LE/2(ID:7057/)

Version 2 of the Network language LE
Related languages
LE/1 => LE/2   Evolution of
LE/2 => MFS   Influence
References:
  • du Masle, J. "An Evaluation of the LE/1 Network Command Language Designed for the SOC Network" view details Abstract:
    LE/1 is the external command language designed for the SAC network. The aim of its designers is to give the user an easy access to the  network facilities. LE/1 covers most of the common means of file hand  ling and provides remote job entry facilities in a high level language  form. LE/1 relieves the user of all synchronisation problems which are  handled by the compiler.

    Defined by a LL(1) syntax, LE/1 is compiled into an intermediate form LI to be dealt with by a network interpreter part of the SAC system.  LE/1 presents some features of general interest:
         -high level language form
         -syntactic definition
         -compiler architecture
         -localisation of resources
         -copy statement and generalized file concatenation
    On the other hand several mistakes have been made:
         -complicated syntax
         -declaration of remote devices
         -scope of names and values
         -level of interpretation
         -system dependance (IBM 360 oriented)
    After a brief description of the language and its implementation we shall comment on the advantages and disadvantages of LE/1 and present  the main points of our LE/2 project applied to the Cyclades heterogeneous network.
    Extract: Introduction
    Introduction
    We have been studying network command languages for the past three years at the University of Grenoble.
    Two different approaches have been taken:
    - a low level one which consists of implementing basic command languages to for the immediate use of existing networks and to find user's reactions
    - a high level one, a long term approach, by which we expect to be able to propose satisfactory command languages implemented under   new operating systems specially designed for a network environment
    As an immediate implementation is not with low level languages with the hope of being able to propose an adequate solution for the future.
    Since the object of command languages is to allow the users to access network facilities and to obtain services, it is surprising to note our interest in this topic when it has been said very often that what purpose networks serve is not known.
    Our aim consists in providing the users with network facilities, and then getting a feed back by studying their behavior and at the end being able to produce a proper interface.  
    The participation of the University of Grenoble in the SAC project has mainly consisted in the definition and implementation of a prototype command language referred to as External Language (LE/1). Extract: Conclusion
    The LE/1 project has been very instructive. The methods applied in the organisation of the compiler, largely inspired by programming languages techniques, have heen successfully appliped to a command language.

    The independence of the command language from the rest of the project with the internal language as interface has proved very practical, especially in a network project involving participants spread in different places.

    LE/1 reflects the state of our understanding of the problem two years
    ago. The experience gained with LE/1 can be applied directly to the Cyclades project whose interpretative internal language (MFS] will permit us to define a more convenient command language than LE/1.

    Extract: The LE/1 compiler
    The LE/1 compiler
    To avoid a great complexity of the compiler and allow for an easy implementation the LE/1 language is syntactically defined by a grammar in Backus normal form. This definition form, given as a reference to the user, may be different from the implementation form actually used by the compiler.

    For example in LE/1, as given to the user, the end of statement semi-colon is represented only once, but it was advantageous for the compiler to distinguish the semicolon for each statement.

    The choice of the internal form decides which syntactic analysis algorithm must be used.

    The LE/1 grammar is of LL(1) type. A utility program was written in
    Algol W to read the B.N.F. grammar and produce a tree structure representing the grammar in the form of 05/360 Assembler macro calls.

    This program. checks the input grammar for LL (1) compliance, number the generation function calls and produce a cross reference list of all rules and terminal symbols. Recorded as a tree, according to the method used in SPG the LE/1 grammar takes 1700 bytes of core. (This size could be reduced easily). The associated analysis algorithm takes only 200 instructions including all tracing and debugging facilities. The performances are very good and would match interactive requirements.
          in Unger, Claus (Ed.) Command languages: Proceedings of the IFIP Working Conference on Command Languages (Lund, Sweden, August 1974) North-Holland, 1975 Amsterdam, The Netherlands view details