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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "Tie::Memoize 3" | |
132 | .TH Tie::Memoize 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | Tie::Memoize \- add data to hash when needed | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 5 | |
138 | \& require Tie::Memoize; | |
139 | \& tie %hash, 'Tie::Memoize', | |
140 | \& \e&fetch, # The rest is optional | |
141 | \& $DATA, \e&exists, | |
142 | \& {%ini_value}, {%ini_existence}; | |
143 | .Ve | |
144 | .SH "DESCRIPTION" | |
145 | .IX Header "DESCRIPTION" | |
146 | This package allows a tied hash to autoload its values on the first access, | |
147 | and to use the cached value on the following accesses. | |
148 | .PP | |
149 | Only read-accesses (via fetching the value or \f(CW\*(C`exists\*(C'\fR) result in calls to | |
150 | the functions; the modify-accesses are performed as on a normal hash. | |
151 | .PP | |
152 | The required arguments during \f(CW\*(C`tie\*(C'\fR are the hash, the package, and | |
153 | the reference to the \f(CW\*(C`FETCH\*(C'\fRing function. The optional arguments are | |
154 | an arbitrary scalar \f(CW$data\fR, the reference to the \f(CW\*(C`EXISTS\*(C'\fR function, | |
155 | and initial values of the hash and of the existence cache. | |
156 | .PP | |
157 | Both the \f(CW\*(C`FETCH\*(C'\fRing function and the \f(CW\*(C`EXISTS\*(C'\fR functions have the | |
158 | same signature: the arguments are \f(CW\*(C`$key, $data\*(C'\fR; \f(CW$data\fR is the same | |
159 | value as given as argument during \fItie()\fRing. Both functions should | |
160 | return an empty list if the value does not exist. If \f(CW\*(C`EXISTS\*(C'\fR | |
161 | function is different from the \f(CW\*(C`FETCH\*(C'\fRing function, it should return | |
162 | a \s-1TRUE\s0 value on success. The \f(CW\*(C`FETCH\*(C'\fRing function should return the | |
163 | intended value if the key is valid. | |
164 | .SH "Inheriting from \fBTie::Memoize\fP" | |
165 | .IX Header "Inheriting from Tie::Memoize" | |
166 | The structure of the \fItied()\fR data is an array reference with elements | |
167 | .PP | |
168 | .Vb 5 | |
169 | \& 0: cache of known values | |
170 | \& 1: cache of known existence of keys | |
171 | \& 2: FETCH function | |
172 | \& 3: EXISTS function | |
173 | \& 4: $data | |
174 | .Ve | |
175 | .PP | |
176 | The rest is for internal usage of this package. In particular, if | |
177 | \&\s-1TIEHASH\s0 is overwritten, it should call \s-1SUPER::TIEHASH\s0. | |
178 | .SH "EXAMPLE" | |
179 | .IX Header "EXAMPLE" | |
180 | .Vb 6 | |
181 | \& sub slurp { | |
182 | \& my ($key, $dir) = shift; | |
183 | \& open my $h, '<', "$dir/$key" or return; | |
184 | \& local $/; <$h> # slurp it all | |
185 | \& } | |
186 | \& sub exists { my ($key, $dir) = shift; return -f "$dir/$key" } | |
187 | .Ve | |
188 | .PP | |
189 | .Vb 3 | |
190 | \& tie %hash, 'Tie::Memoize', \e&slurp, $directory, \e&exists, | |
191 | \& { fake_file1 => $content1, fake_file2 => $content2 }, | |
192 | \& { pretend_does_not_exists => 0, known_to_exist => 1 }; | |
193 | .Ve | |
194 | .PP | |
195 | This example treats the slightly modified contents of \f(CW$directory\fR as a | |
196 | hash. The modifications are that the keys \fIfake_file1\fR and | |
197 | \&\fIfake_file2\fR fetch values \f(CW$content1\fR and \f(CW$content2\fR, and | |
198 | \&\fIpretend_does_not_exists\fR will never be accessed. Additionally, the | |
199 | existence of \fIknown_to_exist\fR is never checked (so if it does not | |
200 | exists when its content is needed, the user of \f(CW%hash\fR may be confused). | |
201 | .SH "BUGS" | |
202 | .IX Header "BUGS" | |
203 | \&\s-1FIRSTKEY\s0 and \s-1NEXTKEY\s0 methods go through the keys which were already read, | |
204 | not all the possible keys of the hash. | |
205 | .SH "AUTHOR" | |
206 | .IX Header "AUTHOR" | |
207 | Ilya Zakharevich <mailto:perl\-module\-hash\-memoize@ilyaz.org>. |