% add the PATH to retrieve cls and sty beamer styles files \makeatletter \providecommand*{\input@path}{} \edef\input@path{{../../support/beamer/}\input@path}% prepend \makeatother % \documentclass{pharoslides} % \documentclass[aspectratio=169]{pharoslides} % format 16/9 \documentclass[withvideo]{pharoslides} % format 16/9 + incrustation video \graphicspath{{./figures/}{../}{../../support/beamer/imgs/}} %Information to be included in the title page: \title{Avoid Null Checks} \subtitle{} \author{Damien Cassou, Stéphane Ducasse and Luc Fabresse} \institute{http://stephane.ducasse.free.fr} \date{} \slidesid{W7S07} \begin{document} \frame[plain]{\titlepage} \begin{frame}[fragile] \frametitle{Anti If Campaign} \begin{listing}{} Main >> showHappiness: animal animal isDog ifTrue: [ animal shakeTail ]. animal isDuck ifTrue: [ animal quack ]. animal isCat: [ ... ]. \end{listing} Branching (with \code{if}) based on the type of an object is bad: \begin{itemize} \item adding a new type requires modifying all such code \item methods will become very long and full of details \end{itemize} Send messages instead \begin{center} \includegraphics[width=0.6\textwidth]{figures/antifIfCampaign.png}\end{center} \end{frame} \begin{frame}[fragile] \frametitle{Anti If Campaign} \begin{listing}{} Dog >> showHappiness self shakeTail Duck >> showHappiness self quack Cat >> showHappiness ... \end{listing} Branching (with \code{if}) based on the type of an object is bad \begin{itemize} \item adding a new type requires modifying all such code \item methods will become very long and full of details \end{itemize} Send messages instead \begin{center} \includegraphics[width=0.6\textwidth]{figures/antifIfCampaign.png}\end{center} \end{frame} \begin{frame}[fragile] \frametitle{Do Not Return Nil} \begin{listing}{} Inferencer >> rulesForFact: aFact self noRule ifTrue: [ ^ nil ] ^ self rulesAppliedTo: aFact \end{listing} \code{ifTrue: {[} \string^ nil {]}} forces every client to check for \code{nil}: \begin{listing}{} (inferencer rulesForFact: 'a') ifNotNil: [ :rules | rules do: [ :each | ... ] \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{Return Polymorphic Objects} When possible, replace \code{if} by polymorphic objects: \begin{itemize} \item when returning a collection, return an empty one \item when returning a number, return 0 \end{itemize} \begin{listing}{} Inferencer >> rulesForFact: aFact self noRule ifTrue: [ ^ #() ] ^ self rulesAppliedTo: aFact \end{listing} Your clients can just iterate and manipulate the returned value \begin{listing}{} (inferencer rulesForFact: 'a') do: [:each | ... ] \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{For Exceptional Cases, Use Exceptions} For exceptional cases, replace \code{nil} by exceptions: \begin{itemize} \item avoid error codes because they require \code{if} in clients \item exceptions may be handled by the client, or the client's client, or ... \end{itemize} \begin{listing}{} FileStream >> nextPutAll: aByteArray canWrite ifFalse: [ self cantWriteError ]. ... FileStream >> cantWriteError (CantWriteError file: file) signal \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{Initialize Your Object State} Avoid \code{nil} checks by initializing your variables \begin{itemize} \item by default instance variables are initialized with \code{nil} \end{itemize} \begin{listing}{} Archive >> initialize super initialize. members := OrderedCollection new \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{Use Lazy Initialization if Necessary} You can defer initialization of a variable to its first use: \begin{listing}{} FreeTypeFont >> descent ^ cachedDescent ifNil: [ cachedDescent := (self face descender * self pixelSize // self face unitsPerEm) negated ] \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{Sometimes you have to check...} Sometimes you have to check before doing an action \begin{itemize} \item if you can, turn the default case into an object \end{itemize} \begin{listing}{} ToolPalette >> nextAction self selectedTool ifNotNil: [ :tool | tool attachHandles ] ToolPalette >> previousAction self selectedTool ifNotNil: [ :tool | tool detachHandles ] \end{listing} \end{frame} \begin{frame}[fragile] \frametitle{Use NullObject} \begin{listing}{} NoTool >> attachHandles ^ self NoTool >> detachHandles ^ self \end{listing} \begin{listing}{} ToolPalette >> initialize self selectedTool: NoTool new ToolPalette >> nextAction self selectedTool attachHandles ToolPalette >> previousAction self selectedTool detachHandles \end{listing} \begin{itemize} \item a null object proposes a polymorphic API and embeds default actions/values \item Woolf, Bobby (1998). \symbol{34}Null Object\symbol{34}. In Pattern Languages of Program Design 3. Addison-Wesley. \end{itemize} \end{frame} \begin{frame}[fragile] \frametitle{Conclusion} \begin{itemize} \item A message acts as a better \code{if} \item Avoid null checks, return polymorphic objects instead \item Initialize your variables \item If you can, create objects representing default behavior \end{itemize} \begin{center} \includegraphics[width=0.6\textwidth]{figures/antifIfCampaign.png}\end{center} \end{frame} % \setlength{\textwidth}{\originaltextwidth} \setbeamertemplate{background canvas}{\insertBorderForHandout} \begin{frame}[plain] \begin{minipage}{\originaltextwidth} \centering \large \vspace{2em} A course by \vspace{1em} \hspace{-20px}\includegraphics[height=32px]{inriaLogo}\hspace{3em}\raisebox{7px}{and}\hspace{3em}\raisebox{-6px}{\includegraphics[height=30px]{logoutop}} \vspace{2em} in collaboration with \vspace{1em} \includegraphics[height=40px]{unisciellogo}\hskip 1.5em% \raisebox{5px}{\includegraphics[height=30px]{unitlogo}}\hskip 2em% \raisebox{8px}{\includegraphics[height=24px]{univLilleLogo}}\hskip 2em% \raisebox{3px}{\includegraphics[height=29px]{logo-IMT}}\hskip 2em% \raisebox{1px}{\includegraphics[height=32px]{minesDouaiLogo}} \vspace{2.5em} \includegraphics[width=50px]{by-nc-nd} \tiny \hskip 0.5em \raisebox{5px}{Inria 2016} \medskip \tiny Except where otherwise noted, this work is licensed under CC BY-NC-ND 3.0 France \url{https://creativecommons.org/licenses/by-nc-nd/3.0/fr/} \end{minipage} \end{frame} \end{document} %%% Local Variables: %%% mode: latex %%% TeX-master: t %%% End: