Dear Lord - yesterday I partially applied a function, why like this?
SCALA AND FUNCTIONAL-PROGRAMMING
23 May 2014 
Tomer-Ben-David
Tomer-Ben-David
So you want to partially apply a function but don’t know why? This post is for you. Here we will show you why you did need to partially apply a function (why it helped you). And how nice and easy this process can be.
Although in scala the process for partially applying a function is just as complex and as encrypted as far as they could think of we will try to make it easy for you. (they really thought hard how to make it encrypted and un-remembable).
So let’s dig in.
But first I would like to ask you to repeat the mantra:
partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function, partial-applied-function.
Now that you are familiar with it let’s repeat the scala partially applied function mantra.
oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore, oh-lord-its-with-space-and-underscore_.
Let’s continue to dig in.
Let’s say you want to load some configuration from disk and lets say this configuration is in the form of properties. You wish to have a primary file and a secondary, like external configuration file which will override internal. (For real life if you use java I would recommend you to use already existing mechanisms such as spring who would do that for you or scala typesafe config manager which would also do that for you). But for the sake of the example lets use plan properties.
Lets first load the configurations into our properties.
val internalProps = new Properties()
val externalProps = new Properties();
extenralProps.load(new FileInputStream(new File("external.properties")))
defaultProps.load(new FileInputStream(new File("default.properties")))
Now lets load a properties with default - no partially applied func here.
val someValFromConf = externalProperties.get("key", defaultProperties.get("key"))
Now let’s partially apply props to already contain the internal defaults
def getOrDefault(externalProps: Properties, defaultProps: Properties)(key: String) = props.externalProps.getProperty(key, props.defaultProps.getProperty(key))
val get = getOrDefault(externalProps, defaultProps) _ // nasty underscore --> partially applying alarm.
Well here we first defined a method named getOrDefault which is aware of both external and default properties, so far so good, I mean, we are talking here about File, FileInputStream, Properties, def, all is clear and nice and dandy isn’t it???
AND HERE COMES THE TWIST IN THE STORY!! THERE IS AN UNDERSCORE!
The story of a method called only with some of its arguments
getOrDefault accepts three arguments, externalProps and defaultProps, and akey, what would happen if we called it with only a single argument? it would not compile right? but we added THE UNDERSCORE.
So as we added the underscore scala understands it should PARTIALLY APPLY meaning it calls them method with a single argument and as we have an _ it does not run it. Also note we are assigning the return value to aval so we have now got a new method called get and as we already passed to its origin method the first argument, we can call get only with the key.
so we can call key method any number of times we want, isn’t that nice, its already aware of the first two arguments. That’s the functional way of doing so. If we wanted to do so in java we would need to create an object initialize it with some members and call it, here all got shorter, and immutable.
So now we can call get like this:
val value = get("key1") // low and behold, the function is already aware of the external,internal props.
val value = get("key2") // low and behold, the function is already aware of the external,internal props.
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