Imports

By default, only declarations in the same package as the current file or in the simpleml.lang package are accessible. All other declarations must be imported first.

Simple-ML has two kinds of imports, namely a qualified import, which imports a single declaration, and a wildcard import, which imports everything in a package.

Qualified Imports

A qualified import makes a single declaration available. Here is an example that imports the class DecisionTree in the package simpleml.model.regression:

import simpleml.model.regression.DecisionTree

The syntax consists of the following parts:

• The keyword import
• The qualified name of the declaration to import (here simpleml.model.regression.DecisionTree). The qualified name can be obtained by concatenating the name of the package that contains the declaration (i.e. simpleml.model.regression) with the name of the declaration (i.e. DecisionTree). The two parts are separated by a dot.

Once the declaration is imported, we can refer to it by its simple name. This is the last segment of the qualified name, which is the name of the declaration. Here is, for example, a call to the constructor of the DecisionTree class:

DecisionTree()

Qualified Imports with Alias

Sometimes the name of the imported declaration can conflict with other declarations that are imported or that are contained in the importing file. To counter this, declarations can be imported under an alias:

import simpleml.model.regression.DecisionTree as StdlibDecisionTree

Let us take apart the syntax:

• The keyword import.
• The qualified name of the declaration to import (here simpleml.model.regression.DecisionTree). The qualified name can be obtained by concatenating the name of the package that contains the declaration (i.e. simpleml.model.regression) with the name of the declaration (i.e. DecisionTree). The two parts are separated by a dot.
• The keyword as.
• The alias to use (here StdlibDecisionTree). This can be any combination of upper- and lowercase letters, underscores, and numbers, as long as it does not start with a number.

Afterwards, the declaration can only be accessed using the alias. The simple name cannot be used anymore. The next example shows how to create a new instance of the class now by invoking its constructor:

StdlibDecisionTree()

Wildcard Imports

We can also import all declarations in a package with a single import. While this saves some typing, it also increases the likelihood of name conflicts and can make it harder for readers of the code to determine where a declaration comes from. Therefore, this should be used with caution.

Nevertheless, let us look at an example, which imports all declarations from the package simpleml.model.regression:

import simpleml.model.regression.*

Here is the breakdown of the syntax:

• The keyword import.
• The name of the package to import (here simpleml.model.regression).
• A dot.
• A star.

Afterwards, we can again access declarations by their simple name, such as the class DecisionTree:

DecisionTree()

Aliases cannot be used together with wildcard imports.

Note that declarations in subpackages, i. e. packages that have a different name but the same prefix as the imported one, are not imported. Therefore, if we would only import simpleml.model, we could no longer access the class DecisionTree.