SOLVED: Eval not trapping fatal error

[Ea]

I've got a Houdini of an error here. The Camel Book says that this is how you trap fatal errors, BUT IT ESCAPES!

I was testing that constants were really constants as suggested in a review by Neil Bowers and the following doesn't compile.

use constant PI => 3.14;
eval { PI = 3; };
warn "You can't round down PI to 3" if $@;

and the execution aborts with Can't modify constant item in scalar assignment

I've tried it on perls v5.16.3 and v5.18.4 on linux. Any suggestions on what is going on here? Have I missed something?

[Corion]

Perl cannot compile your code because it is invalid. You cannot assign to a constant or function (unless that function is already known as lvalue subroutine).

If you want to convert a "syntax"/semantic error to runtime, you will need to use the string form of eval.

[Ea]

Of course, you're right.

So, while other constant modules may die and be trapped with eval BLOCK, the constant pragma makes assignment a syntax error and needs to be trapped with eval EXPRESSION. Now I understand the line in the book

the code in the BLOCK has to be valid Perl code to make it past the compilation phase

Thanks

[Corion]

I don't think that you can trap other constant modules with a block eval either. The issue lies with Perl, which doesn't allow a function/subroutine on the left-hand side of an assignment, unless that function is already known as assignable through the :lvalue attribute:

> perl -wle "sub foo {}; eval { foo()='baz' }"
Can't modify non-lvalue subroutine call in scalar assignment at -e lin
+e 1, near "'baz' }"
Execution of -e aborted due to compilation errors.

Marking the subroutine with :lvalue lets Perl know that we intend to let the function appear on the left-hand side of an assignment:

> perl -wle "sub foo:lvalue {}; eval { foo()='baz' }"

All "constant" modules are syntactic sugar for defining functions that take no parameters, because that's how sigil-less constants work in Perl.

[kcott]

G'day Ea,

I see ++Corion has given plenty of information on why you can't assign to a constant.

Constants, such as the PI you have in your code, are subroutines with a prototype of '()' which allows them to be inlined (see "perlsub: Constant Functions"); as such, they can be redefined.

I really don't recommend that you use the following in any production code but, purely as an academic exercise, consider this:

#!/usr/bin/env perl

use strict;
use warnings;

use constant PI => 3.14;

eval {
    no warnings 'redefine';
    sub PI () { 3 }
};

warn "You can't round down PI to 3" if $@;

print 'PI now set to ', PI, "\n";

When run, this outputs:

PI now set to 3

No errors or warnings are emitted.