memoize.3

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.Ve
.PP
Each of these options is optional; you can include some, all, or none
of them.
.Sh "\s-1INSTALL\s0"
.IX Subsection "INSTALL"
If you supply a function name with \f(CW\*(C`INSTALL\*(C'\fR, memoize will install
the new, memoized version of the function under the name you give.
For example,
.PP
.Vb 1
\&        memoize(\*(Aqfib\*(Aq, INSTALL => \*(Aqfastfib\*(Aq)
.Ve
.PP
installs the memoized version of \f(CW\*(C`fib\*(C'\fR as \f(CW\*(C`fastfib\*(C'\fR; without the
\&\f(CW\*(C`INSTALL\*(C'\fR option it would have replaced the old \f(CW\*(C`fib\*(C'\fR with the
memoized version.
.PP
To prevent \f(CW\*(C`memoize\*(C'\fR from installing the memoized version anywhere, use
\&\f(CW\*(C`INSTALL => undef\*(C'\fR.
.Sh "\s-1NORMALIZER\s0"
.IX Subsection "NORMALIZER"
Suppose your function looks like this:
.PP
.Vb 6
\&        # Typical call: f(\*(Aqaha!\*(Aq, A => 11, B => 12);
\&        sub f {
\&          my $a = shift;
\&          my %hash = @_;
\&          $hash{B} ||= 2;  # B defaults to 2
\&          $hash{C} ||= 7;  # C defaults to 7
\&
\&          # Do something with $a, %hash
\&        }
.Ve
.PP
Now, the following calls to your function are all completely equivalent:
.PP
.Vb 6
\&        f(OUCH);
\&        f(OUCH, B => 2);
\&        f(OUCH, C => 7);
\&        f(OUCH, B => 2, C => 7);
\&        f(OUCH, C => 7, B => 2);
\&        (etc.)
.Ve
.PP
However, unless you tell \f(CW\*(C`Memoize\*(C'\fR that these calls are equivalent,
it will not know that, and it will compute the values for these
invocations of your function separately, and store them separately.
.PP
To prevent this, supply a \f(CW\*(C`NORMALIZER\*(C'\fR function that turns the
program arguments into a string in a way that equivalent arguments
turn into the same string.  A \f(CW\*(C`NORMALIZER\*(C'\fR function for \f(CW\*(C`f\*(C'\fR above
might look like this:
.PP
.Vb 5
\&        sub normalize_f {
\&          my $a = shift;
\&          my %hash = @_;
\&          $hash{B} ||= 2;
\&          $hash{C} ||= 7;
\&
\&          join(\*(Aq,\*(Aq, $a, map ($_ => $hash{$_}) sort keys %hash);
\&        }
.Ve
.PP
Each of the argument lists above comes out of the \f(CW\*(C`normalize_f\*(C'\fR
function looking exactly the same, like this:
.PP
.Vb 1
\&        OUCH,B,2,C,7
.Ve
.PP
You would tell \f(CW\*(C`Memoize\*(C'\fR to use this normalizer this way:
.PP
.Vb 1
\&        memoize(\*(Aqf\*(Aq, NORMALIZER => \*(Aqnormalize_f\*(Aq);
.Ve
.PP
\&\f(CW\*(C`memoize\*(C'\fR knows that if the normalized version of the arguments is
the same for two argument lists, then it can safely look up the value
that it computed for one argument list and return it as the result of
calling the function with the other argument list, even if the
argument lists look different.
.PP
The default normalizer just concatenates the arguments with character
28 in between.  (In \s-1ASCII\s0, this is called \s-1FS\s0 or control\-\e.)  This
always works correctly for functions with only one string argument,
and also when the arguments never contain character 28.  However, it
can confuse certain argument lists:
.PP
.Vb 3
\&        normalizer("a\e034", "b")
\&        normalizer("a", "\e034b")
\&        normalizer("a\e034\e034b")
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.PP
for example.
.PP
Since hash keys are strings, the default normalizer will not
distinguish between \f(CW\*(C`undef\*(C'\fR and the empty string.  It also won't work
when the function's arguments are references.  For example, consider a
function \f(CW\*(C`g\*(C'\fR which gets two arguments: A number, and a reference to
an array of numbers:
.PP
.Vb 1
\&        g(13, [1,2,3,4,5,6,7]);
.Ve
.PP
The default normalizer will turn this into something like
\&\f(CW"13\e034ARRAY(0x436c1f)"\fR.  That would be all right, except that a
subsequent array of numbers might be stored at a different location
even though it contains the same data.  If this happens, \f(CW\*(C`Memoize\*(C'\fR
will think that the arguments are different, even though they are
equivalent.  In this case, a normalizer like this is appropriate:
.PP
.Vb 1
\&        sub normalize { join \*(Aq \*(Aq, $_[0], @{$_[1]} }
.Ve
.PP
For the example above, this produces the key \*(L"13 1 2 3 4 5 6 7\*(R".
.PP
Another use for normalizers is when the function depends on data other
than those in its arguments.  Suppose you have a function which
returns a value which depends on the current hour of the day:
.PP
.Vb 10
\&        sub on_duty {
\&          my ($problem_type) = @_;
\&          my $hour = (localtime)[2];
\&          open my $fh, "$DIR/$problem_type" or die...;
\&          my $line;
\&          while ($hour\-\- > 0)
\&            $line = <$fh>;
\&          } 
\&          return $line;
\&        }
.Ve
.PP
At 10:23, this function generates the 10th line of a data file; at
3:45 \s-1PM\s0 it generates the 15th line instead.  By default, \f(CW\*(C`Memoize\*(C'\fR
will only see the \f(CW$problem_type\fR argument.  To fix this, include the
current hour in the normalizer:
.PP
.Vb 1
\&        sub normalize { join \*(Aq \*(Aq, (localtime)[2], @_ }
.Ve
.PP
The calling context of the function (scalar or list context) is
propagated to the normalizer.  This means that if the memoized
function will treat its arguments differently in list context than it
would in scalar context, you can have the normalizer function select
its behavior based on the results of \f(CW\*(C`wantarray\*(C'\fR.  Even if called in
a list context, a normalizer should still return a single string.
.ie n .Sh """SCALAR_CACHE""\fP, \f(CW""LIST_CACHE"""
.el .Sh "\f(CWSCALAR_CACHE\fP, \f(CWLIST_CACHE\fP"
.IX Subsection "SCALAR_CACHE, LIST_CACHE"
Normally, \f(CW\*(C`Memoize\*(C'\fR caches your function's return values into an
ordinary Perl hash variable.  However, you might like to have the
values cached on the disk, so that they persist from one run of your
program to the next, or you might like to associate some other
interesting semantics with the cached values.
.PP
There's a slight complication under the hood of \f(CW\*(C`Memoize\*(C'\fR: There are
actually \fItwo\fR caches, one for scalar values and one for list values.
When your function is called in scalar context, its return value is
cached in one hash, and when your function is called in list context,
its value is cached in the other hash.  You can control the caching
behavior of both contexts independently with these options.
.PP
The argument to \f(CW\*(C`LIST_CACHE\*(C'\fR or \f(CW\*(C`SCALAR_CACHE\*(C'\fR must either be one of
the following four strings:
.PP
.Vb 4
\&        MEMORY
\&        FAULT
\&        MERGE
\&        HASH
.Ve
.PP
or else it must be a reference to a list whose first element is one of
these four strings, such as \f(CW\*(C`[HASH, arguments...]\*(C'\fR.
.ie n .IP """MEMORY""" 4
.el .IP "\f(CWMEMORY\fR" 4
.IX Item "MEMORY"
\&\f(CW\*(C`MEMORY\*(C'\fR means that return values from the function will be cached in
an ordinary Perl hash variable.  The hash variable will not persist
after the program exits.  This is the default.
.ie n .IP """HASH""" 4
.el .IP "\f(CWHASH\fR" 4
.IX Item "HASH"
\&\f(CW\*(C`HASH\*(C'\fR allows you to specify that a particular hash that you supply
will be used as the cache.  You can tie this hash beforehand to give
it any behavior you want.
.Sp
A tied hash can have any semantics at all.  It is typically tied to an
on-disk database, so that cached values are stored in the database and
retrieved from it again when needed, and the disk file typically
persists after your program has exited.  See \f(CW\*(C`perltie\*(C'\fR for more
complete details about \f(CW\*(C`tie\*(C'\fR.
.Sp
A typical example is:
.Sp
.Vb 3
\&        use DB_File;
\&        tie my %cache => \*(AqDB_File\*(Aq, $filename, O_RDWR|O_CREAT, 0666;
\&        memoize \*(Aqfunction\*(Aq, SCALAR_CACHE => [HASH => \e%cache];
.Ve
.Sp
This has the effect of storing the cache in a \f(CW\*(C`DB_File\*(C'\fR database
whose name is in \f(CW$filename\fR.  The cache will persist after the
program has exited.  Next time the program runs, it will find the
cache already populated from the previous run of the program.  Or you
can forcibly populate the cache by constructing a batch program that
runs in the background and populates the cache file.  Then when you
come to run your real program the memoized function will be fast
because all its results have been precomputed.
.ie n .IP """TIE""" 4
.el .IP "\f(CWTIE\fR" 4
.IX Item "TIE"
This option is no longer supported.  It is still documented only to
aid in the debugging of old programs that use it.  Old programs should
be converted to use the \f(CW\*(C`HASH\*(C'\fR option instead.
.Sp
.Vb 1
\&        memoize ... [TIE, PACKAGE, ARGS...]
.Ve
.Sp
is merely a shortcut for
.Sp
.Vb 5
\&        require PACKAGE;
\&        { my %cache;
\&          tie %cache, PACKAGE, ARGS...;
\&        }
\&        memoize ... [HASH => \e%cache];
.Ve
.ie n .IP """FAULT""" 4
.el .IP "\f(CWFAULT\fR" 4
.IX Item "FAULT"
\&\f(CW\*(C`FAULT\*(C'\fR means that you never expect to call the function in scalar
(or list) context, and that if \f(CW\*(C`Memoize\*(C'\fR detects such a call, it
should abort the program.  The error message is one of
.Sp
.Vb 2
\&        \`foo\*(Aq function called in forbidden list context at line ...
\&        \`foo\*(Aq function called in forbidden scalar context at line ...
.Ve
.ie n .IP """MERGE""" 4
.el .IP "\f(CWMERGE\fR" 4
.IX Item "MERGE"
\&\f(CW\*(C`MERGE\*(C'\fR normally means the function does not distinguish between list
and sclar context, and that return values in both contexts should be
stored together.  \f(CW\*(C`LIST_CACHE => MERGE\*(C'\fR means that list context
return values should be stored in the same hash that is used for
scalar context returns, and \f(CW\*(C`SCALAR_CACHE => MERGE\*(C'\fR means the
same, mutatis mutandis.  It is an error to specify \f(CW\*(C`MERGE\*(C'\fR for both,
but it probably does something useful.
.Sp
Consider this function:
.Sp
.Vb 1
\&        sub pi { 3; }
.Ve
.Sp
Normally, the following code will result in two calls to \f(CW\*(C`pi\*(C'\fR:
.Sp
.Vb 3
\&    $x = pi();
\&    ($y) = pi();
\&    $z = pi();
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.Sp
The first call caches the value \f(CW3\fR in the scalar cache; the second
caches the list \f(CW\*(C`(3)\*(C'\fR in the list cache.  The third call doesn't call
the real \f(CW\*(C`pi\*(C'\fR function; it gets the value from the scalar cache.
.Sp
Obviously, the second call to \f(CW\*(C`pi\*(C'\fR is a waste of time, and storing
its return value is a waste of space.  Specifying \f(CW\*(C`LIST_CACHE =>
MERGE\*(C'\fR will make \f(CW\*(C`memoize\*(C'\fR use the same cache for scalar and list
context return values, so that the second call uses the scalar cache
that was populated by the first call.  \f(CW\*(C`pi\*(C'\fR ends up being called only
once, and both subsequent calls return \f(CW3\fR from the cache, regardless
of the calling context.
.Sp
Another use for \f(CW\*(C`MERGE\*(C'\fR is when you want both kinds of return values
stored in the same disk file; this saves you from having to deal with
two disk files instead of one.  You can use a normalizer function to
keep the two sets of return values separate.  For example: