

@item
The procedure is called with the @var{init} expressions as the formal
arguments.
@end itemize

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

@lisp
(let lp ((x 1000))
  (if (positive? x)
      (lp (- x 1))
      x))
@result{}
0
@end lisp
@end deffn


@node Continuations
@section Continuations
@cindex continuations

A ``continuation'' is the code that will execute when a given function
or expression returns.  For example, consider

@example
(define (foo)
  (display "hello\n")
  (display (bar)) (newline)
  (exit))
@end example

The continuation from the call to @code{bar} comprises a
@code{display} of the value returned, a @code{newline} and an
@code{exit}.  This can be expressed as a function of one argument.

@example
(lambda (r)
  (display r) (newline)
  (exit))
@end example

In Scheme, continuations are represented as special procedures just
like this.  The special property is that when a continuation is called
it abandons the current program location and jumps directly to that
represented by the continuation.

A continuation is like a dynamic label, capturing at run-time a point
in program execution, including all the nested calls that have lead to
it, or rather the code that will execute when those calls return.

Continuations are created with the following functions.

@deffn {Scheme Procedure} call-with-current-continuation proc
@deffnx {Scheme Procedure} call/cc proc
@rnindex call-with-current-continuation
Capture the current continuation and call @code{(@var{proc}
@var{cont})} with it.  The return value is the value returned by
@var{proc}, or when @code{(@var{cont} @var{value})} is later invoked,
the return is the @var{value} passed.

Normally @var{cont} should be called with one argument, but when the
location resumed is expecting multiple values (@pxref{Multiple
Values}) then they should be passed as multiple arguments, for
instance @code{(@var{cont} @var{x} @var{y} @var{z})}.

@var{cont} may only be used from the dynamic root in which it was
created (@pxref{Dynamic Roots}), and in a multi-threaded program only
from the thread in which it was created, since each thread is a
separate dynamic root.

The call to @var{proc} is not part of the continuation captured, it runs
only when the continuation is created.  Often a program will want to
store @var{cont} somewhere for later use; this can be done in
@var{proc}.

The @code{call} in the name @code{call-with-current-continuation}
refers to the way a call to @var{proc} gives the newly created
continuation.  It's not related to the way a call is used later to
invoke that continuation.

@code{call/cc} is an alias for @code{call-with-current-continuation}.
This is in common use since the latter is rather long.
@end deffn

@deftypefn {C Function} SCM scm_make_continuation (int *first)
Capture the current continuation as described above.  The return value
is the new continuation, and @var{*first} is set to 1.

When the continuation is invoked, @code{scm_make_continuation} will
return again, this time returning the value (or set of multiple
values) passed in that invocation, and with @var{*first} set to 0.
@end deftypefn

@sp 1
@noindent
Here is a simple example,

@example
(define kont #f)
(format #t "the return is ~a\n"
        (call/cc (lambda (k)
                   (set! kont k)
                   1)))
@result{} the return is 1

(kont 2)
@result{} the return is 2
@end example

@code{call/cc} captures a continuation in which the value returned is
going to be displayed by @code{format}.  The @code{lambda} stores this
in @code{kont} and gives an initial return @code{1} which is
displayed.  The later invocation of @code{kont} resumes the captured
point, but this time returning @code{2}, which is displayed.

When Guile is run interactively, a call to @code{format} like this has
an implicit return back to the read-eval-print loop.  @code{call/cc}
captures that like any other return, which is why interactively
@code{kont} will come back to read more input.

@sp 1
C programmers may note that @code{call/cc} is like @code{setjmp} in
the way it records at runtime a point in program execution.  A call to
a continuation is like a @code{longjmp} in that it abandons the
present location and goes to the recorded one.  Like @code{longjmp},
the value passed to the continuation is the value returned by
@code{call/cc} on resuming there.  However @code{longjmp} can only go
up the program stack, but the continuation mechanism can go anywhere.

When a continuation is invoked, @code{call/cc} and subsequent code
effectively ``returns'' a second time.  It can be confusing to imagine
a function returning more times than it was called.  It may help
instead to think of it being stealthily re-entered and then program
flow going on as normal.

The functions @code{dynamic-wind} (@pxref{Dynamic Wind}) can be used to
ensure setup and cleanup code is run when a program locus is resumed or
abandoned through the continuation mechanism.  C code can use
@dfn{frames} (@pxref{Frames}).

@sp 1
Continuations are a powerful mechanism, and can be used to implement
almost any sort of control structure, such as loops, coroutines, or
exception handlers.

However the implementation of continuations in Guile is not as
efficient as one might hope, because Guile is designed to cooperate
with programs written in other languages, such as C, which do not know
about continuations.  Basically continuations are captured by a block
copy of the stack, and resumed by copying back.

For this reason, generally continuations should be used only when
there is no other simple way to achieve the desired result, or when
the elegance of the continuation mechanism outweighs the need for
performance.

Escapes upwards from loops or nested functions are generally best
handled with exceptions (@pxref{Exceptions}).  Coroutines can be
efficiently implemented with cooperating threads (a thread holds a
full program stack but doesn't copy it around the way continuations
do).


@node Multiple Values
@section Returning and Accepting Multiple Values

@cindex multiple values
@cindex receive

Scheme allows a procedure to return more than one value to its caller.
This is quite different to other languages which only allow
single-value returns.  Returning multiple values is different from
returning a list (or pair or vector) of values to the caller, because
conceptually not @emph{one} compound object is returned, but several
distinct values.

The primitive procedures for handling multiple values are @code{values}
and @code{call-with-values}.  @code{values} is used for returning
multiple values from a procedure.  This is done by placing a call to
@code{values} with zero or more arguments in tail position in a
procedure body.  @code{call-with-values} combines a procedure returning
multiple values with a procedure which accepts these values as
parameters.

@rnindex values
@deffn {Scheme Procedure} values arg1 @dots{} argN
@deffnx {C Function} scm_values (args)
Delivers all of its arguments to its continuation.  Except for
continuations created by the @code{call-with-values} procedure,
all continuations take exactly one value.  The effect of
passing no value or more than one value to continuations that
were not created by @code{call-with-values} is unspecified.

For @code{scm_values}, @var{args} is a list of arguments and the
return is a multiple-values object which the caller can return.  In
the current implementation that object shares structure with
@var{args}, so @var{args} should not be modified subsequently.
@end deffn

@rnindex call-with-values
@deffn {Scheme Procedure} call-with-values producer consumer
Calls its @var{producer} argument with no values and a
continuation that, when passed some values, calls the
@var{consumer} procedure with those values as arguments.  The
continuation for the call to @var{consumer} is the continuation
of the call to @code{call-with-values}.

@example
(call-with-values (lambda () (values 4 5))
                  (lambda (a b) b))
@result{} 5

@end example
@example
(call-with-values * -)
@result{} -1
@end example
@end deffn

In addition to the fundamental procedures described above, Guile has a
module which exports a syntax called @code{receive}, which is much more
convenient.  If you want to use it in your programs, you have to load
the module @code{(ice-9 receive)} with the statement

@lisp
(use-modules (ice-9 receive))
@end lisp

@deffn {library syntax} receive formals expr body @dots{}
Evaluate the expression @var{expr}, and bind the result values (zero or
more) to the formal arguments in the formal argument list @var{formals}.
@var{formals} must have the same syntax like the formal argument list
used in @code{lambda} (@pxref{Lambda}).  After binding the variables,
the expressions in @var{body} @dots{} are evaluated in order.
@end deffn


@node Exceptions
@section Exceptions
@cindex error handling
@cindex exception handling

A common requirement in applications is to want to jump
@dfn{non-locally} from the depths of a computation back to, say, the
application's main processing loop.  Usually, the place that is the
target of the jump is somewhere in the calling stack of procedures that
called the procedure that wants to jump back.  For example, typical
logic for a key press driven application might look something like this:

@example
main-loop:
  read the next key press and call dispatch-key

dispatch-key:
  lookup the key in a keymap and call an appropriate procedure,
  say find-file

find-file:
  interactively read the required file name, then call
  find-specified-file

find-specified-file:
  check whether file exists; if not, jump back to main-loop
  @dots{}
@end example

The jump back to @code{main-loop} could be achieved by returning through
the stack one procedure at a time, using the return value of each
procedure to indicate the error condition, but Guile (like most modern
programming languages) provides an additional mechanism called
@dfn{exception handling} that can be used to implement such jumps much
more conveniently.

@menu
* Exception Terminology::       Different ways to say the same thing.
* Catch::                       Setting up to catch exceptions.
* Throw::                       Throwing an exception.
* Lazy Catch::                  Catch without unwinding the stack.
* Exception Implementation::    How Guile implements exceptions.
@end menu


@node Exception Terminology
@subsection Exception Terminology

There are several variations on the terminology for dealing with
non-local jumps.  It is useful to be aware of them, and to realize
that they all refer to the same basic mechanism.

@itemize @bullet
@item
Actually making a non-local jump may be called @dfn{raising an
exception}, @dfn{raising a signal}, @dfn{throwing an exception} or
@dfn{doing a long jump}.  When the jump indicates an error condition,
people may talk about @dfn{signalling}, @dfn{raising} or @dfn{throwing}
@dfn{an error}.

@item
Handling the jump at its target may be referred to as @dfn{catching} or
@dfn{handling} the @dfn{exception}, @dfn{signal} or, where an error
condition is involved, @dfn{error}.
@end itemize

Where @dfn{signal} and @dfn{signalling} are used, special care is needed
to avoid the risk of confusion with POSIX signals.

This manual prefers to speak of throwing and catching exceptions, since
this terminology matches the corresponding Guile primitives.


@node Catch
@subsection Catching Exceptions

@code{catch} is used to set up a target for a possible non-local jump.
The arguments of a @code{catch} expression are a @dfn{key}, which
restricts the set of exceptions to which this @code{catch} applies, a
thunk that specifies the code to execute and a @dfn{handler} procedure
that says what to do if an exception is thrown while executing the code.
Note that if the execution thunk executes @dfn{normally}, which means
without throwing any exceptions, the handler procedure is not called at
all.

When an exception is thrown using the @code{throw} function, the first
argument of the @code{throw} is a symbol that indicates the type of the
exception.  For example, Guile throws an exception using the symbol
@code{numerical-overflow} to indicate numerical overflow errors such as
division by zero:

@lisp
(/ 1 0)
@result{}
ABORT: (numerical-overflow)
@end lisp

The @var{key} argument in a @code{catch} expression corresponds to this
symbol.  @var{key} may be a specific symbol, such as
@code{numerical-overflow}, in which case the @code{catch} applies
specifically to exceptions of that type; or it may be @code{#t}, which
means that the @code{catch} applies to all exceptions, irrespective of
their type.

The second argument of a @code{catch} expression should be a thunk
(i.e. a procedure that accepts no arguments) that specifies the normal
case code.  The @code{catch} is active for the execution of this thunk,
including any code called directly or indirectly by the thunk's body.
Evaluation of the @code{catch} expression activates the catch and then
calls this thunk.

The third argument of a @code{catch} expression is a handler procedure.
If an exception is thrown, this procedure is called with exactly the
arguments specified by the @code{throw}.  Therefore, the handler
procedure must be designed to accept a number of arguments that
corresponds to the number of arguments in all @code{throw} expressions
that can be caught by this @code{catch}.

@deffn {Scheme Procedure} catch key thunk handler
@deffnx {C Function} scm_catch (key, thunk, handler)
Invoke @var{thunk} in the dynamic context of @var{handler} for
exceptions matching @var{key}.  If thunk throws to the symbol
@var{key}, then @var{handler} is invoked this way:
@lisp
(handler key args ...)
@end lisp

@var{key} is a symbol or @code{#t}.

@var{thunk} takes no arguments.  If @var{thunk} returns
normally, that is the return value of @code{catch}.

Handler is invoked outside the scope of its own @code{catch}.
If @var{handler} again throws to the same key, a new handler
from further up the call chain is invoked.

If the key is @code{#t}, then a throw to @emph{any} symbol will
match this call to @code{catch}.
@end deffn

If the handler procedure needs to match a variety of @code{throw}
expressions with varying numbers of arguments, you should write it like
this:

@lisp
(lambda (key . args)
  @dots{})
@end lisp

@noindent
The @var{key} argument is guaranteed always to be present, because a
@code{throw} without a @var{key} is not valid.  The number and
interpretation of the @var{args} varies from one type of exception to
another, but should be specified by the documentation for each exception
type.

Note that, once the handler procedure is invoked, the catch that led to
the handler procedure being called is no longer active.  Therefore, if
the handler procedure itself throws an exception, that exception can
only be caught by another active catch higher up the call stack, if
there is one.

@sp 1
@deftypefn {C Function} SCM scm_internal_catch (SCM tag, scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data)
The above @code{scm_catch} takes Scheme procedures as body and handler
arguments.  @code{scm_internal_catch} is an equivalent taking C
functions.

@var{body} is called as @code{@var{body} (@var{body_data})} with a
catch on exceptions of the given @var{tag} type.  If an exception is
caught, @var{handler} is called @code{@var{handler}
(@var{handler_data}, @var{key}, @var{args})}.  @var{key} and
@var{args} are the @code{SCM} key and argument list from the
@code{throw}.

@tpindex scm_t_catch_body
@tpindex scm_t_catch_handler
@var{body} and @var{handler} should have the following prototypes.
@code{scm_t_catch_body} and @code{scm_t_catch_handler} are pointer
typedefs for these.

@example
SCM body (void *data);
SCM handler (void *data, SCM key, SCM args);
@end example

The @var{body_data} and @var{handler_data} parameters are passed to
the respective calls so an application can communicate extra
information to those functions.

If the data consists of an @code{SCM} object, care should be taken
that it isn't garbage collected while still required.  If the
@code{SCM} is a local C variable, one way to protect it is to pass a
pointer to that variable as the data parameter, since the C compiler
will then know the value must be held on the stack.  Another way is to
use @code{scm_remember_upto_here_1} (@pxref{Remembering During
Operations}).
@end deftypefn


@node Throw
@subsection Throwing Exceptions

The @code{throw} primitive is used to throw an exception.  One argument,
the @var{key}, is mandatory, and must be a symbol; it indicates the type
of exception that is being thrown.  Following the @var{key},
@code{throw} accepts any number of additional arguments, whose meaning
depends on the exception type.  The documentation for each possible type
of exception should specify the additional arguments that are expected
for that kind of exception.

@deffn {Scheme Procedure} throw key . args
@deffnx {C Function} scm_throw (key, args)
Invoke the catch form matching @var{key}, passing @var{args} to the
@var{handler}.  

@var{key} is a symbol.  It will match catches of the same symbol or of
@code{#t}.

If there is no handler at all, Guile prints an error and then exits.
@end deffn

When an exception is thrown, it will be caught by the innermost
@code{catch} expression that applies to the type of the thrown
exception; in other words, the innermost @code{catch} whose @var{key} is
@code{#t} or is the same symbol as that used in the @code{throw}
expression.  Once Guile has identified the appropriate @code{catch}, it
handles the exception by applying that @code{catch} expression's handler
procedure to the arguments of the @code{throw}.

If there is no appropriate @code{catch} for a thrown exception, Guile
prints an error to the current error port indicating an uncaught
exception, and then exits.  In practice, it is quite difficult to
observe this behaviour, because Guile when used interactively installs a
top level @code{catch} handler that will catch all exceptions and print
an appropriate error message @emph{without} exiting.  For example, this
is what happens if you try to throw an unhandled exception in the
standard Guile REPL; note that Guile's command loop continues after the
error message:

@lisp
guile> (throw 'badex)
<unnamed port>:3:1: In procedure gsubr-apply @dots{}
<unnamed port>:3:1: unhandled-exception: badex
ABORT: (misc-error)
guile> 
@end lisp

The default uncaught exception behaviour can be observed by evaluating a
@code{throw} expression from the shell command line:

@example
$ guile -c "(begin (throw 'badex) (display \"here\\n\"))"
guile: uncaught throw to badex: ()
$ 
@end example

@noindent
That Guile exits immediately following the uncaught exception
is shown by the absence of any output from the @code{display}
expression, because Guile never gets to the point of evaluating that
expression.


@node Lazy Catch
@subsection Catch Without Unwinding

A @dfn{lazy catch} is used in the same way as a normal @code{catch},
with @var{key}, @var{thunk} and @var{handler} arguments specifying the
exception type, normal case code and handler procedure, but differs in
one important respect: the handler procedure is executed without
unwinding the call stack from the context of the @code{throw} expression
that caused the handler to be invoked.

@deffn {Scheme Procedure} lazy-catch key thunk handler
@deffnx {C Function} scm_lazy_catch (key, thunk, handler)
This behaves exactly like @code{catch}, except that it does
not unwind the stack before invoking @var{handler}.
The @var{handler} procedure is not allowed to return:
it must throw to another catch, or otherwise exit non-locally.
@end deffn

@deftypefn {C Function} SCM scm_internal_lazy_catch (SCM tag, scm_t_catch_body body, void *body_data, scm_t_catch_handler handler, void *handler_data)
The above @code{scm_lazy_catch} takes Scheme procedures as body and
handler arguments.  @code{scm_internal_lazy_catch} is an equivalent
taking C functions.  See @code{scm_internal_catch} (@pxref{Catch}) for
a description of the parameters, the behaviour however of course
follows @code{lazy-catch}.
@end deftypefn

Typically, @var{handler} should save any desired state associated with
the stack at the point where the corresponding @code{throw} occurred,
and then throw an exception itself --- usually the same exception as the
one it caught.  If @var{handler} is invoked and does @emph{not} throw an
exception, Guile itself throws an exception with key @code{misc-error}.

Not unwinding the stack means that throwing an exception that is caught
by a @code{lazy-catch} is @emph{almost} equivalent to calling the
@code{lazy-catch}'s handler inline instead of each @code{throw}, and
then omitting the surrounding @code{lazy-catch}.  In other words,

@lisp
(lazy-catch 'key
  (lambda () @dots{} (throw 'key args @dots{}) @dots{})
  handler)
@end lisp

@noindent
is @emph{almost} equivalent to

@lisp
((lambda () @dots{} (handler 'key args @dots{}) @dots{}))
@end lisp

@noindent
But why only @emph{almost}?  The difference is that with
@code{lazy-catch} (as with normal @code{catch}), the dynamic context is
unwound back to just outside the @code{lazy-catch} expression before
invoking the handler.  (For an introduction to what is meant by dynamic
context, @xref{Dynamic Wind}.)

Then, when the handler @emph{itself} throws an exception, that exception
must be caught by some kind of @code{catch} (including perhaps another
@code{lazy-catch}) higher up the call stack.

The dynamic context also includes @code{with-fluids} blocks (REFFIXME),
so the effect of unwinding the dynamic context can also be seen in fluid
variable values.  This is illustrated by the following code, in which
the normal case thunk uses @code{with-fluids} to temporarily change the
value of a fluid:

@lisp
(define f (make-fluid))
(fluid-set! f "top level value")

(define (handler . args)
  (cons (fluid-ref f) args))

(lazy-catch 'foo
            (lambda ()
              (with-fluids ((f "local value"))
                (throw 'foo)))
            handler)
@result{}
("top level value" foo)

((lambda ()
   (with-fluids ((f "local value"))
     (handler 'foo))))
@result{}
("local value" foo)
@end lisp

@noindent
In the @code{lazy-catch} version, the unwinding of dynamic context
restores @code{f} to its value outside the @code{with-fluids} block
before the handler is invoked, so the handler's @code{(fluid-ref f)}
returns the external value.

@code{lazy-catch} is useful because it permits the implementation of
debuggers and other reflective programming tools that need to access the
state of the call stack at the exact point where an exception or an
error is thrown.  For an example of this, see REFFIXME:stack-catch.


@node Exception Implementation
@subsection How Guile Implements Exceptions

It is traditional in Scheme to implement exception systems using
@code{call-with-current-continuation}.  Continuations
(@pxref{Continuations}) are such a powerful concept that any other
control mechanism --- including @code{catch} and @code{throw} --- can be
implemented in terms of them.

Guile does not implement @code{catch} and @code{throw} like this,
though.  Why not?  Because Guile is specifically designed to be easy to
integrate with applications written in C.  In a mixed Scheme/C
environment, the concept of @dfn{continuation} must logically include
``what happens next'' in the C parts of the application as well as the
Scheme parts, and it turns out that the only reasonable way of
implementing continuations like this is to save and restore the complete
C stack.

So Guile's implementation of @code{call-with-current-continuation} is a
stack copying one.  This allows it to interact well with ordinary C
code, but means that creating and calling a continuation is slowed down
by the time that it takes to copy the C stack.

The more targeted mechanism provided by @code{catch} and @code{throw}
does not need to save and restore the C stack because the @code{throw}
always jumps to a location higher up the stack of the code that executes
the @code{throw}.  Therefore Guile implements the @code{catch} and
@code{throw} primitives independently of
@code{call-with-current-continuation}, in a way that takes advantage of
this @emph{upwards only} nature of exceptions.


@node Error Reporting
@section Procedures for Signaling Errors

Guile provides a set of convenience procedures for signaling error
conditions that are implemented on top of the exception primitives just
described.

@deffn {Scheme Procedure} error msg args @dots{}
Raise an error with key @code{misc-error} and a message constructed by
displaying @var{msg} and writing @var{args}.
@end deffn

@deffn {Scheme Procedure} scm-error key subr message args data
@deffnx {C Function} scm_error_scm (key, subr, message, args, data)
Raise an error with key @var{key}.  @var{subr} can be a string
naming the procedure associated with the error, or @code{#f}.
@var{message} is the error message string, possibly containing
@code{~S} and @code{~A} escapes.  When an error is reported,
these are replaced by formatting the corresponding members of
@var{args}: @code{~A} (was @code{%s} in older versions of
Guile) formats using @code{display} and @code{~S} (was
@code{%S}) formats using @code{write}.  @var{data} is a list or
@code{#f} depending on @var{key}: if @var{key} is
@code{system-error} then it should be a list containing the
Unix @code{errno} value; If @var{key} is @code{signal} then it
should be a list containing the Unix signal number; otherwise
it will usually be @code{#f}.
@end deffn

@deffn {Scheme Procedure} strerror err
@deffnx {C Function} scm_strerror (err)
Return the Unix error message corresponding to @var{err}, which
must be an integer value.
@end deffn

@c begin (scm-doc-string "boot-9.scm" "false-if-exception")
@deffn syntax false-if-exception expr
Returns the result of evaluating its argument; however
if an exception occurs then @code{#f} is returned instead.
@end deffn
@c end


@node Dynamic Wind
@section Dynamic Wind

@rnindex dynamic-wind
@deffn {Scheme Procedure} dynamic-wind in_guard thunk out_guard
@deffnx {C Function} scm_dynamic_wind (in_guard, thunk, out_guard)
All three arguments must be 0-argument procedures.
@var{in_guard} is called, then @var{thunk}, then
@var{out_guard}.

If, any time during the execution of @var{thunk}, the
dynamic extent of the @code{dynamic-wind} expression is escaped
non-locally, @var{out_guard} is called.  If the dynamic extent of
the dynamic-wind is re-entered, @var{in_guard} is called.  Thus
@var{in_guard} and @var{out_guard} may be called any number of
times.
@lisp
(define x 'normal-binding)
@result{} x
(define a-cont  (call-with-current-continuation
                  (lambda (escape)
                     (let ((old-x x))
                       (dynamic-wind
                          ;; in-guard:
                          ;;
                          (lambda () (set! x 'special-binding))

                          ;; thunk
                          ;;
                          (lambda () (display x) (newline)
                                     (call-with-current-continuation escape)
                                     (display x) (newline)
                                     x)

                          ;; out-guard:
                          ;;
                          (lambda () (set! x old-x)))))))

;; Prints:
special-binding
;; Evaluates to:
@result{} a-cont
x
@result{} normal-binding
(a-cont #f)
;; Prints:
special-binding
;; Evaluates to:
@result{} a-cont  ;; the value of the (define a-cont...)
x
@result{} normal-binding
a-cont
@result{} special-binding
@end lisp
@end deffn

@node Frames
@section Frames

For Scheme code, the fundamental procedure to react to non-local entry
and exits of dynamic contexts is @code{dynamic-wind}.  C code could use
@code{scm_internal_dynamic_wind}, but since C does not allow the
convenient construction of anonymous procedures that close over lexical
variables, this will be, well, inconvenient.  Instead, C code can use
@dfn{frames}.

Guile offers the functions @code{scm_frame_begin} and
@code{scm_frame_end} to delimit a dynamic extent.  Within this dynamic
extent, which is called a @dfn{frame}, you can perform various
@dfn{frame actions} that control what happens when the frame is entered
or left.  For example, you can register a cleanup routine with
@code{scm_frame_unwind} that is executed when the frame is left.  There are
several other more specialized frame actions as well, for example to
temporarily block the execution of asyncs or to temporarily change the
current output port.  They are described elsewhere in this manual.

Here is an example that shows how to prevent memory leaks.

@example

/* Suppose there is a function called FOO in some library that you
   would like to make available to Scheme code (or to C code that
   follows the Scheme conventions).

   FOO takes two C strings and returns a new string.  When an error has
   occurred in FOO, it returns NULL.
*/

char *foo (char *s1, char *s2);

/* SCM_FOO interfaces the C function FOO to the Scheme way of life.
   It takes care to free up all temporary strings in the case of
   non-local exits.

   It uses SCM_TO_STRING as a helper procedure.
 */

char *
scm_to_string (SCM obj)
@{
  if (SCM_STRINGP (obj))
    @{
      char *res = scm_malloc (SCM_STRING_LENGTH (obj)+1);
      strcpy (res, SCM_STRING_CHARS (obj));
      scm_remember_upto_here_1 (obj);
      return res;
    @}
  else
    scm_wrong_type_arg ("scm_to_string", 1, obj);
@}

SCM
scm_foo (SCM s1, SCM s2)
@{
  char *c_s1, *c_s2, *c_res;

  scm_frame_begin (0);

  c_s1 = scm_to_string (s1);
  scm_frame_unwind_handler (free, c_s1, SCM_F_WIND_EXPLICITLY);

  c_s2 = scm_to_string (s2);
  scm_frame_unwind_handler (free, c_s2, SCM_F_WIND_EXPLICITLY);

  c_res = foo (c_s1, c_s2);
  if (c_res == NULL)
    scm_memory_error ("foo");

  scm_frame_end ();

  return scm_take0str (res);
@}
@end example

@deftp {C Type} scm_t_frame_flags
This is an enumeration of several flags that modify the behavior of
@code{scm_begin_frame}.  The flags are listed in the following table.

@table @code
@item SCM_F_FRAME_REWINDABLE
The frame is @dfn{rewindable}.  This means that it can be reentered
non-locally (via the invokation of a continuation).  The default is that
a frame can not be reentered non-locally.
@end table

@end deftp

@deftypefn {C Function} void scm_frame_begin (scm_t_frame_flags flags)
The function @code{scm_begin_frame} starts a new frame and makes it the
`current' one.  

The @var{flags} argument determines the default behavior of the frame.
For normal frames, use 0.  This will result in a frame that can not be
reentered with a captured continuation.  When you are prepared to handle
reentries, include @code{SCM_F_FRAME_REWINDABLE} in @var{flags}.

Being prepared for reentry means that the effects of unwind handlers
can be undone on reentry.  In the example above, we want to prevent a
memory leak on non-local exit and thus register an unwind handler that
frees the memory.  But once the memory is freed, we can not get it
back on reentry.  Thus reentry can not be allowed.

The consequence is that continuations become less useful when
non-reenterable frames are captured, but you don't need to worry about
that too much.

The frame is ended either implicitly when a non-local exit happens, or
explicitly with @code{scm_end_frame}.  You must make sure that a frame
is indeed ended properly.  If you fail to call @code{scm_end_frame}
for each @code{scm_begin_frame}, the behavior is undefined.
@end deftypefn

@deftypefn {C Function} void scm_frame_end ()
End the current frame explicitly and make the previous frame current.
@end deftypefn

@deftp {C Type} scm_t_wind_flags
This is an enumeration of several flags that modify the behavior of
@code{scm_on_unwind_handler} and @code{scm_on_rewind_handler}.  The
flags are listed in the following table.

@table @code
@item SCM_F_WIND_EXPLICITLY
The registered action is also carried out when the frame is entered or
left locally.
@end table
@end deftp

@deftypefn {C Function} void scm_frame_unwind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
@deftypefnx {C Function} void scm_frame_unwind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
Arranges for @var{func} to be called with @var{data} as its arguments
when the current frame ends implicitly.  If @var{flags} contains
@code{SCM_F_WIND_EXPLICITLY}, @var{func} is also called when the frame
ends explicitly with @code{scm_frame_end}.

The function @code{scm_frame_unwind_handler_with_scm} takes care that
@var{data} is protected from garbage collection.
@end deftypefn

@deftypefn {C Function} void scm_frame_rewind_handler (void (*func)(void *), void *data, scm_t_wind_flags flags)
@deftypefnx {C Function} void scm_frame_rewind_handler_with_scm (void (*func)(SCM), SCM data, scm_t_wind_flags flags)
Arrange for @var{func} to be called with @var{data} as its argument when
the current frame is restarted by rewinding the stack.  When @var{flags}
contains @code{SCM_F_WIND_EXPLICITLY}, @var{func} is called immediately
as well.

The function @code{scm_frame_rewind_handler_with_scm} takes care that
@var{data} is protected from garbage collection.
@end deftypefn


@node Handling Errors
@section How to Handle Errors

Error handling is based on @code{catch} and @code{throw}.  Errors are
always thrown with a @var{key} and four arguments:

@itemize @bullet
@item
@var{key}: a symbol which indicates the type of error.  The symbols used
by libguile are listed below.

@item
@var{subr}: the name of the procedure from which the error is thrown, or
@code{#f}.

@item
@var{message}: a string (possibly language and system dependent)
describing the error.  The tokens @code{~A} and @code{~S} can be
embedded within the message: they will be replaced with members of the
@var{args} list when the message is printed.  @code{~A} indicates an
argument printed using @code{display}, while @code{~S} indicates an
argument printed using @code{write}.  @var{message} can also be
@code{#f}, to allow it to be derived from the @var{key} by the error
handler (may be useful if the @var{key} is to be thrown from both C and
Scheme).

@item
@var{args}: a list of arguments to be used to expand @code{~A} and
@code{~S} tokens in @var{message}.  Can also be @code{#f} if no
arguments are required.

@item
@var{rest}: a list of any additional objects required. e.g., when the
key is @code{'system-error}, this contains the C errno value.  Can also
be @code{#f} if no additional objects are required.
@end itemize

In addition to @code{catch} and @code{throw}, the following Scheme
facilities are available:

@deffn {Scheme Procedure} display-error stack port subr message args rest
@deffnx {C Function} scm_display_error (stack, port, subr, message, args, rest)
Display an error message to the output port @var{port}.
@var{stack} is the saved stack for the error, @var{subr} is
the name of the procedure in which the error occurred and
@var{message} is the actual error message, which may contain
formatting instructions. These will format the arguments in
the list @var{args} accordingly.  @var{rest} is currently
ignored.
@end deffn

The following are the error keys defined by libguile and the situations
in which they are used:

@itemize @bullet
@item
@cindex @code{error-signal}
@code{error-signal}: thrown after receiving an unhandled fatal signal
such as SIGSEGV, SIGBUS, SIGFPE etc.  The @var{rest} argument in the throw
contains the coded signal number (at present this is not the same as the
usual Unix signal number).

@item
@cindex @code{system-error}
@code{system-error}: thrown after the operating system indicates an
error condition.  The @var{rest} argument in the throw contains the
errno value.

@item
@cindex @code{numerical-overflow}
@code{numerical-overflow}: numerical overflow.

@item
@cindex @code{out-of-range}
@code{out-of-range}: the arguments to a procedure do not fall within the
accepted domain.

@item
@cindex @code{wrong-type-arg}
@code{wrong-type-arg}: an argument to a procedure has the wrong type.

@item
@cindex @code{wrong-number-of-args}
@code{wrong-number-of-args}: a procedure was called with the wrong number
of arguments.

@item
@cindex @code{memory-allocation-error}
@code{memory-allocation-error}: memory allocation error.

@item
@cindex @code{stack-overflow}
@code{stack-overflow}: stack overflow error.

@item
@cindex @code{regular-expression-syntax}
@code{regular-expression-syntax}: errors generated by the regular
expression library.

@item
@cindex @code{misc-error}
@code{misc-error}: other errors.
@end itemize


@subsection C Support

In the following C functions, @var{SUBR} and @var{MESSAGE} parameters
can be @code{NULL} to give the effect of @code{#f} described above.

@deftypefn {C Function} SCM scm_error (SCM @var{key}, char *@var{subr}, char *@var{message}, SCM @var{args}, SCM @var{rest})
Throw an error, as per @code{scm-error} above.
@end deftypefn

@deftypefn {C Function} void scm_syserror (char *@var{subr})
@deftypefnx {C Function} void scm_syserror_msg (char *@var{subr}, char *@var{message}, SCM @var{args})
Throw an error with key @code{system-error} and supply @code{errno} in
the @var{rest} argument.  For @code{scm_syserror} the message is
generated using @code{strerror}.

Care should be taken that any code in between the failing operation
and the call to these routines doesn't change @code{errno}.
@end deftypefn

@deftypefn {C Function} void scm_num_overflow (char *@var{subr})
@deftypefnx {C Function} void scm_out_of_range (char *@var{subr}, SCM @var{bad_value})
@deftypefnx {C Function} void scm_wrong_num_args (SCM @var{proc})
@deftypefnx {C Function} void scm_wrong_type_arg (char *@var{subr}, int @var{argnum}, SCM @var{bad_value})
@deftypefnx {C Function} void scm_memory_error (char *@var{subr})
Throw an error with the various keys described above.

For @code{scm_wrong_num_args}, @var{proc} should be a Scheme symbol
which is the name of the procedure incorrectly invoked.
@end deftypefn


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