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 another.

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 problem.

-Ralph Johnson

On Sun, Sep 10, 2017 at 6:33 AM, Lorenz Köhl <rainbowtwigs@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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