Scala Functional Programming Generics

Question

I'm trying to implement a generic compose combinator

Recall that the composition of two functions—f and g-- is h(x) = f(g(x))

    def inc(x: Double) = x + 1
    def double(x: Double) = 2 * x

    def compose[T](f: T, g: T, x: Int) = f(g(x))

    println(compose(double, inc, 2))

I'm getting errors no matter what I try to do, I think I have a misunderstanding on generics or passing in functions...

What I want the compose function to do is take in two functions and execute f(g(x)) which would do g(x) first, then f(result of g(x)). In my example the result should be: 2(2+1) = 6

Question: How can I refactor my code to do this?

You need to specify that f and g are function types. T could be anything — puhlen
– puhlen, Commented Sep 29, 2017 at 19:40
def inc[T <: Number](x: Int) = x + 1 def dbl[T <: Number](x: Int) = 2 * x — Pavel
– Pavel, Commented Sep 29, 2017 at 19:45

fcat · Accepted Answer · 2017-09-30 03:25:36Z

3

if you want to compose the functions, you have to make sure that the types match.

def compose[A,B,C](f: B => C, g: A => B, x: A): C = f(g(x))
println(compose(double, inc, 2.toDouble))

Then you can compose them. These combinators are already provided in Scala so you could do:

val inc = (v: Double) => v +1
val double = (v: Double) => v * 2
inc.andThen(double)

edited Sep 30, 2017 at 3:25

fcat

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answered Sep 29, 2017 at 19:48

Mikel San Vicente

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4 Comments

Phil Over a year ago

Your solution works! But I think I do have a misunderstanding on generics. Why do we have to set C = f(g(x))? And the parameters B => C , A => B.. Is there a more simple way to refactor this or is it pretty simplified as is?

Phil Over a year ago

Oh that's not C = f(g(x)), you were defining the type of the function, right?

Mikel San Vicente Over a year ago

if you want to make the function completely generic to any input and output you need to make it generic for the input A, the intermediary state B and the result C, the type parameters in the function are making sure that the output of g is of the same type as the input of f, otherwise it would fail

Denis Makarenko Over a year ago

> Is there a more simple way to refactor this or is it pretty simplified as is?I don't think so. B => C is just a syntactic sugar for f: Function1[B, C] so you can write it as "def compose[A,B,C](f: Function1[B, C], g: Function1[A, B], x: A): C = return f(g(x))" which is more verbose.

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