On Wednesday, May 18, 2016 11:40 PM, Brandon Allbery wrote:
On Wed, May 18, 2016 at 11:29 AM, Brandon Allbery <allber...@gmail.com
<mailto:allber...@gmail.com>> wrote:
On Wed, May 18, 2016 at 11:27 AM, mt1957 <mt1...@gmail.com
<mailto:mt1...@gmail.com>> wrote:
This has something to do with lazy evaluation. It triggers the
calculation when it wants to show the value in $r.
IIRC it doesn't throw, it returns a Failure (deferred/lazy
exception that throws when accessed).
And, since this does not seem to be documented in obvious places yet:
"use fatal;" pragma changes these lazy exceptions to strict ones.
"use fatal;" does not change the behaviour. The Failure is only thrown
when referred to. But if the reference is outside the block that
contains the CATCH block to trap the error, then the error is not caught.
The problem is something like:
{ #some code ...
{ # block with error prone code
# Failure created
# no statement like 'say' to get the failure thrown
CATCH { # code to trap error
}
} # end of block with error prone code
# more code
say .... ; # code that causes the Failure to throw an exception
}
Since the exception is thrown outside the block with the CATCH block, it
is not caught.
How can the lazy / deferred Failure be trapped?
A suggestion:
Would it be possible to implement a C( try ) block so that it forces all
Failures to throw Exceptions that can be caught in the CATCH?
If it is necessary to have blocks with an explicit CATCH to have
deferred exceptions, then this behaviour could be kept for ordinary
blocks, whilst try blocks force the exception.
For the code I was originally working on, which was for arithmetic
expressions, a work around is to coerce to Num. But it seems to me this
is not a general solution.
use fatal;
my $r;
for 0..4 -> $s {
try {
$r = 5 / (3 - $s);
CATCH {
when X::Numeric::DivideByZero { $r = 65 }
default { $r = 55 }
}
}
say "At line $?LINE r is $r";
}
## results in
At line 11 r is 1.666667
At line 11 r is 2.5
At line 11 r is 5
Attempt to divide 5 by zero using div
in block <unit> at test.pl line 11
Actually thrown at:
in block <unit> at test.pl line 11
### But
my $r;
for 0..4 -> $s {
$r = (5 / (3 - $s)).Num;
say "At line $?LINE r is $r";
}
#### results in
At line 4 r is 1.66666666666667
At line 4 r is 2.5
At line 4 r is 5
At line 4 r is Inf
At line 4 r is -5
