ch09-errors.asciidoc

Handling Errors

Note	You can learn more about error handling in Chapters 3 and 17 of Erlang Programming, Chapter 4 and Section 18.2 of Programming Erlang, Section 2.8 and Chapters 5 and 7 of Erlang and OTP in Action, and Chapters 7 and 12 of Learn You Some Erlang For Great Good!.

Étude 9-1: try and catch

Update the stats module that you wrote in Étude 7-3 so that it will catch errors in the minimum/1, maximum/1, mean/1 and stdv/1 functions.

Here is some sample output.

1> c(stats).
{ok,stats}
2> stats:minimum([]).
{error,badarg}
3> stats:mean([]).
{error,badarith}
4> stats:mean(["123", 456]).
{error,badarith}
5> stats:stdv([]).
{error,badarith}

See a suggested solution in Appendix A.

Étude 9-2: Logging Errors

Write a module named bank that contains a function account/1. The function takes a numeric Balance, which gives the current balance in the account in imaginary dollars.

The function will repeatedly ask for a transaction (deposit, withdraw, balance inquiry, or quit). If a deposit or withdrawal, it asks for the amount to deposit or withdraw, and then does that transaction. If a deposit is more than $10,000, the deposit may be subject to hold.

Provide output to the customer, and also use error_logger to write to a log file (which, in this case, will go to your terminal). Choose any form of input prompts and feedback and logging messages that you desire. Handle the following situations:

• Deposits and withdrawals cannot be negative numbers (error)

• Deposits of $10,000 or more might be subject to hold (warning)

• All other transactions are successful (informational)

Use get_number/1 from Étude 5-1 to allow either integer or float input.

Here is sample output. Due to Erlang’s asynchronous nature, the user prompts and logging are often interleaved in the most inconvenient places.

1> c(bank).
{ok,bank}
2> bank:account(2000).
D)eposit, W)ithdraw, B)alance, Q)uit: D
Amount to deposit: 300
Your new balance is 2300
D)eposit, W)ithdraw, B)alance, Q)uit:
=INFO REPORT==== 26-Jan-2013::06:42:52 ===
Successful deposit 300
W
Amount to withdraw: -200
Withdrawals may not be less than zero.
=ERROR REPORT==== 26-Jan-2013::06:42:56 ===
Negative withdrawal amount -200
D)eposit, W)ithdraw, B)alance, Q)uit: D
Amount to deposit: 15000
Your deposit of $15000 may be subject to hold.
=ERROR REPORT==== 26-Jan-2013::06:43:05 ===
Excessive deposit 15000
Your new balance is 17300
D)eposit, W)ithdraw, B)alance, Q)uit: W
Amount to withdraw: 32767
You cannot withdraw more than your current balance of 17300.

=ERROR REPORT==== 26-Jan-2013::06:43:17 ===
Overdraw 32767 from balance 17300
D)eposit, W)ithdraw, B)alance, Q)uit: W
Amount to withdraw: 150.25
Your new balance is 17149.75

=INFO REPORT==== 26-Jan-2013::06:43:29 ===
Successful withdrawal 150.25
D)eposit, W)ithdraw, B)alance, Q)uit: B
D)eposit, W)ithdraw, B)alance, Q)uit:
=INFO REPORT==== 26-Jan-2013::06:43:35 ===
Balance inquiry 17149.75
X
Unknown command X
D)eposit, W)ithdraw, B)alance, Q)uit: Q
true

See a suggested solution in Appendix A.