Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!nrl-cmf!ames!pasteur!ucbvax!decwrl!sun!aeras!elxsi!beatnix!corbett
From: corbett@beatnix.UUCP (Bob Corbett)
Newsgroups: comp.lang.pascal
Subject: Modules and separate compilation
Keywords: Extended Pascal, modules, separate compilation
Message-ID: <2483@elxsi.UUCP>
Date: 15 Feb 89 03:04:56 GMT
Sender: news@elxsi.UUCP
Reply-To: uunet!elxsi!corbett (Bob Corbett)
Distribution: usa
Organization: ELXSI Super Computers, San Jose
Lines: 76


     In an earlier posting, I described a scheme for implementing
separate compilation for Extended Pascal.  My proposal was based on
having the compiler maintain catalogue files.  Catalogue files
would contain the names of files submitted to and created by the
compiler.  The response to my proposal has been overwhelmingly negative.
I am now formulating new proposals for separate compilation.

     Few of the people who have written or spoken to me have read the
proposed standard.  Most of the schemes that have been offered in
opposition to my proposals violate the proposed standard.  If anyone
feels that he or she knows the right way to implement separate
compilation, he or she should let the standards committee know about it.
Otherwise, he or she is likely to find that his or her method is
prohibited by the new standard.

     I am exploring a way of making separate compilation for Extended
Pascal look as much as possible like separate compilation for languages
such as C and Fortran.  Under this scheme, each source file consists of
zero or more program components (a program component is either a
module-declaration or a main-program-declaration).  When a source file is
compiled, the compiler will produce an object file.  The object file will
contain an abstract syntax tree (AST) for each program component in the
source file.  Semantic information will be maintained separately.  Both
source and object files may be submitted to the compiler.  If one of the
files contains a main- program-declaration and all names referenced
directly or indirectly from the main program can be resolved, an executable
file will be generated.

     A drawback of this scheme is that it is impossible complete semantic
checking until link time.  When a program component that imports an
interface is compiled, the compiler does not have access to the declarations
of the imported names unless the module declaration containing the interface
is contained in the source file.  The compiler may not even know which names
have been imported.  I fear that users will be upset when they get what
appear to be compilation errors at link time.

     The problem can be avoided by requiring each source file to be compiled
in sequence and having the resulting object file retain information
about previously compiled files.  Suppose a programmer is writing a linear
algebra program.  He might write a source file main.p containing his main
program.  The main program might import an interface dgemat contained in a
module in a file named linpack.p.  The module containing the declaration
of the interface dgemat may itself import an interface named dblas1 contained
in a file named blas.p.  Then the programmer could compile his program using
the command

	          pascal blas.p linpack.p main.p

Assuming all references could be resolved, the compiler would produce an
executable file named a.out.  If some references could not be resolved, it
would produce a file main.o, which would contain compiled code for blas.p,
linpack.p, and main.p.  If the programmmer wished use separate compilation,
he could first use the command

		  pascal blas.p

to produce an object file blas.o.  Next he could use the command

		  pascal blas.o linpack.p

to produce an object file for blas.p and linpack.p.  Finally, he could
complete the compilation with the command

		  pascal linpack.o main.p.

However, he could not compile linpack.p without blas.o or blas.p, and he
could not compile main.p without linpack.o or linpack.p.  This approach
eliminates the need to store abstract syntax trees, since code can be
generated at each stage of compilation.

     I welcome comments and suggestions regarding separate compilation
for Extended Pascal in general or my scheme in particular.

					      Yours truly,
					      Bob Corbett