![Vaadin-based language
HelloWorld {
Component content = VerticalLayout {
Component myLabel = Label {
String Label caption = “Hello world!”;
}
Component myButton = Button {
String Button caption = “Push Me!”;
Function changeCaption = () {
caption = “Pushed!”;
} listens to click;
}
}
}
HelloWorld {
VerticalLayout {
Label “Hello world!”;
Button “Push Me!” {
when click: caption = “Pushed”;
}
}
}
TouchScript (H. Westergård, 2013)
layout vertically:
label “Hello world!”
button “Push Me!” as myButton
events for myButton:
click { caption = “Pushed”; }
“Hello world!”
[“Push Me!”]
when click on [“Push Me!”] -> [“Pushed”]
numerous options on how to define a DSL
[ ] – represent a button
no names for components
“transformation operator”
similar to REBOL syntaxview layout [text "Hello world!" button "Quit" [quit]]](https://image.slidesharecdn.com/2-what-why-how-180123200340/85/DSLs-what-why-how-6-320.jpg)
DSLs: what, why, how
- 1. DSLs: what, why, how?
- 2. What is a DSL? DSL computer programming language limited expressiveness particular domain used by humans to instruct a computer bare minimum of features entire system can’t be built in a DSL sense of fluency domain focus makes a limited language worthwhile executable by a computer language nature
- 3. Internal vs. external DSLs internal DSL external DSL a particular way to use a GPL embedded into GPL script in internal DSL valid code in its host GPL only uses a subset of GPL’s features has a feel of custom language rather than its host language Ruby on Rails often seen as a collection of DSLs a separate language has custom syntax script in external DSL parsed using text parsing techniques Unix little languages Graph g = graph("example1") .directed() .with( node("a") .link( node("b") ) );
- 4. Command-query APIs methods on an object command query commands queries returns a value does not change the state of the system may change state of the system should not return a value doesn’t have side effects can be called multiple times in any order without changing the results does have side effects should make sense in stand-alone contextnames of methods obj.getName() obj.removeAt(1) obj.toString() obj.setName(’a’)
- 5. Command-query APIs Vaadin Framework domain rich Internet applications import com.vaadin.server.VaadinRequest; import com.vaadin.ui.Label; import com.vaadin.ui.UI; @Title("My UI") public class HelloWorld extends UI { @Override protected void init(VaadinRequest request) { // Create the content root layout for the UI VerticalLayout content = new VerticalLayout(); setContent(content); // Display the greeting content.addComponent(new Label("Hello World!")); // Have a clickable button content.addComponent(new Button("Push Me!", click -> Notification.show("Pushed!"))); } } semantic model http://vaadin.com
- 6. Vaadin-based language HelloWorld { Component content = VerticalLayout { Component myLabel = Label { String Label caption = “Hello world!”; } Component myButton = Button { String Button caption = “Push Me!”; Function changeCaption = () { caption = “Pushed!”; } listens to click; } } } HelloWorld { VerticalLayout { Label “Hello world!”; Button “Push Me!” { when click: caption = “Pushed”; } } } TouchScript (H. Westergård, 2013) layout vertically: label “Hello world!” button “Push Me!” as myButton events for myButton: click { caption = “Pushed”; } “Hello world!” [“Push Me!”] when click on [“Push Me!”] -> [“Pushed”] numerous options on how to define a DSL [ ] – represent a button no names for components “transformation operator” similar to REBOL syntaxview layout [text "Hello world!" button "Quit" [quit]]
- 7. Why use a DSL? tool with limited focusDSL very specific tool for very particular conditions typical project uses a dozen of DSLs improving development productivity communication with domain experts change in execution context alternative computation model shift logic from compile time to runtime
- 8. The Why: development productivity communicate intent of a part of a system easier to understand DSLDSL vs. command-query API aestheticsnot only code is easier to read easier to find mistakes easier to modify the system limited expressiveness harder to express wrong things DSL to wrap awkward third-party library
- 9. The Why: domain experts domain experts read code write code understand what the system thinks it’s doing spot mistakes talk more effectively to programmers focus on reading code not writing DSL shouldn’t necessarily be implemented useful to describe the system communication issues involving domain experts is difficult but has a high payoff
- 10. The Why: alternative computation model mainstream programming imperative what to do and in what sequence conditionals loopscontrol flow declarative what should happen rather than how it should happen state machine decision table
- 11. Problems with DSLs (1) language “hell” cost of building languages are hard to learn far easier to learnDSL simpler than GPL people aren’t used to building DSLs cost of a DSL model help think about the model reduce cost of building itDSL too many DSLs bad DSLs good DSL wrap a bad library make it easier to deal with waste of resources to maintain cost over the cost of
- 12. Problems with DSLs (2) ghetto language limited abstraction the whole system built in a language it’s a GPL! DSL GPLmight evolve into a DSL should be focused on a narrow problem ideally no features that fall outside that DSL provides abstraction to think about subject area behavior of the domain expressed easier than using lower-level constructs the world is fit into abstraction instead of the other way around DSL something evolving unfinished