A namespace is a type of value that groups together several variables (fields) from the same scope. A block or file returns a namespace if it contains any export arrows ⇐ at the top level, and fields from namespaces can be accessed with either dot syntax or destructuring assignment. A namespace can be mutable only if any of its source code uses ↩ to change the value of a field.
The following quick example shows a few ways to use a namespace returned by •Import:
ns ← •Import "file.bqn" ⟨something, abbr⇐abbreviation⟩ ← ns # Destructure ns.DoThing 6 # Dot syntax
An here's how the contents of file.bqn might look in order to define the variables used above:
⟨something, DoThing⟩⇐ # Declare exports abbreviation ⇐ "sth" # Define and export _something ← {𝕗} # Separate definition DoThing ← "TODO"⊸!
The features of namespaces that make them useful in BQN programming are encapsulation and mutability. But these are exactly the same features that closures provide! In fact a namespace is not much more than a closure with a name lookup system. Consequently namespaces don't expand the basic functionality of the language, but just make it easier to use.
Namespaces improve encapsulation by allowing many values to be exported at once. With only one way to call them, functions and modifiers aren't such a good way to define a large part of a program. With a namespace you can define lots of things and expose exactly the ones you want to the rest of the world. For example, it's typical for files to define namespaces. A reader can see the exported values just by searching for ⇐, and if you're nice, you might declare them all at the beginning of the file. Careful use of exports can guarantee that potentially dangerous functions are used correctly: if it's only valid to call function B after function A has been called, export AB⇐{A𝕩⋄B𝕩} and don't export B.
Mutability means that the behavior of one namespace can change over the course of the program. Mutability is often a liability, so make sure you really need it before leaning too heavily on this property. While there's no way to tell from the outside that a particular namespace is mutable, you can tell it isn't if the source code doesn't contain ↩, as this is the only way it could modify the variables it contains.
A namespace that makes use of mutability is essentially an object: a collection of state along with operations that act on it. Object-oriented programming is the other major use of namespaces. Contrary to the name, there's never a need to orient your programming around objects, and it's perfectly fine to use an object here or there when you need to, for instance to build a mutable queue of values.
The double arrow ⇐ is used to export variables from a block or file, making the result a namespace instead of the result of the last line. There are two ways to export variables. First, ← in the variable definition can be replaced with ⇐ to export the variable as it's defined. Second, an export statement consisting of an assignment target followed by ⇐, with nothing to the right, exports the variables in the target and does nothing else. These export statements can be placed anywhere in the relevant program or body, including before declaration or on the last line, and a given variable can be exported any number of times. The block in the example below has two statements that export variables, exporting a, b, and c.
example ← { b‿c⇐ # Non-definition exports can go anywhere a⇐2 # Define and export b←1+a c←b‿"str" }
There are also two ways to get values out of a namespace, such as example defined above. First, one field at a time can be retrieved with dot syntax: write the namespace, then a dot ., then another name.
example.b {n⇐7}.n
The part on the left can be anything with a subject role, although it will often need to be parenthesized because . has higher precedence than any operator. So it can be chained like a.b.c if a field has a value that is also a namespace (and so on). The overall role is determined by the last name: for example •math.Sin has a function role.
Second, a namespace might be used in a destructuring assignment like the one below. This assignment's target looks like a list, where each entry specifies one of the names exported by the block and what it should be assigned to. The element can be either a single name, like b, which gives both, or an aliasing expression like b2⇐b. In this case, the value b from the namespace is used, but it's given the name b2 instead of b. Imported names can be repeated—but the variable names defined can't be—and all the names can be spelled with any role (the role is ignored).
⟨alias⇐a, b, c0‿c1⇐c, b2⇐b⟩ ← example
If aliasing with ⇐ is never used, the names can be given as a strand with ‿.
c‿a ← example
The arrow ⇐ used for an alias doesn't export anything or indicate that its block is a namespace. However, a single statement can both import and export, if it's a destructuring assignment and the main assignment arrow is ⇐.
⟨two, vars⟩ ⇐ •Import "stuff.bqn"