The single export pattern
by Edward Z. Yang
Single export refers to a design pattern where a module identifier is overloaded to also represent a function or type inside the module. As far as I can tell, the term “single export” is not particularly widely used outside the ECMAScript TC39 committee; however, the idea shows up in other contexts, so I’m hoping to popularize this particular name (since names are powerful).
var sayHello = require('./sayhello.js'); sayHello.run();
The methods of sayHello are the functions exported by the module. But what about sayHello itself? Because functions are objects too, we could imagine that sayHello was a function as well, and thus:
would be a valid fragment of code, perhaps equivalent to sayHello.run(). Only one symbol can be exported this way, but in many modules, there is an obvious choice (think of jQuery’s $ object, etc).
This pattern is also commonly employed in Haskell, by taking advantage of the fact that types and modules live in different namespaces:
import qualified Data.Map as Map import Data.Map (Map)
Map is now overloaded to be both a type and a module.
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