12.3: Handling Non-Local Returns
- Page ID
- 43731
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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.