Exceptions

The value of on:do: is the value of the protected block, or of the handler block if an exception was handled.

24.1 Signaling and handling

The value of on:do: is the value of the protected block, or of the handler block if an exception was handled.

[ 10 / 0 ] on: ZeroDivide do: [ :ex | 0 ].              "→ 0"
[ #(1 2 3) at: 10 ] on: SubscriptOutOfBounds do: [ :ex | nil ].
[ Dictionary new at: #missing ] on: KeyNotFound do: [ :ex | 'default' ].
[ nil foo ] on: MessageNotUnderstood do: [ :ex | ex message selector ].   "→ #foo"
[ self error: 'boom' ] on: Error do: [ :ex | ex messageText ].           "→ 'boom'"
[ 1/0 ] on: ZeroDivide, MessageNotUnderstood do: [ :ex | -1 ].           "ExceptionSet with ,"

24.2 The exception hierarchy (simplified)

Exception
 ├── Error                         (not resumable by default)
 │    ├── ZeroDivide (ArithmeticError)
 │    ├── MessageNotUnderstood
 │    ├── SubscriptOutOfBounds, NotFound, KeyNotFound
 │    ├── AssertionFailure (TestFailure in SUnit)
 │    ├── FileException, FileDoesNotExistException
 │    └── your own errors
 ├── Notification                  (resumable; unhandled → returns nil, no debugger)
 │    └── Warning                  (resumable; unhandled → debugger with Proceed)
 └── Halt

24.3 Signaling

self error: 'Something went wrong' signals a generic Error; equivalently Error signal: 'msg' or Error new signal: 'msg'. Warning signal: 'Disk almost full' is resumable; any exception class can be signaled with ZeroDivide new signal.

24.4 What a handler can do

Message to ex ex return: value Exit the on:do: expression with value (default if the handler just falls off the end) Re-evaluate the protected block ex retryUsing: aBlock Replace the protected block and retry ex resume: value

Return value from the signal expression and continue (resumable exceptions only)

Re-signal to an outer handler (as if not handled here) Like pass, but if the outer handler resumes, come back here Re-signal the exception ex messageText / ex description ex signalerContext The stack frame where it was signaled

"Retry with a counter"
| attempts result |
attempts := 0.
result := [ attempts := attempts + 1.
            attempts < 3 ifTrue: [ Error signal: 'flaky' ].
            'succeeded after ', attempts printString ]
    on: Error
    do: [ :ex | ex retry ].
result.          "→ 'succeeded after 3'"

"Resume a warning with a value"
[ (Warning signal: 'use default?') ifNil: [ 'no answer' ] ]
    on: Warning do: [ :ex | ex resume: 'yes' ].         "→ 'yes'"

"ZeroDivide is resumable in Pharo"
[ 10 / 0 + 1 ] on: ZeroDivide do: [ :ex | ex resume: 0 ].   "→ 1"

24.5 ensure: and ifCurtailed:

writeStreamDo:/readStreamDo: already wrap ensure: for you.

| stream |
stream := 'data.txt' asFileReference readStream.
[ stream upToEnd ] ensure: [ stream close ].        "always runs (like finally)"

[ self riskyOperation ] ifCurtailed: [ self rollback ].   "runs only on abnormal exit (error, non-local return, process termination)"

24.6 Custom exceptions

Create a hierarchy (BankError → InsufficientFunds, AccountClosed) so clients can catch broad or narrow categories.

Error << #InsufficientFunds
    slots: { #requested . #available };
    package: 'Bank-Core'

InsufficientFunds >> requested: r available: a
    requested := r.
    available := a

InsufficientFunds >> messageText
    ^ 'Insufficient funds: requested ', requested printString,
      ', available ', available printString

InsufficientFunds >> isResumable
    ^ false

BankAccount >> withdraw: anAmount
    anAmount > balance ifTrue: [
        ^ InsufficientFunds new requested: anAmount available: balance; signal ].
    balance := balance - anAmount

[ (BankAccount owner: 'Ada' balance: 10) withdraw: 50 ]
    on: InsufficientFunds
    do: [ :ex | ex messageText ].       "→ 'Insufficient funds: requested 50, available 10'"

24.7 Notifications for context passing

A Notification that nobody handles simply returns nil — handy for implicit parameters:

Pharo also provides DynamicVariable subclasses (MyVar value: x during: [...]) for the same purpose.

Notification << #CurrentUser slots: {}; package: 'App'.

"Deep in the call stack:"
(CurrentUser signal) ifNil: [ 'anonymous' ].

"At the top:"
[ self handleRequest ] on: CurrentUser do: [ :ex | ex resume: 'ada' ].

24.8 Unhandled errors and the debugger

An unhandled Error opens the debugger — not a crash. You can inspect, fix the code, and proceed. In headless mode, unhandled errors are logged (to stdout/stderr or PharoDebug.log) and the process is terminated (or the image exits, depending on the command-line handler).