jstnchng - 1 year ago 119
Scala Question

# Partial Functions in Scala

I just wanted to clarify something about partially defined functions in Scala. I looked at the docs and it said the type of a partial function is

`PartialFunction[A,B]`
, and I can define a partial function such as

``````val f: PartialFunction[Any, Int] = {...}
``````

I was wondering, for the types
`A`
and
`B`
, is
`A`
a parameter, and
`B`
a return type? If I have multiple accepted types, do I use
`orElse`
to chain partial functions together?

In the set theoretic view of a function, if a function can map every value in the domain to a value in the range, we say that this function is a `total function`. There can be situations where a function cannot map some element(s) in the domain to the range; such functions are called `partial functions`.

Taking the example from the Scala docs for partial functions:

``````val isEven: PartialFunction[Int, String] = {
case x if x % 2 == 0 => x+" is even"
}
``````

Here a partial function is defined since it is defined to only map an even integer to a string. So the input to the partial function is an integer and the output is a string.

``````val isOdd: PartialFunction[Int, String] = {
case x if x % 2 == 1 => x+" is odd"
}
``````

`isOdd` is another partial function similarly defined as `isEven` but for odd numbers. Again, the input to the partial function is an integer and the output is a string.

If you had the a list of numbers such as:

``````List(1,2,3,4,5)
``````

and applied the `isEven` partial function on this list you would get as output

``````List(2 is even, 4 is even)
``````

Notice that not all the elements in the original list have been mapped by the partial function. However, there may be situations where you want to apply another function in those cases where a partial function cannot map an element from the domain to the range. In this case we use `orElse`:

``````val numbers = sample map (isEven orElse isOdd)
``````

And now you will get as output:

``````List(1 is odd, 2 is even, 3 is odd, 4 is even, 5 is odd)
``````
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