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.IX Title "Tie::Memoize 3perl"
.TH Tie::Memoize 3perl 2024-01-12 "perl v5.38.2" "Perl Programmers Reference Guide"
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.SH NAME
Tie::Memoize \- add data to hash when needed
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 5
\&  require Tie::Memoize;
\&  tie %hash, \*(AqTie::Memoize\*(Aq,
\&      \e&fetch,                  # The rest is optional
\&      $DATA, \e&exists,
\&      {%ini_value}, {%ini_existence};
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
This package allows a tied hash to autoload its values on the first access,
and to use the cached value on the following accesses.
.PP
Only read-accesses (via fetching the value or \f(CW\*(C`exists\*(C'\fR) result in calls to
the functions; the modify-accesses are performed as on a normal hash.
.PP
The required arguments during \f(CW\*(C`tie\*(C'\fR are the hash, the package, and
the reference to the \f(CW\*(C`FETCH\*(C'\fRing function.  The optional arguments are
an arbitrary scalar \f(CW$data\fR, the reference to the \f(CW\*(C`EXISTS\*(C'\fR function,
and initial values of the hash and of the existence cache.
.PP
Both the \f(CW\*(C`FETCH\*(C'\fRing function and the \f(CW\*(C`EXISTS\*(C'\fR functions have the
same signature: the arguments are \f(CW\*(C`$key, $data\*(C'\fR; \f(CW$data\fR is the same
value as given as argument during \fBtie()\fRing.  Both functions should
return an empty list if the value does not exist.  If \f(CW\*(C`EXISTS\*(C'\fR
function is different from the \f(CW\*(C`FETCH\*(C'\fRing function, it should return
a TRUE value on success.  The \f(CW\*(C`FETCH\*(C'\fRing function should return the
intended value if the key is valid.
.SH "Inheriting from \fBTie::Memoize\fP"
.IX Header "Inheriting from Tie::Memoize"
The structure of the \fBtied()\fR data is an array reference with elements
.PP
.Vb 5
\&  0:  cache of known values
\&  1:  cache of known existence of keys
\&  2:  FETCH  function
\&  3:  EXISTS function
\&  4:  $data
.Ve
.PP
The rest is for internal usage of this package.  In particular, if
TIEHASH is overwritten, it should call SUPER::TIEHASH.
.SH EXAMPLE
.IX Header "EXAMPLE"
.Vb 6
\&  sub slurp {
\&    my ($key, $dir) = shift;
\&    open my $h, \*(Aq<\*(Aq, "$dir/$key" or return;
\&    local $/; <$h>                      # slurp it all
\&  }
\&  sub exists { my ($key, $dir) = shift; return \-f "$dir/$key" }
\&
\&  tie %hash, \*(AqTie::Memoize\*(Aq, \e&slurp, $directory, \e&exists,
\&      { fake_file1 => $content1, fake_file2 => $content2 },
\&      { pretend_does_not_exists => 0, known_to_exist => 1 };
.Ve
.PP
This example treats the slightly modified contents of \f(CW$directory\fR as a
hash.  The modifications are that the keys \fIfake_file1\fR and
\&\fIfake_file2\fR fetch values \f(CW$content1\fR and \f(CW$content2\fR, and
\&\fIpretend_does_not_exists\fR will never be accessed.  Additionally, the
existence of \fIknown_to_exist\fR is never checked (so if it does not
exists when its content is needed, the user of \f(CW%hash\fR may be confused).
.SH BUGS
.IX Header "BUGS"
FIRSTKEY and NEXTKEY methods go through the keys which were already read,
not all the possible keys of the hash.
.SH AUTHOR
.IX Header "AUTHOR"
Ilya Zakharevich <mailto:perl\-module\-hash\-memoize@ilyaz.org>.