[Newbies] Polymorphic or: with block argument

Ralph Johnson johnson at cs.uiuc.edu
Sun Sep 10 13:50:02 UTC 2017

Yes, when the compiler sees   exp or: [ ... ] then it assumes that "exp" is
a boolean-valued expression and generates code that fails if it isn't.
 Not all Smalltalk compilers perform this optimization, but it is very
common.  There are a bunch of methods treated like this, ifTrue:ifFalse: is

whileTrue: is an interesting slightly different case.   The compiler
usually optimizes whileTrue: only when both receiver and the argument are
blocks.    So, you can define whileTrue: in other classes without any

-Ralph Johnson

On Sun, Sep 10, 2017 at 6:33 AM, Lorenz Köhl <rainbowtwigs at gmail.com> wrote:

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> I don't understand why Smalltalk doesn't allow me to have an or:
> method that takes a block argument (except on Boolean). For example:
> Set or: 1. ==> MNU as expected.
> Set or: [1]. ==> NonBooleanReceiver exception.
> Set or: [:x | x] ==> Argument has too many arguments
> Is it because the compiler specializes this to boolean when it sees
> the or: [ ... ] syntax? Is this a tradeoff made for performance?
> greetings,
>   Lorenz
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