Cannonball is a Web application that demonstrates using Jellyfish for dependency injection. It is named for the delicious Cannonball Jellyfish.
Cannonball implements a very simple single-user micro blog. Entries can be added in an HTTP POST, and retrieved with a HTTP GET.
To run, Cannonball needs a couple dependencies: a way to connect to a database, and a way to generate unique entry identifiers:
trait DbConnector {
def connect[A](f: Connection => A): A
}
trait IdGenerator {
def generate: String
}These dependencies are used within Program implementations:
def getEntries(offset: Int, limit: Int) = program {
val dbConnector: DbConnector = read[DbConnector]
val entries: Seq[Entry] = dbConnector.connect(Queries.getEntries(offset, limit))
Return(entries)
}
def addEntry(content: String) = program {
val dbConnector: DbConnector = read[DbConnector]
val idGenerator: IdGenerator = read[IdGenerator]
val id: String = idGenerator.generate
val entry: Entry = dbConnector.connect(Commands.addEntry(id, content))
Return(entry)
}A basic type-based pattern match injects the dependencies and continues the program:
def run[A](p: Program): Any = p match {
case Return(a) => a
case With(c, f) if c.isA[DbConnector] => run(f(dbConnector))
case With(c, f) if c.isA[IdGenerator] => run(f(idGenerator))
}First, fire up sbt:
$ sbt
Then launch Cannonball with the container:start sbt command:
> container:start
Once Cannonball is running, throw some requests at it:
$ curl localhost:8080 -d 'content=Hello world!'
<html>
<head>
<title>Cannonball</title>
</head>
<body>
<h1>Entry e25deba7-0f51-4b68-a81a-3178636c5911</h1>
<h2>Tue Jun 04 16:03:35 PDT 2013</h2>
<p>Hello, world!</p>
</body>
</html>
$ curl localhost:8080 -d 'content=Dependency injection!'
<html>
<head>
<title>Cannonball</title>
</head>
<body>
<h1>Entry 32b27b04-8ce4-4b58-a8eb-3f8373b483ed</h1>
<h2>Tue Jun 04 16:04:23 PDT 2013</h2>
<p>Dependency injection!</p>
</body>
</html>
$ curl localhost:8080
<html>
<head>
<title>Cannonball</title>
</head>
<body>
<h1>Entries</h1>
<div>Hello, world! @ 2013-06-04 16:03:35.861</div>
<div>Dependency injection! @ 2013-06-04 16:04:23.781</div>
</body>
</html>