Re: Why isn't a fatal error trappable?

It is trappable:

c:\test>perl -E"chop for 'fred'; print 'here' "
Modification of a read-only value attempted at -e line 1.

c:\test>perl -E"eval{ chop; } for 'fred'; print 'here' "
here
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The only circumstance I'm aware of that it isn't, is if you supply a constant directly to a mutator:

c:\test>perl -E"chop 'fred'; print 'here' "
Can't modify constant item in chop at -e line 1, near "'fred';"
Execution of -e aborted due to compilation errors.

c:\test>perl -E"eval{ chop 'fred' }; print 'here' "
Can't modify constant item in chop at -e line 1, near "'fred' }"
Execution of -e aborted due to compilation errors.
[download]

And that, I've always assumed, is because it is detected at compile time rather than runtime.

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Re^2: Why isn't a fatal error trappable? by ikegami (Patriarch) on Nov 16, 2010 at 18:22 UTC
Interesting. Division by zero is intentionally deferred to compile time. `$ perl -c -e'3/0' -e syntax OK $ perl -e'3/0' Illegal division by zero at -e line 1.` [download] Yet `chop` is constant folded even though that always results in an exception. Update: Ah! The exception is probably not from constant folding. It's probably a compile-time requirement for an lvalue. `$ perl -e'chop "fred"' Can't modify constant item in chop at -e line 1, at EOF Execution of -e aborted due to compilation errors.` [download] runs afoul the same check as `$ perl -e'"fred" = $_' Can't modify constant item in scalar assignment at -e line 1, at EOF Execution of -e aborted due to compilation errors.` [download]	[reply] [d/l] [select]

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Re^2: Why isn't a fatal error trappable?
by ikegami (Patriarch) on Nov 16, 2010 at 18:22 UTC

Interesting. Division by zero is intentionally deferred to compile time.

$ perl -c -e'3/0'
-e syntax OK

$ perl -e'3/0'
Illegal division by zero at -e line 1.
[download]

Yet chop is constant folded even though that always results in an exception.

Update: Ah! The exception is probably not from constant folding. It's probably a compile-time requirement for an lvalue.

$ perl -e'chop "fred"'
Can't modify constant item in chop at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
[download]

runs afoul the same check as

$ perl -e'"fred" = $_'
Can't modify constant item in scalar assignment at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
[download]

[reply]
[d/l]
[select]