📄 sync-var.mldoc

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<!-- ../doc/mldoc/sync-var.mldoc --><!DOCTYPE ML-DOC SYSTEM><COPYRIGHT OWNER="Bell Labs, Lucent Technologies" YEAR=1998><COPYRIGHT OWNER="AT&AMP;T Bell Laboratories" YEAR=1995><VERSION VERID="1.0" YEAR=1995 MONTH=11 DAY=16><TITLE>The SyncVar structure</TITLE><INTERFACE><HEAD>The <CD/SyncVar/ structure</HEAD><SEEALSO>  <STRREF TOPID/CML/</SEEALSO><PP>The <STRREF TOPID NOLINK/SyncVar/ structure provides <BF/Id/-stylesynchronous variables (or memory cells).These variables have two states: <IT/empty/ and <IT/full/.An attempt to read a value from an empty variable blocks the callingthread until there is a value available.An attempt to put a value into a variable that is full results in the<EXNREF STRID="SyncVar"/Put/ exception being raised.There are two kinds of synchronous variables: I-variables are write-once,while M-variables are mutable.<STRUCTURE STRID="SyncVar">  <SIGBODY SIGID="SYNC_VAR" FILE=SYNC-VAR-SIG>    <SPEC>      <EXN>Put      <COMMENT>	<PP>	This exception is raised when an attempt is made to put a	value into a value that is already full (see <VALREF/iPut/ and	<VALREF/mPut/).    <SPEC>      <TYPE><TYPARAM>'a<ID>ivar      <COMMENT>	<PP>	This is the type constructor for I-structured variables.	I-structured variables are write-once variables that provide	synchronization on read operations.	They are especially useful for one-shot communications, such	as reply messages in client/server protocols, and can also be used	to implement shared <EM>incremental</EM> data structures.    <SPEC>      <VAL>iVar<TY>unit -> 'a ivar        <COMMENT>          <PROTOTY>          iVar ()          </PROTOTY>          creates a new empty I-variable.    <SPEC>      <VAL>iPut<TY>('a ivar * 'a) -> unit	<RAISES><EXNREF/Put/        <COMMENT>          <PROTOTY>          iPut (<ARG/iv/, <ARG/x/)          </PROTOTY>	  fills the I-variable <ARG/iv/ with the value <ARG/x/.	  Any threads that are blocked on <ARG/iv/ will be resumed.	  If <ARG/iv/ already has a value in it, then the <EXNREF/Put/	  exception is raised.    <SPEC>      <VAL>iGet<TY>'a ivar -> 'a        <COMMENT>          <PROTOTY>          iGet <ARG/iv/          </PROTOTY>          returns the contents of the I-variable <ARG/iv/.	  If the variable is empty, then the calling thread blocks until	  the variable becomes full.    <SPEC>      <VAL>iGetEvt<TY>'a ivar -> 'a event        <COMMENT>          <PROTOTY>          iGetEvt <ARG/iv/          </PROTOTY>          returns an event-value that represents the <VALREF/iGet/	  operation on <ARG/iv/.    <SPEC>      <VAL>iGetPoll<TY>'a ivar -> 'a option        <COMMENT>	<PP>	  This is a non-blocking version of <VALREF/iGet/.	  If the corresponding blocking form would block, then it returns	  <CONREF DOCUMENT=SML-BASIS-DOC STRID="Option"/NONE/; otherwise	  it returns <CONREF DOCUMENT=SML-BASIS-DOC STRID="Option"/SOME/	  of the variable's contents.    <SPEC>      <VAL>sameIVar<TY>('a ivar * 'a ivar) -> bool        <COMMENT>          <PROTOTY>          sameIVar (<ARG/iv1/, <ARG/iv2/)          </PROTOTY>          returns <CD/true/, if <ARG/iv1/ and <ARG/iv2/ are the	  same I-variable.    <SPEC>      <TYPE><TYPARAM>'a<ID>mvar      <COMMENT>	<PP>	This is the type constructor for M-structured variables.	Unlike <TYREF/ivar/ values, M-structured variables may be	updated multiple times.	Like I-variables, however, they may only be written if they	are empty.    <SPEC>      <VAL>mVar<TY>unit -> 'a mvar        <COMMENT>          <PROTOTY>          mVar ()          </PROTOTY>          creates a new empty M-variable.    <SPEC>      <VAL>mVarInit<TY>'a -> 'a mvar        <COMMENT>          <PROTOTY>          mVarInit <ARG/x/          </PROTOTY>          creates a new M-variable initialized to <ARG/x/.    <SPEC>      <VAL>mPut<TY>('a mvar * 'a) -> unit	<RAISES><EXNREF/Put/        <COMMENT>          <PROTOTY>          mPut (<ARG/mv/, <ARG/x/)          </PROTOTY>	  fills the M-variable <ARG/mv/ with the value <ARG/x/.	  Any threads that are blocked on <ARG/mv/ will be resumed.	  If <ARG/mv/ already has a value in it, then the <EXNREF/Put/	  exception is raised.    <SPEC>      <VAL>mTake<TY>'a mvar -> 'a      <COMMENT>        <PROTOTY>          mTake <ARG/mv/        </PROTOTY>	removes and returns the contents of the M-variable <ARG/mv/	making it empty.	If the variable is already empty, then the calling thread	is blocked until a value is available.    <SPEC>      <VAL>mTakeEvt<TY>'a mvar -> 'a event        <COMMENT>          <PROTOTY>          mTakeEvt <ARG/mv/          </PROTOTY>          returns an event-value that represents the <VALREF/mTake/	  operation on <ARG/mv/.    <SPEC>      <VAL>mGet<TY>'a mvar -> 'a        <COMMENT>          <PROTOTY>          mGet <ARG/mv/          </PROTOTY>          returns the contents of the M-variable <ARG/mv/ without emptying	  the variable; if the variable is empty, then the thread blocks until	  a value is available.	  It is equivalent to the code:	  <CODE>let val x = <VALREF/mTake/ <ARG/mv/ in <VALREF/mPut/(<ARG/mv/, x); x end	  </CODE>    <SPEC>      <VAL>mGetEvt<TY>'a mvar -> 'a event        <COMMENT>          <PROTOTY>          mGetEvt <ARG/mv/          </PROTOTY>          returns an event-value that represents the <VALREF/mGet/	  operation on <ARG/mv/.    <SPEC>      <VAL>mTakePoll<TY>'a mvar -> 'a option      <VAL>mGetPoll<TY>'a mvar -> 'a option        <COMMENT>	<PP>	  These are non-blocking versions of <VALREF/mTake/ and	  <VALREF/mGet/ (respectively).	  If the corresponding blocking form would block, then they return	  <CONREF DOCUMENT=SML-BASIS-DOC STRID="Option"/NONE/; otherwise	  they return <CONREF DOCUMENT=SML-BASIS-DOC STRID="Option"/SOME/	  of the variable's contents.    <SPEC>      <VAL>mSwap<TY>('a mvar * 'a) -> 'a        <COMMENT>          <PROTOTY>          mSwap (<ARG/mv/, <ARG/newV/)          </PROTOTY>	  puts the value <ARG/newV/ into the M-variable <ARG/mv/ and returns	  the previous contents.	  If the variable is empty, then the thread blocks until	  a value is available.	  It is equivalent to the code:	  <CODE>let val x = <VALREF/mTake/ <ARG/mv/ in <VALREF/mPut/(<ARG/mv/, <ARG/newV/); x end	  </CODE>	  except that <CD/mSwap/ is executed atomically.    <SPEC>      <VAL>mSwapEvt<TY>('a mvar * 'a) -> 'a event        <COMMENT>          <PROTOTY>          mSwapEvt (<ARG/mv/, <ARG/newV/)          </PROTOTY>          returns an event-value that represents the <VALREF/mSwap/	  operation on <ARG/mv/ and <ARG/newV/.    <SPEC>      <VAL>sameMVar<TY>('a mvar * 'a mvar) -> bool        <COMMENT>          <PROTOTY>          sameMVar (<ARG/mv1/, <ARG/mv2/)          </PROTOTY>          returns <CD/true/, if <ARG/mv1/ and <ARG/mv2/ are the same	  M-variable.  </SIGBODY></STRUCTURE><PP>I-variables provide a useful mechanism for implementing thereply communication in request/reply protocols (in cases wherethe server does not care if the reply is accepted).They may also be used to implement incremental data structuresand streams; for example, the <STRREF>Multicast</STRREF> structureuses I-variables to implement its multicast channels.<PP>A disciplined use of M-variables can provide an atomicread-modify-write operation.</INTERFACE>
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