Programming Flix

Invoking Object Methods

We can use the import mechanism to invoke methods on objects.

For example:

import new java.io.File(String): ##java.io.File \ IO as newFile;
import java.io.File.exists(): Bool \ IO as fileExists;
let f = newFile("HelloWorld.txt");
fileExists(f)

Here we import the java.io.File.exists method under the name fileExists.

If the Java method name is a legal Flix name and we want to reuse it, we can also import the method without an as clause. For example:

import new java.io.File(String): ##java.io.File \ IO as newFile;
import java.io.File.exists(): Bool \ IO;
let f = newFile("HelloWorld.txt");
exists(f)

Here we import the method under the name exists.

When a Java method is imported, we must annotate it with its effect. Most commonly, a Java method has a side-effect (such as deleting a file), and hence must be annotated with the IO effect.

In rare cases where a method is pure, we can import it as such by writing the empty effect set: {}. For example:

import java.lang.String.startsWith(String): Bool \ {};
startsWith("Hello World", "Hello")

And as another example:

import java.lang.String.charAt(Int32): Char \ {};
charAt("Hello World", 2)

Type signatures should use Flix type names and not Java type names for primitive types. For example, if a Java method takes a Double its signature should use the Flix type Float64. Similarly, if a Java method takes a Boolean its signature should use the Flix type Bool. This goes for return types, too.

Invoking Static Methods

We can invoke a static method by writing the static keyword after import:

import static java.lang.String.valueOf(Bool): String \ {};
valueOf(true)