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.\" ========================================================================
.\"
.IX Title "Tie::Memoize 3"
.TH Tie::Memoize 3 "2019-10-24" "perl v5.30.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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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 \s-1TRUE\s0 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:  $data
.Ve
.PP
The rest is for internal usage of this package.  In particular, if
\&\s-1TIEHASH\s0 is overwritten, it should call \s-1SUPER::TIEHASH.\s0
.SH "EXAMPLE"
.IX Header "EXAMPLE"
.Vb 6
\&  sub slurp {
\&    my ($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"
\&\s-1FIRSTKEY\s0 and \s-1NEXTKEY\s0 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>.