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 @settitle The FreeM Manual  @settitle The FreeM Manual
   
 @copying  @copying
 This manual is for FreeM, (version 0.64.0-rc1), which is a free software@footnote{FreeM subscribes to the software licensing philosophy described in @emph{Free Software, Free Society:  This manual is for FreeM, (version 0.64.0-rc1), which is a free software implementation of the M programming language.
 Selected Essays of Richard M. Stallman}.} implementation of the M programming language.  
   
   Print-optimized versions of this book are typeset in @emph{Computer Modern} by the author using the @emph{GNU Texinfo} tools.
   
 Copyright @copyright{} 2014-2025 Coherent Logic Development LLC  Copyright @copyright{} 2014-2025 Coherent Logic Development LLC
   
Line 44  This is the official manual for the Free Line 44  This is the official manual for the Free
 @menu  @menu
 * Introduction::                        About the FreeM Project, its history, and its goals.  * Introduction::                        About the FreeM Project, its history, and its goals.
 * Document Conventions::                Conventions used in this manual.  * Document Conventions::                Conventions used in this manual.
   * A Note on Standards::                 FreeM positioning regarding the M Development Committee.
 * FreeM Invocation::                    How to invoke FreeM from the command line.  * FreeM Invocation::                    How to invoke FreeM from the command line.
 * The FreeM Environment Daemon::        Managing shared resources in the FreeM environment.  * The FreeM Environment Daemon::        Managing shared resources in the FreeM environment.
 * The FreeM Direct-Mode Environment::   Executing M programs interactively.  * The FreeM Direct-Mode Environment::   Executing M programs interactively.
Line 117  When GT.M was open-sourced in late 1999, Line 118  When GT.M was open-sourced in late 1999,
   
 In February of 2020, work began in earnest to build a development and support infrastructure for FreeM and begin the careful process of refining it into a more stable and robust application.  In February of 2020, work began in earnest to build a development and support infrastructure for FreeM and begin the careful process of refining it into a more stable and robust application.
   
   For more information on FreeM history, see @code{@emph{$PREFIX}/share/freem/doc/freem_history.*} (distributed in PostScript, PDF, and plain text formats).
   
 @section Production Readiness  @section Production Readiness
   
 FreeM is not yet production-ready. There are several show-stopping bugs that preclude a general release for public use:  FreeM is not yet production-ready. There are several show-stopping bugs that preclude a general release for public use:
Line 276  Refers to the name of an M structured sy Line 279  Refers to the name of an M structured sy
 Refers to a truth-valued expression.  Refers to a truth-valued expression.
 @end table  @end table
   
   @node A Note on Standards
   @chapter A Note on Standards
   @cindex MDC
   @cindex M Development Committee
   @cindex ANSI X11.1
   @cindex standards, ANSI
   @cindex standards, MDC
   
   FreeM attempts to implement as many features as possible from the M Development Committee's unpublished @emph{Millennium Draft Standard}, as well as its predecessors.
   
   The maintainer of FreeM (who is also the author of this book) is largely in favor of standardization efforts, and hopes that the MDC will resume activities, and will happily participate if it does so in an open, public, transparent, and democratic manner. Until then, however, FreeM will attempt to improve the M language, in cooperation with other free software M implementers where possible. Any breaking changes introduced in future MDC releases of the @emph{Standard} (such as the rumored @emph{M5}) which prove incompatible with FreeM will be handled via the @code{$DIALECT} special variable (to be changed to @code{$ZDIALECT} in a coming release).
   
   The conformance document required per the @emph{Standard} should be installed as a @code{man} page on any computer system where FreeM is made available. Simply type @code{man freem_conformance} to access.
   
 @node FreeM Invocation  @node FreeM Invocation
 @chapter FreeM Invocation  @chapter FreeM Invocation
 @cindex invocation, command-line  @cindex invocation, command-line
Line 681  Please note that FreeM is not entirely s Line 698  Please note that FreeM is not entirely s
 * $SYSTEM::                  MDC system ID of FreeM.  * $SYSTEM::                  MDC system ID of FreeM.
 * $TEST::                    Truth value of last conditional expression or result of certain @code{LOCK} operations.  * $TEST::                    Truth value of last conditional expression or result of certain @code{LOCK} operations.
 * $TLEVEL::                  Current level of transaction nesting.  * $TLEVEL::                  Current level of transaction nesting.
 * $WITH::                    Current variable prefix set by @code{WITH} command.  * $WITH::                    Current variable prefix set by @code{ZWITH} command.
 * $X::                       Current horizontal screen position.  * $X::                       Current horizontal screen position.
 * $Y::                       Current vertical screen position.  * $Y::                       Current vertical screen position.
 * $ZA::                      Current position of file on @code{$IO}.  * $ZA::                      Current position of file on @code{$IO}.
Line 877  uncommitted transactions exist. Line 894  uncommitted transactions exist.
 @cindex $WITH  @cindex $WITH
 @cindex intrinsic special variables, $WITH  @cindex intrinsic special variables, $WITH
   
 Returns the variable prefix set by the @code{@ref{WITH}} command.  Returns the variable prefix set by the @code{@ref{ZWITH}} command.
   
 @node $X  @node $X
 @section $X  @section $X
Line 2000  Identical to @ref{$TRANSLATE()}, except Line 2017  Identical to @ref{$TRANSLATE()}, except
 * AUNBLOCK::                            Decrement the block counter for one or more event classes.  * AUNBLOCK::                            Decrement the block counter for one or more event classes.
 * BREAK::                               Interrupt a running routine to allow interactive debugging.  * BREAK::                               Interrupt a running routine to allow interactive debugging.
 * CLOSE::                               Close an input/output device.  * CLOSE::                               Close an input/output device.
 * CONST::                               Define a constant that cannot be altered after initial definition.  
 * DO::                                  Transfer program control to one or more subroutines or introduces a new execution level.  * DO::                                  Transfer program control to one or more subroutines or introduces a new execution level.
 * ELSE::                                Execute the remainder of a line if @code{@ref{$TEST}} evaluates @emph{false}.  * ELSE::                                Execute the remainder of a line if @code{@ref{$TEST}} evaluates @emph{false}.
 * FOR::                                 Repeat execution of a line or block of code.  * FOR::                                 Repeat execution of a line or block of code.
Line 2013  Identical to @ref{$TRANSLATE()}, except Line 2029  Identical to @ref{$TRANSLATE()}, except
 * KSUBSCRIPTS::                         Kill only the descendant subscripts of a local, global, global, or structured system variable.  * KSUBSCRIPTS::                         Kill only the descendant subscripts of a local, global, global, or structured system variable.
 * KVALUE::                              Kill only the value of a local, global, or structured system variable.  * KVALUE::                              Kill only the value of a local, global, or structured system variable.
 * LOCK::                                Control advisory locking for concurrency control.  * LOCK::                                Control advisory locking for concurrency control.
 * MAP::                                 Map a global name to a non-default namespace.  
 * MERGE::                               Merge contents of one local, global, or structured system variable into another.  * MERGE::                               Merge contents of one local, global, or structured system variable into another.
 * NEW::                                 Introduce a new scope for a specified local variable or intrinsic special variable or instantiate an object.  * NEW::                                 Introduce a new scope for a specified local variable or intrinsic special variable or instantiate an object.
 * OPEN::                                Open a sequential or socket input/output device.  * OPEN::                                Open a sequential or socket input/output device.
Line 2022  Identical to @ref{$TRANSLATE()}, except Line 2037  Identical to @ref{$TRANSLATE()}, except
 * SET::                                 Set the value of a local variable, global, intrinsic special variable, or structured system variable.  * SET::                                 Set the value of a local variable, global, intrinsic special variable, or structured system variable.
 * TCOMMIT::                             Commit a transaction.  * TCOMMIT::                             Commit a transaction.
 * THEN::                                Preserve @code{@ref{$TEST}} until the end of the current line.  * THEN::                                Preserve @code{@ref{$TEST}} until the end of the current line.
 * THROW::                               Programmatically raise an error condition.  
 * TROLLBACK::                           Roll back all pending transactions.  * TROLLBACK::                           Roll back all pending transactions.
 * TSTART::                              Introduce a new transaction processing level.  * TSTART::                              Introduce a new transaction processing level.
 * UNMAP::                               Remove a mapping of a global to a non-default namespace.  
 * USE::                                 Set the currently-active input/output device.  * USE::                                 Set the currently-active input/output device.
 * VIEW::                                Modify FreeM internal parameters.  * VIEW::                                Modify FreeM internal parameters.
 * WATCH::                               Enable or disable watchpoints, or set or clear watchpoints on specified globals, locals, or structured system variables.  
 * WITH::                                Set prefix for future variable references.  
 * WRITE::                               Write output to current input/output device.  * WRITE::                               Write output to current input/output device.
 * XECUTE::                              Interpret string as M code.  * XECUTE::                              Interpret string as M code.
 * ZBREAK::                              Unknown.  * ZBREAK::                              Unknown.
   * ZCONST::                              Define a constant that cannot be altered after initial definition.
 * ZGO::                                 Unknown.  * ZGO::                                 Unknown.
 * ZHALT::                               Unknown.  * ZHALT::                               Unknown.
 * ZINSERT::                             Insert code into routine buffer.  * ZINSERT::                             Insert code into routine buffer.
 * ZJOB::                                Unknown.  * ZJOB::                                Invokes a job, ignoring any timeouts.
 * ZLOAD::                               Load routine into routine buffer.  * ZLOAD::                               Load routine into routine buffer.
   * ZMAP::                                Map a global name to a non-default namespace.
 * ZNEW::                                Unknown.  * ZNEW::                                Unknown.
 * ZPRINT::                              Print contents of routine buffer.  * ZPRINT::                              Print contents of routine buffer.
 * ZQUIT::                               Unknown.  * ZQUIT::                               Quits multiple stack levels at once.
 * ZREMOVE::                             Remove code from routine buffer.  * ZREMOVE::                             Remove code from routine buffer.
 * ZSAVE::                               Save routine buffer to disk.  * ZSAVE::                               Save routine buffer to disk.
   * ZTHROW::                              Programmatically raise an error condition.
 * ZTRAP::                               Unknown.  * ZTRAP::                               Unknown.
   * ZUNMAP::                              Remove a mapping of a global to a non-default namespace.
   * ZWATCH::                              Enable or disable watchpoints, or set or clear watchpoints on specified globals, locals, or structured system variables.
   * ZWITH::                               Set prefix for future variable references.
 * ZWRITE::                              Write local variable, global, or structured system variable to @code{@ref{$IO}}.  * ZWRITE::                              Write local variable, global, or structured system variable to @code{@ref{$IO}}.
 @end menu  @end menu
   
Line 2341  In its argumentless form, @code{CLOSE} c Line 2358  In its argumentless form, @code{CLOSE} c
   
 In its single-argument form, @code{CLOSE} closes the I/O device associated with channel @emph{channel}, provided that @emph{channel} represents a currently-open device, and the optional @emph{postcondition} is @emph{true} or omitted.  In its single-argument form, @code{CLOSE} closes the I/O device associated with channel @emph{channel}, provided that @emph{channel} represents a currently-open device, and the optional @emph{postcondition} is @emph{true} or omitted.
   
 @node CONST  
 @section CONST  
 @cindex CONST  
 @cindex commands, CONST  
 @cindex commands, non-standard  
 @emph{FreeM Extension}  
   
 Defines a local @emph{constant}, or variable that cannot be altered after its initial definition, provided the optional @emph{postcondition} is @emph{true} or omitted.  
   
 Constants must only be locals, and globals are not supported.  
   
 @emph{Syntax}  
   
 @example  
 @code{CONST@emph{:postcondition} @emph{mref1}=@emph{initial-value1},...,@emph{mrefN}=@emph{initial-valueN}}  
 @end example  
   
 @node DO  @node DO
 @section DO  @section DO
 @cindex DO  @cindex DO
Line 2404  ELSE[@emph{:postcondition}] Line 2404  ELSE[@emph{:postcondition}]
 @quotation  @quotation
 @emph{Non-Standard Behavior}  @emph{Non-Standard Behavior}
   
 FreeM allows a @emph{postcondition} on @code{ELSE}. While explicitly forbidden in the @emph{standard}--and for good reason--it was decided that FreeM should allow postconditions everywhere, both for the sake of foolish consistency (the likes of which Emerson warned against), and for the benefit of entrants to a hypothetical future obfuscated M contest, and those with a Machiavellian predisposition to wicked perversions and undue cleverness.  FreeM allows a @emph{postcondition} on @code{ELSE}. While explicitly forbidden in the @emph{Standard}, it was decided that FreeM should allow postconditions everywhere, both for the sake of foolish consistency (the likes of which Emerson warned against), and for the benefit of entrants to a hypothetical future obfuscated M contest, and those with a Machiavellian predisposition to wicked perversions and undue cleverness.
   
 Using postconditions on @code{ELSE} should be strictly avoided in production code, as they have no practical use, and may contribute to technical debt, hardening of the arteries, hobgoblins, a small mind, a surfeit of logic, climate change, @emph{Daily WTF} rants, or the meltdown of global financial markets.  Using postconditions on @code{ELSE} should be strictly avoided in production code, as they have no practical use, and may contribute to technical debt, hardening of the arteries, hobgoblins, a small mind, a surfeit of logic, climate change, @emph{Daily WTF} rants, or the meltdown of global financial markets.
 @end quotation  @end quotation
Line 2559  IF[@emph{:postcondition}] @emph{tvexpr}[ Line 2559  IF[@emph{:postcondition}] @emph{tvexpr}[
   
 In its argumentless form, allows the remainder of the line of code following @code{IF} to execute only if @code{$TEST} evaluates to @emph{1}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.  In its argumentless form, allows the remainder of the line of code following @code{IF} to execute only if @code{$TEST} evaluates to @emph{1}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.
   
   
 @emph{Syntax}  @emph{Syntax}
 @example  @example
 IF[@emph{:postcondition}]  IF[@emph{:postcondition}]  @emph{command...}
 @end example  @end example
   
   @cartouche
   @quotation
   @emph{Style Recommendation}
   
   In the interest of readability and maintainability, we recommend avoiding the argumentless form of @code{IF} in new code. It is an obsolete relic of an era when routine sizes were severely limited, and can be difficult to spot, as the use of whitespace (@code{IF  @emph{command}}) makes the intent of its use non-obvious at a glance. It is also far too easy to inadvertently delete the extra space, leading to program errors easily avoided otherwise.
   
   We recommend explicitly checking the value of @code{$TEST} instead, as in @code{IF $TEST @emph{command}} or @code{@emph{command}:$TEST ...}, as this makes the intent immediately clear both to M newcomers and seasoned experts, and sacrifices nothing of value, even on the oldest computer systems where FreeM can be used today.
   @end quotation
   @end cartouche
   
 @node JOB  @node JOB
 @section JOB  @section JOB
 @cindex JOB  @cindex JOB
Line 2713  If @code{LOCK} succeeds within @emph{tim Line 2724  If @code{LOCK} succeeds within @emph{tim
 LOCK[@emph{:postcondition}] @emph{name}[:@emph{timeout}][,...@emph{name}[:@emph{timeout}]]  LOCK[@emph{:postcondition}] @emph{name}[:@emph{timeout}][,...@emph{name}[:@emph{timeout}]]
 @end example  @end example
   
 @node MAP  
 @section MAP  
 @cindex MAP  
 @cindex commands, MAP  
 @cindex commands, implementation-specific  
 @cindex commands, non-standard  
   
 Maps global name @code{gvn} to be mapped to the non-default namespace @emph{expr V namespace}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.  
   
 @emph{Syntax}  
   
 @example  
 MAP[@emph{:postcondition}] GLOBAL @emph{gvn}=@emph{expr V namespace}  
 @end example  
   
 @node MERGE  @node MERGE
 @section MERGE  @section MERGE
 @cindex MERGE  @cindex MERGE
Line 2977  Saves the value of @code{$TEST} until th Line 2973  Saves the value of @code{$TEST} until th
 IF 1 THEN  WRITE "HELLO!",!  IF 1 THEN  WRITE "HELLO!",!
 @end example  @end example
   
 @node THROW  
 @section THROW  
 @cindex THROW  
 @cindex commands, THROW  
 @cindex commands, non-standard  
 @emph{FreeM Extension}  
   
 Raises an error condition as long as the optional @emph{postcondition} is @emph{true} or omitted.  
   
 @emph{Syntax}  
   
 @example  
 @code{THROW@emph{:postcondition} @emph{expr V error-code}}  
 @end example  
   
 @emph{Example}  
   
 @example  
 @code{THROW "M102"}  
 @end example  
   
 @node TROLLBACK  @node TROLLBACK
 @section TROLLBACK  @section TROLLBACK
 @cindex TROLLBACK  @cindex TROLLBACK
Line 3062  If you are using more than one transacti Line 3037  If you are using more than one transacti
 TSTART (FOO,BAR):(SERIAL,TRANSACTIONID="FOO")  TSTART (FOO,BAR):(SERIAL,TRANSACTIONID="FOO")
 @end example  @end example
   
 @node UNMAP  
 @section UNMAP  
 @cindex UNMAP  
 @cindex commands, UNMAP  
 @cindex commands, implementation-specific  
 @cindex commands, non-standard  
   
 Removes any mapping connecting @emph{gvn} to a non-default namespace, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.  
   
 @emph{Syntax}  
   
 @example  
 UNMAP GLOBAL @emph{gvn}  
 @end example  
   
 @node USE  @node USE
 @section USE  @section USE
Line 3380  VIEW 133:@emph{tvexpr} Line 3341  VIEW 133:@emph{tvexpr}
   
 @end table  @end table
   
 @node WATCH  
 @section WATCH  
 @cindex WATCH  
 @cindex commands, WATCH  
 @cindex commands, debugging  
 @cindex commands, implementation-specific  
 @cindex commands, non-standard  
 @emph{FreeM Extension}  
   
 Sets a watchpoint on a global, local, or SSVN node.  
   
 @emph{Syntax}  
   
   
 In its @emph{argumentless} form, @code{WATCH} toggles watchpoints on and off, provided the optional @emph{postcondition} is @emph{true} or omitted.  
   
 @example  
 WATCH[@emph{:postcondition}]  
 @end example  
   
 In its @emph{inclusive} form, @code{WATCH} adds, removes, or examines watchpoints, provided the optional @emph{postcondition} is @emph{true} or omitted.  
   
 A @code{+} adds a new watchpoint to the following variable.  
   
 A @code{-} removes an existing watchpoint for the following variable.  
   
 A @code{?} examines the status of a watchpoint for the following variable.  
   
 @example  
 WATCH[@emph{:postcondition}] [+|-|?]@emph{var1}...,[+|-|?]@emph{varN}  
 @end example  
   
   
 The following example demonstrates turning watchpoint processing on and adding a watchpoint for global variable @code{^snw(1)}. It then changes the value of @code{^snw(1)}.  
   
 @example  
 DEFAULT.USER> WATCH  
    
 Watchpoints enabled.  
    
 DEFAULT.USER> WATCH +^SNW(1)  
    
 Added '^SNW("1")' to the watchlist.  
    
 DEFAULT.USER> SET ^SNW(1)="new value"  
    
 >> WATCHPOINT:  ^SNW("1") => 'new value' (changed 1 times)  
   
 @end example  
   
 The following example will remove that watchpoint:  
   
 @example  
 DEFAULT.USER> WATCH -^SNW(1)  
    
 Removed '^SNW("1")' from the watchlist.  
    
 DEFAULT.USER> WATCH ?^SNW(1)  
    
 '^SNW("1")' is not being watched.  
 @end example  
   
 @node WITH  
 @section WITH  
 @cindex WITH  
 @cindex commands, WITH  
 @cindex commands, non-standard  
 @emph{FreeM Extension}  
   
 NOTE: This command may be deprecated and removed in future FreeM releases.  
   
 Sets a prefix to be applied to all subsequent local variable or constant references.  
   
 @emph{Syntax}  
   
 @example  
 @code{WITH@emph{:postcondition} @emph{var-prefix}}  
 @end example  
   
 In the above single-argument form, sets the @code{$WITH} prefix to @emph{var-prefix}, provided that the optional @emph{postcondition} is either @emph{true} or omitted.  
   
 The @emph{var-prefix} argument may be a string literal or any valid FreeM expression.  
   
 @example  
 @code{WITH@emph{:postcondition}}  
 @end example  
   
 In the above argumentless form, clears the @code{$WITH} prefix, provided the optional @emph{postcondition} is either @emph{true} or omitted. Equivalent to @code{WITH ""}.  
   
   
 @node WRITE  @node WRITE
Line 3489  In the above argumentless form, clears t Line 3362  In the above argumentless form, clears t
 @cindex commands, non-standard  @cindex commands, non-standard
 @emph{FreeM Extension}  @emph{FreeM Extension}
   
   @node ZCONST
   @section ZCONST
   @cindex ZCONST
   @cindex commands, ZCONST
   @cindex commands, non-standard
   @emph{FreeM Extension}
   
   Defines a local @emph{constant}, or variable that cannot be altered after its initial definition, provided the optional @emph{postcondition} is @emph{true} or omitted.
   
   Constants must only be locals, and globals are not supported.
   
   @emph{Syntax}
   
   @example
   @code{ZCONST@emph{:postcondition} @emph{mref1}=@emph{initial-value1},...,@emph{mrefN}=@emph{initial-valueN}}
   @end example
   
 @node ZGO  @node ZGO
 @section ZGO  @section ZGO
 @cindex ZGO  @cindex ZGO
Line 3541  Loads routine @emph{<routine-name>} into Line 3431  Loads routine @emph{<routine-name>} into
 ZLOAD@emph{:postcondition} @emph{<routine-name>}  ZLOAD@emph{:postcondition} @emph{<routine-name>}
 @end example  @end example
   
   
   @node ZMAP
   @section ZMAP
   @cindex ZMAP
   @cindex commands, ZMAP
   @cindex commands, implementation-specific
   @cindex commands, non-standard
   
   Maps global name @code{gvn} to be mapped to the non-default namespace @emph{expr V namespace}, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.
   
   @emph{Syntax}
   
   @example
   ZMAP[@emph{:postcondition}] GLOBAL @emph{gvn}=@emph{expr V namespace}
   @end example
   
 @node ZNEW  @node ZNEW
 @section ZNEW  @section ZNEW
 @cindex ZNEW  @cindex ZNEW
Line 3599  In its argumentless form, quits from @co Line 3505  In its argumentless form, quits from @co
 @cindex commands, non-standard  @cindex commands, non-standard
 @emph{FreeM Extension}  @emph{FreeM Extension}
   
   
   @node ZTHROW
   @section ZTHROW
   @cindex ZTHROW
   @cindex commands, ZTHROW
   @cindex commands, non-standard
   @emph{FreeM Extension}
   
   Raises an error condition as long as the optional @emph{postcondition} is @emph{true} or omitted.
   
   @emph{Syntax}
   
   @example
   @code{ZTHROW@emph{:postcondition} @emph{expr V error-code}}
   @end example
   
   @emph{Example}
   
   @example
   @code{ZTHROW "M102"}
   @end example
   
 @node ZTRAP  @node ZTRAP
 @section ZTRAP  @section ZTRAP
 @cindex ZTRAP  @cindex ZTRAP
Line 3608  In its argumentless form, quits from @co Line 3536  In its argumentless form, quits from @co
 @cindex commands, non-standard  @cindex commands, non-standard
 @emph{FreeM Extension}  @emph{FreeM Extension}
   
   @node ZUNMAP
   @section ZUNMAP
   @cindex ZUNMAP
   @cindex commands, ZUNMAP
   @cindex commands, implementation-specific
   @cindex commands, non-standard
   
   Removes any mapping connecting @emph{gvn} to a non-default namespace, provided the optional @emph{postcondition} evaluates to @emph{true} or is omitted.
   
   @emph{Syntax}
   
   @example
   ZUNMAP GLOBAL @emph{gvn}
   @end example
   
   @node ZWATCH
   @section ZWATCH
   @cindex ZWATCH
   @cindex commands, ZWATCH
   @cindex commands, debugging
   @cindex commands, implementation-specific
   @cindex commands, non-standard
   @emph{FreeM Extension}
   
   Sets a watchpoint on a global, local, or SSVN node.
   
   @emph{Syntax}
   
   
   In its @emph{argumentless} form, @code{ZWATCH} toggles watchpoints on and off, provided the optional @emph{postcondition} is @emph{true} or omitted.
   
   @example
   ZWATCH[@emph{:postcondition}]
   @end example
   
   In its @emph{inclusive} form, @code{ZWATCH} adds, removes, or examines watchpoints, provided the optional @emph{postcondition} is @emph{true} or omitted.
   
   A @code{+} adds a new watchpoint to the following variable.
   
   A @code{-} removes an existing watchpoint for the following variable.
   
   A @code{?} examines the status of a watchpoint for the following variable.
   
   @example
   ZWATCH[@emph{:postcondition}] [+|-|?]@emph{var1}...,[+|-|?]@emph{varN}
   @end example
   
   
   The following example demonstrates turning watchpoint processing on and adding a watchpoint for global variable @code{^snw(1)}. It then changes the value of @code{^snw(1)}.
   
   @example
   DEFAULT.USER> ZWATCH
    
   Watchpoints enabled.
    
   DEFAULT.USER> ZWATCH +^SNW(1)
    
   Added '^SNW("1")' to the watchlist.
    
   DEFAULT.USER> SET ^SNW(1)="new value"
    
   >> WATCHPOINT:  ^SNW("1") => 'new value' (changed 1 times)
   
   @end example
   
   The following example will remove that watchpoint:
   
   @example
   DEFAULT.USER> ZWATCH -^SNW(1)
    
   Removed '^SNW("1")' from the watchlist.
    
   DEFAULT.USER> ZWATCH ?^SNW(1)
    
   '^SNW("1")' is not being watched.
   @end example
   
   @node ZWITH
   @section ZWITH
   @cindex ZWITH
   @cindex commands, ZWITH
   @cindex commands, non-standard
   @emph{FreeM Extension}
   
   NOTE: This command may be deprecated and removed in future FreeM releases.
   
   Sets a prefix to be applied to all subsequent local variable or constant references.
   
   @emph{Syntax}
   
   @example
   @code{ZWITH@emph{:postcondition} @emph{var-prefix}}
   @end example
   
   In the above single-argument form, sets the @code{$WITH} prefix to @emph{var-prefix}, provided that the optional @emph{postcondition} is either @emph{true} or omitted.
   
   The @emph{var-prefix} argument may be a string literal or any valid FreeM expression.
   
   @example
   @code{ZWITH@emph{:postcondition}}
   @end example
   
   In the above argumentless form, clears the @code{$WITH} prefix, provided the optional @emph{postcondition} is either @emph{true} or omitted. Equivalent to @code{ZWITH ""}.
   
   
   
   
   
   
   
   
 @node ZWRITE  @node ZWRITE
 @section ZWRITE  @section ZWRITE
 @cindex ZWRITE  @cindex ZWRITE
Line 4550  SET MYSTRING="This is a ""string literal Line 4589  SET MYSTRING="This is a ""string literal
 @cindex types, custom  @cindex types, custom
 @cindex classes  @cindex classes
   
 See @ref{Classes}.  See @ref{Object-Oriented Programming}.
   
 @node Globals  @node Globals
 @chapter Globals  @chapter Globals
Line 4672  Transaction processing allows applicatio Line 4711  Transaction processing allows applicatio
 @section Transaction Processing  @section Transaction Processing
 @cindex transaction processing  @cindex transaction processing
   
 FreeM implements a significant subset of the transaction processing features from @emph{ANSI X11.1-1995}. This allows a series of global operations to be conducted all at once, either in batch mode (where concurrency is not disturbed), or in serial mode (where writes are guaranteed to be atomic, consistent, isolated, and durable).  FreeM implements a significant subset of the transaction processing features from @emph{ANSI X11.1-1995}. This allows a series of global operations to be conducted all at once, either in batch mode (where concurrent operation is not interrupted until the last possible moment), or in serial mode (where writes are guaranteed to be atomic, consistent, isolated, and durable).
   
 @subsection Theory of Operation  @subsection Theory of Operation
   
Line 4751  The effect of this is that the operation Line 4790  The effect of this is that the operation
 @cindex variables, local  @cindex variables, local
 @cindex local variables  @cindex local variables
   
   @menu
   * Local Variables Overview::            The basics of locals.
   * Creating Local Variables::            Creating locals with SET.
   * Removing Local Variables::            Removing locals with KILL.
   @end menu
   
   @node Local Variables Overview
   @section Local Variables Overview
   
   FreeM @emph{local variables} have the same data structure as global variables, but are scoped to a single FreeM process, and stored in memory.
   
   Each local comprises three elements:
   
   @itemize @bullet
   @item
   An alphabetic name beginning with a letter or a percent sign (@code{%})
   @item
   Optionally, one or more comma-delimited subscripts, enclosed in parentheses
   @item
   A value of up to 255 characters in length
   @end itemize
   
   @node Creating Local Variables
   @section Creating Local Variables
   @cindex local variables, creating
   
   To create a local variable, use the @code{SET} command:
   
   @example
   SET MYLOCAL("foo","bar")="this is the data value"
   @end example
   
   @node Removing Local Variables
   @section Removing Local Variables
   @cindex local variables, removing
   
   To remove an entire local variable, you can use the @code{KILL} command with the unsubscripted name of the variable:
   
   @example
   KILL MYLOCAL
   @end example
   
   If you only want to remove part of a local variable, i.e., beginning at a certain subscript level, use the @code{KILL} command with a subscripted name:
   
   @example
   KILL MYLOCAL("foo")
   @end example
   
   This will remove only the @code{"foo"} subscript and all of its children.
   
   If you only want to remove the data value at a specific subscript level, leaving the subscript itself intact, use @code{KVALUE}:
   
   @example
   KVALUE MYLOCAL("foo")
   @end example
   
 @node Scoping  @node Scoping
 @chapter Scoping  @chapter Scoping
 @cindex scoping  @cindex scoping
   
   By default, FreeM local variables and their values are scoped to the entire process, meaning that any function or subroutine can access and modify their values. This can lead to pernicious bugs.
   
   M provides the @code{NEW} command to work around these issues. When @code{NEW} is called with a local variable as its argument, FreeM will scope the variable to the process stack frame in which the @code{NEW} command occured. When exiting the stack frame (i.e. with the @code{QUIT} command), FreeM will restore the variable to its value prior to being @code{NEW}ed.
   
   @emph{Example}
   
   @example
   MYRTN ;
     S J=1 ; set local variable J to 1
     W J,! ; this will output "1"
     D X   ; execute subroutine X
     W J,! ; this will output "1", as the value of J was restored
     Q
     ;;
   X ;
     N J   ; stack J
     S J=6 ; set its value to 6
     W J,! ; this will output "6"
     Q     ; quit from the subroutine, destroying its stack frame
     ;;
   @end example
   
   @section Scoping Considerations for $TEST
   
   In M, the truth value of comparisons, logic operations, and certain forms of @code{LOCK} is stored in the @code{$TEST} intrinsic special variable, which follows the same rules as any M local variable.
   
   This is probably the most significant design flaw of the language, as the side effects of logic on @code{$TEST} lead to incredibly difficult bugs. However, M allows @code{$TEST} to be @code{NEW}ed, and FreeM provides the @code{THEN} command@footnote{From MDC Type A extension X11/1998-31} to help in the case of conditionals. @code{THEN} stacks @code{$TEST} to the end of the line.
   
   When writing new M code in FreeM, we strongly suggest using @code{THEN} as follows:
   
   @example
   MYRTN ;
     IF MYVAR=1 THEN DO SUBRT
   @end example
   
   This is instead of the traditional form:
   
   @example
   MYRTN ;
     IF MYVAR=1 DO SUBR
   @end example
   
   @cartouche
   @quotation
   @emph{Style Recommendation}
   
   Note that @code{THEN} is not in any currently published version of the @emph{Standard}, but is part of MDC Type A extension X11/1998-31. However, we recommend using @code{THEN} instead of favoring portability, as there is no defensible reason for this incredibly simple feature @emph{not} to be ubiquitous.
   
   If you use other M implementations, you should bug the implementers to implement @code{THEN}, as it at least partially mitigates an inexcusable flaw in the design of M.
   @end quotation
   @end cartouche
   
 @node Decision Constructs  @node Decision Constructs
 @chapter Decision Constructs  @chapter Decision Constructs
 @cindex decision constructs  @cindex decision constructs
Line 4798  The effect of this is that the operation Line 4945  The effect of this is that the operation
 @cindex programming, object-oriented  @cindex programming, object-oriented
   
 @menu  @menu
 * Classes::                         The basis of object-oriented programming.  * Classes::                             The organizing concept of object-oriented programming.
 * Inheritance::                     Basing one class upon another.  * Inheritance::                         Making one class derive from another.
 * Methods::                         Attaching code to a class.  * Methods::                             Attach code to classes.
 * Public and Private Variables::    Managing class member access.  * Public and Private Variables::        Determining accessibility.
 * Instantiating Objects::           Creating instances of classes.  * Instantiating Objects::               Creating instances of classes.
 * Determining Object Class::        Getting object information at runtime.  
 @end menu  @end menu
   
 @node Classes  @node Classes
 @section Classes  @section Classes
   
 @menu  
 * Class Overview::                  Class basics.  
 * Constructors::                    Managing object creation.  
 * Destructors::                     Cleaning up.  
 * Runtime Polymorphism::            Selecting methods at runtime.  
 @end menu  
   
 @node Class Overview  
 @subsection Class Overview  @subsection Class Overview
   
 A @emph{class} is the primary organizing concept of FreeM support for object-oriented programming, and in FreeM, is simply an M routine with a few special properties:  A @emph{class} is the primary organizing concept of FreeM support for object-oriented programming, and in FreeM, is simply an M routine with a few special properties:
Line 4834  DESTROY(THIS) ; This is the destructor Line 4971  DESTROY(THIS) ; This is the destructor
   
 The above example demonstrates general class syntax.  The above example demonstrates general class syntax.
   
 @node Constructors  
 @subsection Constructors  @subsection Constructors
   
 A @emph{constructor} is an M entry point that is called when a new instance of a class is created.  A @emph{constructor} is an M entry point that is called when a new instance of a class is created.
Line 4859  A constructor looks like this: Line 4995  A constructor looks like this:
   
 In the above example, @emph{<superclass>} represents the name of a class from which this class should inherit. In this case, the @code{FRACTION} class inherits from the @code{OBJECT} class. Note that this is not strictly necessary in this case, as all classes in FreeM automatically inherit from @code{OBJECT}.  In the above example, @emph{<superclass>} represents the name of a class from which this class should inherit. In this case, the @code{FRACTION} class inherits from the @code{OBJECT} class. Note that this is not strictly necessary in this case, as all classes in FreeM automatically inherit from @code{OBJECT}.
   
 @node Destructors  
 @subsection Destructors  @subsection Destructors
 A @code{destructor} is called when you @code{KILL} an instance variable. Its tag must be @code{DESTROY}, and it must take one argument (@code{THIS}).  A @code{destructor} is called when you @code{KILL} an instance variable. Its tag must be @code{DESTROY}, and it must take one argument (@code{THIS}).
   
Line 4886  Inheritance is achieved by specifying th Line 5021  Inheritance is achieved by specifying th
 CLASS(THIS,INIT):SUPERCLASS  CLASS(THIS,INIT):SUPERCLASS
 @end example  @end example
   
 @node Runtime Polymorphism  
 @subsection Runtime Polymorphism  @subsection Runtime Polymorphism
   
 You can achieve runtime polymorphism by subclassing, and defining methods in the subclass that match the names of existing methods in the superclass. Following FreeM inheritance rules, the overridden method in the subclass will be called, and the method in the superclass will not.  You can achieve runtime polymorphism by subclassing, and defining methods in the subclass that match the names of existing methods in the superclass. Following FreeM inheritance rules, the overridden method in the subclass will be called, and the method in the superclass will not.
Line 4916  DEFAULT.USER> W MYOBJ.MYMETHOD() Line 5050  DEFAULT.USER> W MYOBJ.MYMETHOD()
 VALUE  VALUE
 @end example  @end example
   
 @node Public and Private Variables   @node Public and Private Variables
 @section Public and Private Variables  @section Public and Private Variables
   
 FreeM supports private fields with the @code{:PRIVATE} specifier in the @code{SET} command, enforcing classical object-oriented data encapsulation. The @code{:PUBLIC} specifier is provided for completeness, and is the default.  FreeM supports private fields with the @code{:PRIVATE} specifier in the @code{SET} command, enforcing classical object-oriented data encapsulation. The @code{:PUBLIC} specifier is provided for completeness, and is the default.
Line 4950  NEW MYSTR=$#^%STRING("myString") Line 5084  NEW MYSTR=$#^%STRING("myString")
   
 This will create a local variable called MYSTR of type STRING, and initialize it with the value myString.   This will create a local variable called MYSTR of type STRING, and initialize it with the value myString. 
   
 @node Determining Object Class  @subsection Determining Object Class
 @section Determining Object Class  
   
 To determine the class of any FreeM local variable, you will use the @code{$$TYPE()} method:  To determine the class of any FreeM local variable, you will use the @code{$$TYPE()} method:
   
Removed from v.1.37  
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