12.3: Handling Non-Local Returns

Last updated
Save as PDF

Page ID: 43731

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

The message ifCurtailed: is typically used for “cleaning” actions. It is similar to ensure:, but instead of ensuring that its argument block is evaluated even if the receiver terminates abnormally, ifCurtailed: does so only if the receiver fails or returns.

In the following example, the receiver of ifCurtailed: performs an early return, so the following statement is never reached. In Smalltalk, this is referred to as a non-local return. Nevertheless the argument block will be executed.

[^ 10] ifCurtailed: [Transcript show: 'We see this'].
Transcript show: 'But not this'.

In the following example, we can see clearly that the argument to ifCurtailed: is evaluated only when the receiver terminates abnormally.

[Error signal] ifCurtailed: [Transcript show: 'Abandoned'; cr].
Transcript show: 'Proceeded'; cr.

\(\bigstar\) Open a transcript and evaluate the code above in a workspace. When the pre-debugger windows open, first try selecting Proceed and then Abandon. Note that the argument to ifCurtailed: is evaluated only when the receiver terminates abnormally. What happens when you select Debug?

Here are some examples of ifCurtailed: usage: the text of the Transcript show: describes the situation:

[^ 10] ifCurtailed: [Transcript show: 'This is displayed'; cr]

[10] ifCurtailed: [Transcript show: 'This is not displayed'; cr]

[1 / 0] ifCurtailed: [Transcript show: 'This is displayed after selecting Abandon in the
    debugger'; cr]

Although in Pharo ifCurtailed: and ensure: are implemented using a a marker primitive (described at the end of the chapter), in principle ifCurtailed: could be implemented using ensure: as follows:

ifCurtailed: curtailBlock
    | result curtailed |
    curtailed := true.
    [ result := self value.
        curtailed := false ] ensure: [ curtailed ifTrue: [ curtailBlock value ] ].
    ^ result

In a similar fashion, ensure: could be implemented using ifCurtailed: as follows:

ensure: ensureBlock
    | result |
    result := self ifCurtailed: ensureBlock.
    "If we reach this point, then the receiver has not been curtailed,
    so ensureBlock still needs to be evaluated"
    ensureBlock value.
    ^ result

Both ensure: and ifCurtailed: are very useful for making sure that important “cleanup” code is executed, but are not by themselves sufficient for handling all exceptional situations. Now let’s look at a more general mechanism for handling exceptions.