When do I use brackets (parentheses) in Capytalk / Smalltalk?

Question

I've been almost too embarassed to ask this question and it's really a general Smalltalk question rather than a Kyma one, but it's been bugging me for ages... and I think I finally understand it.

Whenever a Capytalk expression didn't do what I was expected I just kept adding brackets until it worked. This gave me a disastified feeling of not really understanding what I was doing. Although it seems this is a recognised approach, quoting Kyma X Revealed:

When in doubt, use parentheses to make your intentions clear

Googling "Smalltalk brackets" didn't immediately give me the answer. What I should have been googling was "Smalltalk Precedence". There's a brief description of Smalltalk Precedence in Kyma X Revealed on page 216, which I now understand having read this I found online:

Smalltalk Precedence  (see sections 7.6.4 and 7.6.5)

which is a slightly longer explanation and finally I think I understand it.

I've kind of answered my own question here but I thought this might be useful if anyone else out there was struggling with this the way I was.

Answer 1

There are 3 different kinds of messages in Smalltalk (and Capytalk):

• a message with a receiver only and no arguments (unary): !x abs
• a message (composed of one or two non-alphabetic characters) with a receiver and a single argument (binary): 1 - !x
• a message with a receiver and one or more arguments (keyword): !KeyDown bpm: !BPM dutyCycle: 0.1

The order of evaluation (without parentheses) for messages is first unary, then binary, and finally keyword.

This means that !KeyDown bpm: !BPM dutyCycle: 1 - !x abs means:

• evaluate !x abs
• evaluate 1 - (!x abs)
• evaluate !KeyDown bpm: !BPM dutyCycle: (1 - (!x abs))

When using more than one keyword message in an expression you will always need to use parentheses:

(!KeyDown bpm: !BPM) triggerEvery: !Count

Without the parentheses, Smalltalk will be trying to send the message bpm:triggerEvery:, which is not defined.

If you are sending more than one unary (binary) message in a row, the order of evaluation is left to right. For example, !x abs negated is evaluated as (!x abs) negated and !KeyPitch + 24 / 48 is evaluated as (!KeyPitch + 24) / 48. This simple rule for binary expressions is different than the complex precedence rules you may have learned for arithmetic in other computer languages.

Comments

Very clear explanation, thanks SSC.

I find that parentheses in Smalltalk and Capytalk is easy to visualise as the demarcation boundaries of an object, which is what every message is in Smalltalk in fact.