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POCreating subs on the fly from eval-ed string in perl
 

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    <p>I need to transform data structures from a list of arrays into a tree-like one. I know the depth of the tree before I start processing the data, but I want to keep things flexible so I can re-use the code.</p> <p>So I landed upon the idea of generating a subref on the fly (from within a Moose-based module) to go from array to tree. Like this (in a simplified way):</p> <pre><code>use Data::Dump qw/dump/; sub create_tree_builder { my $depth = shift; return eval join '', 'sub { $_[0]-&gt;{$_[', join(']}-&gt;{$_[', (1..$depth)), ']} = $_[', $depth + 1 , '] }'; } my $s = create_tree_builder(5); my $tree = {}; $s-&gt;($tree, qw/one two three four five/, 'a value'); print dump $tree; # prints # { # one =&gt; { two =&gt; { three =&gt; { four =&gt; { five =&gt; "a value" } } } }, # } </code></pre> <p>This opened up worlds to me, and I'm finding cool uses for this process of eval-in a parametrically generated string into a function all over the place (clearly, a solution in search of problems).</p> <p>However, it feels a little too good to be true, almost.</p> <p>Any advice <em>against</em> this practice? Or suggestion for improvements?</p> <p>I can see clearly that eval-ing arbitrary input might not be the safest thing, but what else?</p> <h1>Follow up</h1> <p>Thanks for all the answers. I used amon's code and benchmarked a bit, like this: </p> <pre><code>use Benchmark qw(:all) ; $\ = "\n"; sub create_tree_builder { my $depth = shift; return eval join '', 'sub { $_[0]-&gt;{$_[', join(']}-&gt;{$_[', (1..$depth)), ']} = $_[', $depth + 1 , '] }'; } my $s = create_tree_builder(5); $t = sub { $_[0] //= {}; my ($tree, @keys) = @_; my $value = pop @keys; $tree = $tree-&gt;{shift @keys} //= {} while @keys &gt; 1; $tree-&gt;{$keys[0]} = $value; }; cmpthese(900000, { 'eval' =&gt; sub { $s-&gt;($tree, qw/one two three four five/, 'a value') }, 'build' =&gt; sub { $t-&gt;($tree, qw/one two three four five/, 'a value') }, }); </code></pre> <p>The results are clearly in favour of building the tree, not of the eval'ed factory:</p> <pre><code> Rate build eval build 326087/s -- -79% eval 1525424/s 368% -- </code></pre> <p>I'll admit I could have done that before. I'll try with more random trees (rather than assigning the same element over and over) but I see no reason that the results should be different. </p> <p>Thanks a lot for the help.</p>
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    1. CO@amon Thanks for the edit - I accidentally deleted the brace in trying to fit in the window (it looked OK in the editor)
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