5.2: Three Types of Messages
- Page ID
- 39592
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Pharo distinguishes between three kinds of messages to reduce mandatory parentheses. A few simple rules based on the such different message determine the order in which the messages are sent.
There are three different types of messages:
- Unary messages are messages that are sent to an object without any other information. For example, in
3 factorial,factorialis a unary message. - Binary messages are messages consisting of operators (often arithmetic). They are binary because they always involve only two objects: the receiver and the argument object. For example in
10 + 20, + is a binary message sent to the receiver 10 with argument 20. - Keyword messages are messages consisting of one or more keywords, each ending with a colon (:) and taking an argument. For example in
anArray at: 1 put: 10, the keyword at: takes the argument 1 and the keywordput:takes the argument 10.
Unary messages
Unary messages are messages that do not require any argument. They follow the syntactic template: receiver messageName. The selector is simply made up of a succession of characters not containing : (e.g., factorial, open, class).
89 sin >>> 0.860069405812453
3 sqrt >>> 1.732050807568877
Float pi >>> 3.141592653589793
'blop' size >>> 4
true not >>> false
Object class >>> Object class "The class of Object is Object class (BANG)"
Important
Unary messages are messages that do not require any argument. They follow the syntactic template: receiver selector.
Binary messages
Binary messages are messages that require exactly one argument and whose selector consists of a sequence of one or more characters from the set: +, -, *, /, &, =, >, |, <, ~, and @. Note that -- (double minus) is not allowed for parsing reasons.
100@100 >>> 100@100 "creates a Point object"
3+4 >>> 7
10 - 1 >>> 9
4 <= 3 >>> false
(4/3) * 3 == 4 >>> true "equality is just a binary message, and Fractions are exact"
(3/4) == (3/4) >>> false "two equal Fractions are not the same object"
Important
Binary messages are messages that require exactly one argument and whose selector is composed of a sequence of characters from: +, -, *, /, &, =, >, |, <, ~, and @. -- is not possible. They follow the syntactic template: receiver selector argument.
Keyword messages
Keyword messages are messages that require one or more arguments and whose selector consists of one or more keywords each ending in :. Keyword messages follow the syntactic template: receiver selectorWordOne: argumentOne wordTwo: argumentTwo.
Each keyword takes an argument. Hence r:g:b: is a method with three arguments, playFileNamed: and at: are methods with one argument, and at:put: is a method with two arguments. To create an instance of the class Color one can use the method r:g:b: (as in Color r: 1 g: 0 b: 0), which creates the color red. Note that the colons are part of the selector.
Note
In Java or C++, the method invocation Color r: 1 g: 0 b: 0 would be written Color.rgb(1, 0, 0).
1 to: 10 >>> (1 to: 10) "creates an interval"
Color r: 1 g: 0 b: 0 >>> Color red "creates a new color"
12 between: 8 and: 15 >>> true
nums := Array newFrom: (1 to: 5). nums at: 1 put: 6. nums >>> #(6 2 3 4 5)
Important
Keyword messages are messages that require one or more arguments. Their selector consists of one or more keywords each ending in a colon (:). They follow the syntactic template: receiver selectorWordOne: argumentOne wordTwo: argumentTwo.