error_handling.md

Error handling

Catch the error

Once an error is raised, the evaluation terminates. If you want to catch and handle the error, you should use Obj#try.

5.try.{|n| n / 0}.val # nil (error occurred)
5.try.{|n| n / 0}.err # [ZeroDivisionErr: cannot be divided by 0]

Obj#try wraps the receiver by Either. As the name implies, it contains either of an evaluated value or an error.

#                      value, error
5.try.{|n| n / 2}.A # [2.500000, nil]
5.try.{|n| n / 0}.A # [nil, [ZeroDivisionErr: cannot be divided by 0]]

If either contains an error, it ignores any calls. You can handle all errors at the end of the whole chain.

# any call to eitherErr is no operation

# error not raised
20.try.{|n| n + 5}.{|n| 100 // n}.sqrt.A.{|v, err| return err.p if err; v.p} # 2.0
#            err
"a".try.{|n| n + 5}.{|n| 100 // n}.sqrt.A.{|v, err| return err.p if err; v.p} # [TypeErr: 5 cannot be treated as str]
#                       err
-5.try.{|n| n + 5}.{|n| 100 // n}.sqrt.A.{|v, err| return err.p if err; v.p} # [ZeroDivisionErr: cannot be divided by 0]
#                                 err
-6.try.{|n| n + 5}.{|n| 100 // n}.sqrt.A.{|v, err| return err.p if err; v.p} # [ValueErr: sqrt of -100 is not a real number]

Error objects generated by either can be treated as normal objects (no errors are raised).

6.try.{|n| n / 0}.err.kindOf?(ZeroDivisionErr) # true

Why is either introduced?

Ordinal try-catch statements is not suitable for Pangaea one-liners. Also, either keeps chain flow even though error is raised inside.

Thoughtful chain vs Either

While thoughtful chain ~ just ignores the raised error, either can handle caught errors.

Raise caught error object again

Error objects generated by either is just an object. If you want to raise it again, use raise statement.

err := 6.try.{|n| n / 0}.err
raise err