I am a perl newb.
Unfortunately, eval doesn't work the way I'm expecting
My mock example:
my $filter = 'return false;';
my $filterFunc = sub{ eval{ $filter } } ;
while (my $entry = readNextEntry()){
print "Eval:".$filterFunc->( $entry )."\n";
}
When I run this, I get the literal "return false;" being returned from every pass (rather than the function getting evaluated). I've tried a couple of variations, but I'm not hitting the magic combination.
A note on security implications:
As I am the user, I don't have security concerns about what the passed in code does.
My intent is to pull in multiple sources and filter out stuff based on parameters, since I don't know what parameters are going to be useful, I thought I could pass in some text, eval the text into an anonymous function, and run the function for each record (a filter function). As I am the user, I don't have security concerns about what the passed in code does.
You need to do string eval, not block eval.
my $filterFunc = sub{ eval{ $filter } } ;
The eval BLOCK is like a try/catch mechanism. It does not evaluate arbitrary code. It catches errors in the code inside the block. What you want is string eval.
my $filterFunc = sub{ eval $filter };
Here's an example implementation of what I think you are trying to do:
$ perl -E 'my $filter = sub { my $f = shift; eval $ARGV[0]; }; for ( 1 .. 10 ) { say $_ if $filter->($_) }' '$f % 2'
1
3
5
7
9
However, there is no false in Perl. That's not a keyword. Unless you have sub false somewhere, this might give you a syntax error, depending on if you have use strict or not.
You should read up on eval.
If all you want is a $filterFunc that returns something false, use 0 instead. Note that the literal string "return false;" is true in Perl.
Related
eval is slow when done on a string: The string first has to be parsed before it can be executed.
I am looking for a way to cache the parsing, so that I can reuse the parsed string for yet another eval. The next eval will be the same code, but will not eval to the same value, so I cannot simply cache the results.
From the description I am looking for ceval from Eval::Compile.
But I cannot use Eval::Compile, as that requires a C compiler for the platform, and it is not given that the user has a C compiler.
So can I do something similar to ceval in pure Perl?
Background
GNU Parallel lets the user give Perl expressions that will be eval'ed on every argument. Currently the Perl expressions are given as strings by the user and eval'ed for every argument. The Perl expressions remain unchanged for each argument. It is therefore a waste to recompile the expression as the recompilation will not change anything.
Profiling of the code shows that the eval is one of the bottlenecks.
Example
The user enters: $_ .= "foo" and s/a/b/g
A user's scripts are stored in $usereval1 and $usereval2.
The user gives 10000 random arguments (strings) stored in #arguments.
sub replace {
my ($script, $arg) = #_;
local $_;
$_ = $arg;
# This is where I would like to cache the parsed $script.
eval $script;
return $_;
}
for my $arg (#arguments) {
# Loads of indirect code (in the order of 1000 lines) that
# call subs calling subs calling subs that eventually does:
$replaced1 = replace($usereval1, $arg);
$replaced2 = replace($usereval2, $arg);
# Yet more code that does stuff with $replaced1 $replaced2
}
You can store a subroutine ref like this:
perl -lwe 'my $x = eval(q( sub { my $foo = shift; $foo*2; } )); print $x->(12);'
This prints 24. You can reuse the code without the need to recompile it.
I have perl function I dont what does it do?
my what does min in perl?
#ARVG what does mean?
sub getArgs
{
my $argCnt=0;
my %argH;
for my $arg (#ARGV)
{
if ($arg =~ /^-/) # insert this entry and the next in the hash table
{
$argH{$ARGV[$argCnt]} = $ARGV[$argCnt+1];
}
$argCnt++;
}
return %argH;}
Code like that makes David sad...
Here's a reformatted version of the code doing the indentations correctly. That makes it so much easier to read. I can easily tell where my if and loops start and end:
sub getArgs {
my $argCnt = 0;
my %argH;
for my $arg ( #ARGV ) {
if ( $arg =~ /^-/ ) { # insert this entry and the next in the hash table
$argH{ $ARGV[$argCnt] } = $ARGV[$argCnt+1];
}
$argCnt++;
}
return %argH;
}
The #ARGV is what is passed to the program. It is an array of all the arguments passed. For example, I have a program foo.pl, and I call it like this:
foo.pl one two three four five
In this case, $ARGV is set to the list of values ("one", "two", "three", "four", "five"). The name comes from a similar variable found in the C programming language.
The author is attempting to parse these arguments. For example:
foo.pl -this that -the other
would result in:
$arg{"-this"} = "that";
$arg{"-the"} = "other";
I don't see min. Do you mean my?
This is a wee bit of a complex discussion which would normally involve package variables vs. lexically scoped variables, and how Perl stores variables. To make things easier, I'm going to give you a sort-of incorrect, but technically wrong answer: If you use the (strict) pragma, and you should, you have to declare your variables with my before they can be used. For example, here's a simple two line program that's wrong. Can you see the error?
$name = "Bob";
print "Hello $Name, how are you?\n";
Note that when I set $name to "Bob", $name is with a lowercase n. But, I used $Name (upper case N) in my print statement. As it stands, now. Perl will print out "Hello, how are you?" without a care that I've used the wrong variable name. If it's hard to spot an error like this in a two line program, imagine what it would be like in a 1000 line program.
By using strict and forcing me to declare variables with my, Perl can catch that error:
use strict;
use warnings; # Another Pragma that should always be used
my $name = "Bob";
print "Hello $Name, how are you doing\n";
Now, when I run the program, I get the following error:
Global symbol "$Name" requires explicit package name at (line # of print statement)
This means that $Name isn't defined, and Perl points to where that error is.
When you define variables like this, they are in scope with in the block where it's defined. A block could be the code contained in a set of curly braces or a while, if, or for statement. If you define a variable with my outside of these, it's defined to the end of the file.
Thus, by using my, the variables are only defined inside this subroutine. And, the $arg variable is only defined in the for loop.
One more thing:
The person who wrote this should have used the Getopt::Long module. There's a major bug in their code:
For example:
foo.pl -this that -one -two
In this case, my hash looks like this:
$args{'-this'} = "that";
$args{'-one'} = "-two";
$args{'-two'} = undef;
If I did this:
if ( defined $args{'-two'} ) {
...
}
I would not execute the if statement.
Also:
foo.pl -this=that -one -two
would also fail.
#ARGV is a special variable (refer to perldoc perlvar):
#ARGV
The array #ARGV contains the command-line arguments intended for the
script. $#ARGV is generally the number of arguments minus one, because
$ARGV[0] is the first argument, not the program's command name itself.
See $0 for the command name.
Perl documentation is also available from your command line:
perldoc -v #ARGV
I'm just starting in Perl and I'm quite enjoying it. I'm writing some basic functions, but what I really want to be able to do is to use those functions intelligently using console commands. For example, say I have a function adding two numbers. I'd want to be able to type in console "add 2, 4" and read the first word, then pass the two numbers as parameters in an "add" function. Essentially, I'm asking for help in creating some basic scripting using Perl ^^'.
I have some vague ideas about how I might do this in VB, but Perl, I have no idea where I'd start, or what functions would be useful to me. Is there something like VB.net's "Split" function where you can break down the contents of a scalar into an array? Is there a simple way to analyse one word at a time in a scalar, or iterate through a scalar until you hit a separator, for example?
I hope you can help, any suggestions are appreciated! Bear in mind, I'm no expert, I started Perl all of a few weeks ago, and I've only been doing VB.net half a year.
Thank you!
Edit: If you're not sure what to suggest and you know any simple/intuitive resources that might be of help, that would also be appreciated.
Its rather easy to make a script which dispatches to a command by name. Here is a simple example:
#!/usr/bin/env perl
use strict;
use warnings;
# take the command name off the #ARGV stack
my $command_name = shift;
# get a reference to the subroutine by name
my $command = __PACKAGE__->can($command_name) || die "Unknown command: $command_name\n";
# execute the command, using the rest of #ARGV as arguments
# and print the return with a trailing newline
print $command->(#ARGV);
print "\n";
sub add {
my ($x, $y) = #_;
return $x + $y;
}
sub subtract {
my ($x, $y) = #_;
return $x - $y;
}
This script (say its named myscript.pl) can be called like
$ ./myscript.pl add 2 3
or
$ ./myscript.pl subtract 2 3
Once you have played with that for a while, you might want to take it further and use a framework for this kind of thing. There are several available, like App::Cmd or you can take the logic shown above and modularize as you see fit.
You want to parse command line arguments. A space serves as the delimiter, so just do a ./add.pl 2 3 Something like this:
$num1=$ARGV[0];
$num2=$ARGV[1];
print $num1 + $num2;
will print 5
Here is a short implementation of a simple scripting language.
Each statement is exactly one line long, and has the following structure:
Statement = [<Var> =] <Command> [<Arg> ...]
# This is a regular grammar, so we don't need a complicated parser.
Tokens are seperated by whitespace. A command may take any number of arguments. These can either be the contents of variables $var, a string "foo", or a number (int or float).
As these are Perl scalars, there is no visible difference between strings and numbers.
Here is the preamble of the script:
#!/usr/bin/perl
use strict;
use warnings;
use 5.010;
strict and warnings are essential when learning Perl, else too much weird stuff would be possible. The use 5.010 is a minimum version, it also defines the say builtin (like a print but appends a newline).
Now we declare two global variables: The %env hash (table or dict) associates variable names with their values. %functions holds our builtin functions. The values are anonymous functions.
my %env;
my %functions = (
add => sub { $_[0] + $_[1] },
mul => sub { $_[0] * $_[1] },
say => sub { say $_[0] },
bye => sub { exit 0 },
);
Now comes our read-eval-loop (we don't print by default). The readline operator <> will read from the file specified as the first command line argument, or from STDIN if no filename is provided.
while (<>) {
next if /^\s*\#/; # jump comment lines
# parse the line. We get a destination $var, a $command, and any number of #args
my ($var, $command, #args) = parse($_);
# Execute the anonymous sub specified by $command with the #args
my $value = $functions{ $command }->(#args);
# Store the return value if a destination $var was specified
$env{ $var } = $value if defined $var;
}
That was fairly trivial. Now comes some parsing code. Perl “binds” regexes to strings with the =~ operator. Regexes may look like /foo/ or m/foo/. The /x flags allows us to include whitespace in our regex that doesn't match actual whitespace. The /g flag matches globally. This also enables the \G assertion. This is where the last successful match ended. The /c flag is important for this m//gc style parsing to consume one match at a time, and to prevent the position of the regex engine in out string to being reset.
sub parse {
my ($line) = #_; # get the $line, which is a argument
my ($var, $command, #args); # declare variables to be filled
# Test if this statement has a variable declaration
if ($line =~ m/\G\s* \$(\w+) \s*=\s* /xgc) {
$var = $1; # assign first capture if successful
}
# Parse the function of this statement.
if ($line =~ m/\G\s* (\w+) \s*/xgc) {
$command = $1;
# Test if the specified function exists in our %functions
if (not exists $functions{$command}) {
die "The command $command is not known\n";
}
} else {
die "Command required\n"; # Throw fatal exception on parse error.
}
# As long as our matches haven't consumed the whole string...
while (pos($line) < length($line)) {
# Try to match variables
if ($line =~ m/\G \$(\w+) \s*/xgc) {
die "The variable $1 does not exist\n" if not exists $env{$1};
push #args, $env{$1};
}
# Try to match strings
elsif ($line =~ m/\G "([^"]+)" \s*/xgc) {
push #args, $1;
}
# Try to match ints or floats
elsif ($line =~ m/\G (\d+ (?:\.\d+)? ) \s*/xgc) {
push #args, 0+$1;
}
# Throw error if nothing matched
else {
die "Didn't understand that line\n";
}
}
# return our -- now filled -- vars.
return $var, $command, #args;
}
Perl arrays can be handled like linked list: shift removes and returns the first element (pop does the same to the last element). push adds an element to the end, unshift to the beginning.
Out little programming language can execute simple programs like:
#!my_little_language
$a = mul 2 20
$b = add 0 2
$answer = add $a $b
say $answer
bye
If (1) our perl script is saved in my_little_language, set to be executable, and is in the system PATH, and (2) the above file in our little language saved as meaning_of_life.mll, and also set to be executable, then
$ ./meaning_of_life
should be able to run it.
Output is obviously 42. Note that our language doesn't yet have string manipulation or simple assignment to variables. Also, it would be nice to be able to call functions with the return value of other functions directly. This requires some sort of parens, or precedence mechanism. Also, the language requires better error reporting for batch processing (which it already supports).
I am learning perl and understand that it is a common and accepted practice to unpack subroutine arguments using shift. I also understand that it is common and acceptable practice to omit function arguments to use the default #_ array.
Considering these two things, if you call a subroutine without arguments, the #_ can (and will, if using shift) be changed. Does this mean that calling another subroutine with default arguments, or, in fact, using the #_ array after this, is considered bad practice? Consider this example:
sub total { # calculate sum of all arguments
my $running_sum;
# take arguments one by one and sum them together
while (#_) {
$running_sum += shift;
}
$running_sum;
}
sub avg { calculate the mean of given arguments
if (#_ == 0) { return }
my $sum = &total; # gets the correct answer, but changes #_
$sum / #_ # causes division by zero, since #_ is now empty
}
My gut feeling tells me that using shift to unpack arguments would actually be bad practice, unless your subroutine is actually supposed to change the passed arguments, but I have read in multiple places, including Stack Overflow, that this is not a bad practice.
So the question is: if using shift is common practice, should I always assume the passed argument list could get changed, as a side-effect of the subroutine (like the &total subroutine in the quoted example)? Is there maybe a way to pass arguments by value, so I can be sure that the argument list does not get changed, so I could use it again (like in the &avg subroutine in the quoted text)?
In general, shifting from the arguments is ok—using the & sigil to call functions isn't. (Except in some very specific situations you'll probably never encounter.)
Your code could be re-written, so that total doesn't shift from #_. Using a for-loop may even be more efficient.
sub total {
my $total = 0;
$total += $_ for #_;
$total;
}
Or you could use the sum function from List::Util:
use List::Util qw(sum);
sub avg { #_ ? sum(#_) / #_ : 0 }
Using shift isn't that common, except for extracting $self in object oriented Perl. But as you always call your functions like foo( ... ), it doesn't matter if foo shifts or doesn't shift the argument array.
(The only thing worth noting about a function is whether it assigns to elements in #_, as these are aliases for the variables you gave as arguments. Assigning to elements in #_ is usually bad.)
Even if you can't change the implementation of total, calling the sub with an explicit argument list is safe, as the argument list is a copy of the array:
(a) &total — calls total with the identical #_, and overrides prototypes.
(b) total(#_) — calls total with a copy of #_.
(c) &total(#_) — calls total with a copy of #_, and overrides prototypes.
Form (b) is standard. Form (c) shouldn't be seen, except in very few cases for subs inside the same package where the sub has a prototype (and don't use prototypes), and they have to be overridden for some obscure reason. A testament to poor design.
Form (a) is only sensible for tail calls (#_ = (...); goto &foo) or other forms of optimization (and premature optimization is the root of all evil).
You should avoid using the &func; style of calling unless you have a really good reason, and trust that others do the same.
To guard your #_ against modification by a callee, just do &func() or func.
Perl is a little too lax sometimes and having multiple ways of accessing input parameters can make smelly and inconsistent code. For want of a better answer, try to impose your own standard.
Here's a few ways I've used and seen
Shifty
sub login
{
my $user = shift;
my $passphrase = shift;
# Validate authentication
return 0;
}
Expanding #_
sub login
{
my ($user, $passphrase) = #_;
# Validate authentication
return 0;
}
Explicit indexing
sub login
{
my user = $_[0];
my user = $_[1];
# Validate authentication
return 0;
}
Enforce parameters with function prototypes (this is not popular however)
sub login($$)
{
my ($user, $passphrase) = #_;
# Validate authentication
return 0;
}
Sadly you still have to perform your own convoluted input validation/taint checking, ie:
return unless defined $user;
return unless defined $passphrase;
or better still, a little more informative
unless (defined($user) && defined($passphrase)) {
carp "Input error: user or passphrase not defined";
return -1;
}
Perldoc perlsub should really be your first port of call.
Hope this helps!
Here are some examples where the careful use of #_ matters.
1. Hash-y Arguments
Sometimes you want to write a function which can take a list of key-value pairs, but one is the most common use and you want that to be available without needing a key. For example
sub get_temp {
my $location = #_ % 2 ? shift : undef;
my %options = #_;
$location ||= $options{location};
...
}
So now if you call the function with an odd number of arguments, the first is location. This allows get_temp('Chicago') or get_temp('New York', unit => 'C') or even get_temp( unit => 'K', location => 'Nome, Ak'). This may be a more convenient API for your users. By shifting the odd argument, now #_ is an even list and may be assigned to a hash.
2. Dispatching
Lets say we have a class that we want to be able to dispatch methods by name (possibly AUTOLOAD could be useful, we will hand roll). Perhaps this is a command line script where arguments are methods. In this case we define two dispatch methods one "clean" and one "dirty". If we call with the -c flag we get the clean one. These methods find the method by name and call it. The difference is how. The dirty one leaves itself in the stack trace, the clean one has to be more cleaver, but dispatches without being in the stack trace. We make a death method which gives us that trace.
#!/usr/bin/env perl
use strict;
use warnings;
package Unusual;
use Carp;
sub new {
my $class = shift;
return bless { #_ }, $class;
}
sub dispatch_dirty {
my $self = shift;
my $name = shift;
my $method = $self->can($name) or confess "No method named $name";
$self->$method(#_);
}
sub dispatch_clean {
my $self = shift;
my $name = shift;
my $method = $self->can($name) or confess "No method named $name";
unshift #_, $self;
goto $method;
}
sub death {
my ($self, $message) = #_;
$message ||= 'died';
confess "$self->{name}: $message";
}
package main;
use Getopt::Long;
GetOptions
'clean' => \my $clean,
'name=s' => \(my $name = 'Robot');
my $obj = Unusual->new(name => $name);
if ($clean) {
$obj->dispatch_clean(#ARGV);
} else {
$obj->dispatch_dirty(#ARGV);
}
So now if we call ./test.pl to invoke the death method
$ ./test.pl death Goodbye
Robot: Goodbye at ./test.pl line 32
Unusual::death('Unusual=HASH(0xa0f7188)', 'Goodbye') called at ./test.pl line 19
Unusual::dispatch_dirty('Unusual=HASH(0xa0f7188)', 'death', 'Goodbye') called at ./test.pl line 46
but wee see dispatch_dirty in the trace. If instead we call ./test.pl -c we now use the clean dispatcher and get
$ ./test.pl -c death Adios
Robot: Adios at ./test.pl line 33
Unusual::death('Unusual=HASH(0x9427188)', 'Adios') called at ./test.pl line 44
The key here is the goto (not the evil goto) which takes the subroutine reference and immediately switches the execution to that reference, using the current #_. This is why I have to unshift #_, $self so that the invocant is ready for the new method.
Refs:
sub refWay{
my ($refToArray,$secondParam,$thirdParam) = #_;
#work here
}
refWay(\#array, 'a','b');
HashWay:
sub hashWay{
my $refToHash = shift; #(if pass ref to hash)
#and i know, that:
return undef unless exists $refToHash->{'user'};
return undef unless exists $refToHash->{'password'};
#or the same in loop:
for (qw(user password etc)){
return undef unless exists $refToHash->{$_};
}
}
hashWay({'user'=>YourName, 'password'=>YourPassword});
I tried a simple example:
#!/usr/bin/perl
use strict;
sub total {
my $sum = 0;
while(#_) {
$sum = $sum + shift;
}
return $sum;
}
sub total1 {
my ($a, $aa, $aaa) = #_;
return ($a + $aa + $aaa);
}
my $s;
$s = total(10, 20, 30);
print $s;
$s = total1(10, 20, 30);
print "\n$s";
Both print statements gave answer as 60.
But personally I feel, the arguments should be accepted in this manner:
my (arguments, #garb) = #_;
in order to avoid any sort of issue latter.
I found the following gem in http://perldoc.perl.org/perlsub.html:
"Yes, there are still unresolved issues having to do with visibility of #_ . I'm ignoring that question for the moment. (But note that if we make #_ lexically scoped, those anonymous subroutines can act like closures... (Gee, is this sounding a little Lispish? (Never mind.)))"
You might have run into one of those issues :-(
OTOH amon is probably right -> +1
I'm looking for a way to do this in Perl:
$a = "60"; $b = "< 80";
if ( $a $b ) { then .... }
Here, $b "holds" an operator... can I do this? Maybe some other way?
It's nice to see how people discover functional programming. :-)
Luckily, Perl has capabilities to create and store functions on-the-fly. For example, the sample in your question will look like this:
$a = "60"; $b = sub { $_[0] < 80 };
if ( $b->($a) ) { .... }
In this example, a reference to the anonymous subroutine is stored in $b, the sub having the same syntax for argument passing as a usual one. -> is then used to call-by-reference (the same syntax you probably use for references to arrays and hashes).
But, of course, if you want just to construct Perl expressions from arbitrary strings, you might want to use eval:
$a = "60"; $b = " < 80";
if ( eval ("$a $b") ) { .... }
However, doing this via eval is not safe, if the string you're eval-ing contains parts that come as user input. Sinan Ünür explained it perfectly in his answer-comment.
How about defining a function that wraps the needed condition:
my $cond = sub { $_[0] < 80 };
if ( $cond->( $a ) ) {
...
}
This should be a comment but comments are too cramped for something like this so I am making it CW.
For the case which you showed where the contents of the variables that are going to be passed to string eval, the accepted solution is correct.
If, however, the contents of $a and $b come from user input, then take a look at the following script:
#!/usr/bin/perl
use strict; use warnings;
my $x = '80';
my $y = '; warn "evil laugh!\n"; exit';
if ( eval ($x . $y) ) {
print "it worked!!!\n";
}
If the strings are entered by the user, there is nothing preventing the user from passing to your program the string ';system "rm -rf /bin"'.
So, the correct solution to your question would require writing or using an expression parser.
BTW, you should not use $a and $b as variable names as the are magical package local variables used by sort and as such they are exempt from strict — and you must always use strict and warnings in your programs.
$a = "60"; $b = "< 80";
if( eval($a. $b)){
print "ok";
}
see perldoc eval for more
I wonder if Number::Compare is of any interest here. From the example:
Number::Compare->new(">1Ki")->test(1025); # is 1025 > 1024
my $c = Number::Compare->new(">1M");
$c->(1_200_000); # slightly terser invocation
Safer form if you trust (or can sufficiently validate) $op and don't trust the safety of the inputs:
my $compare_x = $user_input_x;
my $compare_y = $user_input_y;
my $op = <some safe non-user-input, or otherwise checked against a safe list>;
if ( eval("\$compare_x $op \$compare_y") )
{
...
}