Dynamic Web Development with Seaside

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11.9.1Transforming a Call to a Show

Why is show: useful at all?

  • First of all show: allows one to replace multiple components in one request. This is not possible with call: as it blocks the flow of execution and the developer has no possibility to display another component at the same time.
  • Another reason to use show: is that it is more lightweight and that it uses fewer resources than call:. This means that if the blocking behavior is not needed, then show: is more memory friendly.
  • Finally some Smalltalk dialects cannot implement call: due to limitations in their VM.

If you want or must get rid of the call: statements in a sequence of calls things are relatively simple. Transform code using call:

| a b c |
a := self call: A.
b := self call: B.
c := self call: C.

to the following using show:onAnswer:

self show: A onAnswer: [ :a |
self show: B onAnswer: [ :b |
self show: C onAnswer: [ :c |
... ] ] ]

If you have a loop like the following one, things are slightly more complicated:

[ self call: A ]
whileTrue: [ self call: B ]

The example below shows an equivalent piece of code that uses recursion to implement the loop:

self show: A onAnswer: [ :a |
a ifTrue: [
self show: B onAnswer: [ :b |
self go ] ] ]

The transformation technique applied here is called continuation-passing style or short CPS. The onAnswer: block implements the continuation of the flow after the shown component answered. Unfortunately for more complicated flows CPS lead to messy code pretty quickly.

Copyright © 26 February 2017 Stéphane Ducasse, Lukas Renggli, C. David Shaffer, Rick Zaccone
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