mmhelloworld

main = println "Hello World!"

Idris JVM: Guarding against Java null using Maybe type

Idris JVM now helps avoiding nulls getting into Idris from FFI calls using Maybe type. FFI declarations can have Maybe type in any argument position or in the return type.

Handling null from FFI call

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getProperty : String -> JVM_IO (Maybe String)
getProperty = invokeStatic SystemClass "getProperty" (String -> JVM_IO (Maybe String))

The above function is an FFI call to Java’s method static String getProperty(String key). The method returns a system property value if the property is set otherwise returns null. With Maybe type in the Idris function’s return type, the Idris function returns Nothing if the returned value is null otherwise the value is wrapped in Just.

Example
returningnull.idr
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module Main

import IdrisJvm.IO
import Java.Lang

main : JVM_IO ()
main = printLn !(getProperty "foo")
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$ idrisjvm returningnull.idr -o target

$ java -cp target:/path/to/idris-jvm-runtime-1.0-SNAPSHOT.jar main.Main
Nothing

$ java -cp target:/path/to/idris-jvm-runtime-1.0-SNAPSHOT.jar -Dfoo=hello main.Main
Just "hello"

Passing Maybe values for nullable values in FFI calls

passingnull.idr
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module Main

import IdrisJvm.IO
import Java.Lang

namespace Component
  Component : Type
  Component = JVM_Native $ Class "java/awt/Component"

namespace JOptionPane
  JOptionPaneClass : JVM_NativeTy
  JOptionPaneClass = Class "javax/swing/JOptionPane"

  showMessageDialog : Inherits Object messageTy => Maybe Component -> messageTy -> JVM_IO ()
  showMessageDialog parent message =
    invokeStatic JOptionPaneClass "showMessageDialog" (Maybe Component -> Object -> JVM_IO ()) parent (believe_me message)

main : JVM_IO ()
main = showMessageDialog Nothing "Hello Idris!"

In the above code, the Java method JOptionPane.showMessageDialog(parentComponent, message) takes a nullable parent component and a message. If the parent component is null then the message is displayed in a default frame.

passingnull.idr
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$ idrisjvm passingnull.idr -o target

$ java -cp target:/path/to/idris-jvm-runtime-1.0-SNAPSHOT.jar main.Main

Idris code passes Nothing in the above code so null is passed for the Java method that displays the message in a default frame as shown below.

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