Hi

this is part of a longer meditation, but I want to keep it short:

I want to be able to catch error-objects thrown with die and I want to have an easy syntax:

the JS model is reproducible 1-to-1 with Try::Tiny it's even easier because Perl's $_ is automatically set to JS's e

from try...catch#conditional_catch-blocks

try {
  myroutine(); // may throw three types of exceptions
} catch (e) {
  if (e instanceof TypeError) {
    // statements to handle TypeError exceptions
  } else if (e instanceof RangeError) {
    // statements to handle RangeError exceptions
  } else if (e instanceof EvalError) {
    // statements to handle EvalError exceptions
  } else {
    // statements to handle any unspecified exceptions
    logMyErrors(e); // pass exception object to error handler
  }
}
[download]

But in most cases the final else will just do die $_ to propagate the error to higher call levels, hence it's boilerplate.

Python has a model for this by providing a condition after the 'catch', if non is matched the error is raised again.

from https://pythonbasics.org/try-except/

try: 
    # your code here
except FileNotFoundError: 
    # handle exception 
except IsADirectoryError:
    # handle exception
except:                                  # *
    # all other types of exceptions

print('Should reach here')
[download]

(* the last except must be removed to automatically raise the error again)

So ideally one could define in Perl a prototype for catch (COND) { CODE } where the (COND) part is optional. Alas that's not possible in Perl, even with mandatory (COND) (otherwise experiments with syntax extension were easy)

Three workarounds come into mind

1. a bail out function only {} (or maybe handle {} )

try {
    # your code here
} catch { 
    only { FileNotFoundError };            # °
    # handle exception 
} catch {
    only { IsADirectoryError };
    # handle exception
} catch {                                  # *
    # all other types of exceptions
};

print('Should reach here')
[download]

(* again, if the simple catch is removed the error is automatically thrown again die $_ )

( ° the COND is either a boolean expression or a constant resp. object representing an error-class. Since an error-class is blessed into a type like "ErrorClass" this can be tested by only ... hence a shorthand for $_->isa(FileNotFoundError) ... not sure if Python has the same flexibility ;)

2. extend catch (&;&) {...} with optional second sub {}

try {
    # your code here
} catch { FileNotFoundError } sub { 
    # handle exception 
}, catch { IsADirectoryError } sub {
    # handle exception
}, catch {                                  # *
    # all other types of exceptions
};
print('Should reach here')
[download]

UPDATE: added commas, plz see here why.

3. as a variant of 2. use an "underscore" sub _ as syntactic sugar to chain code-blocks

with sub _ (&;@){ return @_ }

try {
    # your code here
} catch { FileNotFoundError }_ { 
    # handle exception 
} catch { IsADirectoryError }_ {
    # handle exception
} catch {                                  # *
    # all other types of exceptions
};
print('Should reach here')
[download]

This variant could help implementing a future built-in syntax catch () {} by parsing {...}_ as (...)

Like that module authors wanting to be backwards compatible to older versions could keep writing {...}_ without loosing the full performance of newer versions.

And new syntax for compound statements could be experimentally implemented and tested.

finally

I wanted to keep it short and didn't show much implementation detail. I'm more interested in comments regarding the interface... or probably I'm missing a good CPAN module already?

comments? suggestions?

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