@iftex
@finalout
@end iftex
-@comment $Id: user.texinfo,v 1.37 1995/11/29 07:08:59 adams Exp $
+@comment $Id: user.texinfo,v 1.38 1996/02/22 22:12:26 adams Exp $
@comment %**start of header (This is for running Texinfo on a region.)
@setfilename user.info
@settitle MIT Scheme User's Manual
to change the value of the variable @samp{libdir} to be this directory.
@item
-If you have GNU Emacs installed on your system, uncomment the
+If you have GNU Emacs installed on your system, un-comment the
definitions of the variables @samp{infodir}, @samp{emacslisp}, and
@samp{movemail} in the installation script. You will also need to edit
these definitions so that they correctly indicate where the GNU Emacs
The output code should work on virtually any modern
general-purpose architecture with an ANSI-compliant (perhaps even
a traditional K&R) C compiler. We've tried it on several 32-bit
-architectures (HP PA, Sparc, Motorola 68040, RS6000/Power) and a
+architectures (HP PA, SPARC, Motorola 68040, RS6000/Power) and a
64-bit architecture (Alpha).
However, there may very well be assumptions of the following form:
accessed. In particular, after a @code{disk-restore}, previously loaded
modules are still mapped. In addition, at least in some versions of
unix, the shared object file cannot be deleted while it is loaded, so
-recompilation of a module in place will fail if some process has it
+re-compilation of a module in place will fail if some process has it
already loaded.
@item
Multiple loading does not work. This is bad for development. After
loading a compiled module once, it cannot be reloaded (presumably after
-recompilation).
+re-compilation).
@item
@code{fasdump} does not work for compiled code. It gives an error.
@item
DOS version 6. Although we have not tested the DOS version under
-anythig other than MS-DOS 5.0, we have the following anecdotal evidence:
+anything other than MS-DOS 5.0, we have the following anecdotal evidence:
@itemize @bullet
@item
@var{filename} as the initial file to be loaded. If this option isn't
given, a normal load is performed instead. This option may not be used
together with the @code{-band} option. This option is useful only for
-maintainance and development of the MIT Scheme runtime system.
+maintenance and development of the MIT Scheme runtime system.
@end table
@noindent
@end table
@noindent
-In addition to the above, @code{bchscheme} recognises the following
+In addition to the above, @code{bchscheme} recognizes the following
command line options, all of which specify parameters affecting how
@code{bchscheme} uses disk storage to do garbage collection:
@end example
The rest of this section is a summary of the environment variables that
-are specific to MIT Scheme. The environment variables are organised
+are specific to MIT Scheme. The environment variables are organized
according to the parts of MIT Scheme that they affect.
@menu
notified. The default is no notification.
The notification appears as a single line like the following, showing
-how many garbage collections have occured, the time taken to perform the
+how many garbage collections have occurred, the time taken to perform the
garbage collection and the free storage remaining (in words) after
collection.
@subsubheading Internal procedures
Calls to internal procedures are faster than calls to global procedures.
-There are two things tha make internal procedures faster: First, the
+There are two things that make internal procedures faster: First, the
procedure call is compiled to a direct jump to a known location, which
-is mare efficeint that jumping `via' a global binding.
+is more efficient that jumping `via' a global binding.
Second, there is a knock-on effect: since the compiler can see the
-internal procedure it can analyze it and possibly produce better code
-for other expressions in the body of the loop too.
+internal procedure, the compiler can analyze it and possibly produce
+better code for other expressions in the body of the loop too:
@lisp
(define (map f original-lst)
@end lisp
+@subsubheading Internal defines
+
+Internal definitions are a useful tool for structuring larger
+procedures. However, certain internal definitions can thwart compiler
+optimizations. Consider the following two procedures, where
+@code{compute-100} is some unknown procedure that we just know returns
+@samp{100}.
+
+@lisp
+(define (f1)
+ (define v 100)
+ (lambda () v))
+
+(define (f2)
+ (define v (compute-100))
+ (lambda () v))
+@end lisp
+
+The procedure returned by @code{f1} will always give the same result and
+the compiler can prove this. The procedure returned by @code{f2} may
+return different results, even if @code{f2} is only called once.
+Because of this, the compiler has to allocate a memory cell to @code{v}.
+How can the procedure return different results?
+
+The fundamental reason is that the continuation may escape during the
+evaluation of @code{(compute-100)}, allowing the rest of the body of
+@code{f2} to be executed @emph{again}:
+
+@lisp
+(define keep)
+
+(define (compute-100)
+ (call-with-current-continuation
+ (lambda (k)
+ (set! keep k)
+ 100)))
+
+(define p (f2))
+
+(p) @result{} 100
+(keep -999) @result{} p @emph{re-define v and p}
+(p) @result{} -999
+@end lisp
+
+To avoid the inefficiency introduced to handle the general case, the
+compiler must prove that the continuation cannot possibly escape. The
+compiler knows that lambda expressions and constants do not let their
+continuations escape, so order the internal definitions so that
+definitions of the following form come first:
+
+@lisp
+(define x '@emph{something})
+(define x (lambda (...) ...))
+(define (f u v) ...)
+@end lisp
+
@node Global variables, Fixnum arithmetic, Coding style, Efficiency Tips
@subsection Global variables
to that variable. The compiler can't prove this because it assumes that
other, independently compiled, files might be loaded which invalidate
these assumptions.
-If you know that these contitions hold, the following declarations can
+If you know that these conditions hold, the following declarations can
speed up and reduce the size of a program that uses global variables.
@deffn {declaration+} ignore-reference-traps variables
@code{ignore-assignment-traps} differs between version of the compiler.
MIT Scheme version 8.0 (Liar
version 5.0) has true block scoping, thus the declaration takes effect
-only within the procedure or @code{let} in which the declarartion
+only within the procedure or @code{let} in which the declaration
occurs. This makes it possible to control individual variable
references, for example:
Flonums have to be @dfn{boxed} because a 64 bit IEEE floating point
number (the representation the MIT Scheme uses) does not fit in a
regular machine word.
-This is true even on 64 bit archiectures because some extra bits are
+This is true even on 64 bit architectures because some extra bits are
needed to distinguish floating point numbers from other objects like
-pairs ans strigs.
+pairs ans strings.
Values are boxed by storing them in a small record in the heap.
Every floating point value that you see at the REPL is boxed.
Floating point values are unboxed only for short periods of time when
Declarations from outside of an @code{in-package} form do not apply to
the body of the form.
This is because, in general, the new package (environment) could be nay
-packag, including one which contains alternative definitions for the
+package, including one which contains alternative definitions for the
standard procedures.
The declarations in the enclosing text might be meaningless in the
@code{in-package} body.
These commands are implemented by Emacs but not by Edwin. The commands
marked with an asterisk are implemented by the unix version of Edwin but
-not by the PC version. Some of the asterisked comands can work in the
+not by the PC version. Some of the asterisked commands can work in the
PC version but the code to implement them is not loaded in order to save
space; others are unix-specific and are not implemented on the PC.
projects / mit-scheme.git / commitdiff