Editing ICL VME (section)

===SCL===
The command language for VME is known as SCL (System Control Language).

This is much more recognizably a typed [[high-level programming language]] than the job control or shell languages found in most other operating systems: it can be likened to scripting languages such as [[JavaScript]], though its surface syntax is derived from [[Algol 68]].

SCL is designed to allow both line-at-a-time interactive use from a console or from a [[shell script|command file]], and creation of executable scripts or programs (when the language is compiled into object module format in the same way as any other VME programming language). The declaration of a procedure within SCL also acts as the definition of a simple form or template allowing the procedure to be invoked from an interactive terminal, with fields validated according to the data types of the underlying procedure parameters or using the default procedure parameter values.

The built-in command vocabulary uses a consistent naming convention with an imperative verb followed by a noun: for example DELETE_FILE or DISPLAY_LIBRARY_DETAILS. The command can be written in full, or in an abbreviated form that combines standard abbreviations for the verb and noun: for example XF (X for DELETE, F for FILE) or DLBD (D for DISPLAY, LB for LIBRARY, D for DETAILS).

SCL is block-structured, with begin/end blocks serving the dual and complementary roles of defining the lexical scope of variable declarations, and defining the points at which resources acquired from the operating system should be released. Variables in the language (which are accessible from applications in the form of [[environment variable]]s) can have a number of simple types such as strings, superstrings (sequences of strings), booleans, and integers, and are also used to contain references to system resources such as files and network connections.

It is possible to "disassemble" an SCL program from OMF back into SCL source code using the READ_SCL (or RSCL) command. However the output is not always perfect, and will often include errors that would stop re-compilation without user intervention.

A simple code example can be seen on the [[99 bottles of beer]] website.<ref>[http://99-bottles-of-beer.net/language-scl-848.html SCL code example on the 99 bottles of beer website]</ref>

A more realistic example, where SCL is used to compile a program written in [[S3 (programming language)|S3]], is shown below. This example is taken from the [[Columbia University]] Archive of implementations of [[Kermit (protocol)|Kermit]].<ref>{{cite web
|url=http://www.columbia.edu/kermit/archive
|publisher=Columbia University's Kermit Project
|title=Kermit Software Source Code Archive
|date=22 Aug 2011
|access-date=1 March 2013
}}</ref>

<pre>
BEGIN
   WHENEVER
      RESULT GT 0 +_
      THEN        +_
         SEND_RESULT_MESSAGE (RES = RESULT,
                              ACT = "QUIT()")
   FI
 
   INT KMT_SRC, KMT_OMF, KMT_REL
 
   ASSIGN_LIBRARY (NAM = KERMIT.SOURCE,
                   LNA = KMT_SRC)
   ASSIGN_LIBRARY (NAM = KERMIT.OMF,
                   LNA = KMT_OMF)
   ASSIGN_LIBRARY (NAM = KERMIT.REL,
                   LNA = KMT_REL)
 
   BEGIN
      DELETE_FILE (NAM = *KMT_OMF.KMT_DATA_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_DH_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_EH_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_FH_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_HELP_MTM(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_MAIN_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_PH_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_PP_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_SP_MODULE(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_SP_MTM(101))
      DELETE_FILE (NAM = *KMT_OMF.KMT_UI_MODULE(101))
 
      DELETE_FILE (NAM = *KMT_REL.KERMIT(101))
      DELETE_FILE (NAM = *KMT_REL.KERMIT_MODULE(101))
   END
 
   S3_COMPILE_DEFAULTS (LIS = OBJECT & XREF,
                        DIS = ERRORLINES)
 
   S3_COMPILE (INP = *KMT_SRC.KMT_DATA_MODULE(101),
               OMF = *KMT_OMF.KMT_DATA_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_DH_MODULE(101),
               OMF = *KMT_OMF.KMT_DH_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_EH_MODULE(101),
               OMF = *KMT_OMF.KMT_EH_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_FH_MODULE(101),
               OMF = *KMT_OMF.KMT_FH_MODULE(101))
 
   NEW_MESSAGE_TEXT_MODULE (CON = *KMT_SRC.KMT_HELP_MTM(101),
                            OMF = *KMT_OMF.KMT_HELP_MTM(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_MAIN_MODULE(101),
               OMF = *KMT_OMF.KMT_MAIN_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_PH_MODULE(101),
               OMF = *KMT_OMF.KMT_PH_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_PP_MODULE(101),
               OMF = *KMT_OMF.KMT_PP_MODULE(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_SP_MODULE(101),
               OMF = *KMT_OMF.KMT_SP_MODULE(101))
 
   NEW_MESSAGE_TEXT_MODULE (CON = *KMT_SRC.KMT_SP_MTM(101),
                            OMF = *KMT_OMF.KMT_SP_MTM(101))
 
   S3_COMPILE (INP = *KMT_SRC.KMT_UI_MODULE(101),
               OMF = *KMT_OMF.KMT_UI_MODULE(101))
 
   COLLECT ()
----
      INPUT(*KMT_OMF.KMT_DATA_MODULE(101)      &
            *KMT_OMF.KMT_DH_MODULE(101)        &
            *KMT_OMF.KMT_EH_MODULE(101)        &
            *KMT_OMF.KMT_FH_MODULE(101)        &
            *KMT_OMF.KMT_HELP_MTM(101)         &
            *KMT_OMF.KMT_MAIN_MODULE(101)      &
            *KMT_OMF.KMT_PH_MODULE(101)        &
            *KMT_OMF.KMT_PP_MODULE(101)        &
            *KMT_OMF.KMT_SP_MODULE(101)        &
            *KMT_OMF.KMT_SP_MTM(101)           &
            *KMT_OMF.KMT_UI_MODULE(101))
      NEWMODULE(*KMT_REL.KERMIT_MODULE(101))
      SUPPRESS
      RETAIN(KERMIT_THE_FROG)
      LISTMODULE
      PERFORM
++++
 
COMPILE_SCL (INP = *KMT_SRC.KERMIT(101),
             OUT = *KMT_REL.KERMIT(101),
             COD = NOTIFWARNINGS,
             OPT = FIL)
 
END
</pre>

Commands illustrated in this fragment include WHENEVER (declares error handling policy), ASSIGN_LIBRARY (binds a local name for a file directory), DELETE_FILE (Makes a permanent file temporary, and it is then deleted at the END of the block), S3_COMPILE (compiles a program written in S3: this command breaks the usual verb-noun convention), NEW_MESSAGE_TEXT_MODULE (creates a module containing parameterized error messages suitable for localization) and COMPILE_SCL, which compiles an SCL program into object code.

The COLLECT command combines different object code modules into a single module, and is driven by its own local command file which is incorporated inline in the SCL between the delimiters "----" and "++++". The sub-commands INPUT and NEWMODULE identify the names of the input and output modules; SUPPRESS and RETAIN determine the external visibility of named procedures within the collected module; and LISTMODULE requests a report describing the output module.

Note that "." is used to separate the parts of a hierarchic file name. A leading asterisk denotes a local name for a library, bound using the ASSIGN_LIBRARY command. The number in parentheses after a file name is a generation number. The operating system associates a generation number with every file, and requests for a file get the latest generation unless specified otherwise. Creating a new file will by default create the next generation and leave the previous generation intact; this program however is deliberately choosing to create generation 101, to identify a public release.