CODEWITHSUNDEEP
scala
Mikiku
Mikiku

Reputation: 152

Scala compose function

I have a function compose, that I know is correct

def compose[A,B,C](f: B => C, g: A => B): A => C = {
    a: A => f(g(a))
}

But when I try to use it, I get an error

def main(args: Array[String]): Unit = {
    println("Function composition:")
    compose(x: Int => x+1, y: Int => y-1)(10)
}

error: identifier expected but integer literal found.

Can someone please point out what am I doing wrong?

Upvotes: 2

Views: 1619

Answers (2)

Ben Reich
Ben Reich

Reputation: 16324

You have a lot of options here! You can put parentheses around the parameter definitions:

compose((x: Int) => x+1, (y: Int) => y-1)(10)

Or put braces around the whole function:

compose({ x: Int => x+1 }, { y: Int => y-1 })(10)

Or specify the generic parameters explicitly. If you do it this way, you can even use the underscore syntax:

compose[Int, Int, Int](x => x+1, y => y-1)(10)
compose[Int, Int, Int](_+1, _-1)(10)

Or use type ascription, which also enables the underscore syntax:

compose({ x => x+1 }:(Int=>Int), { y => y-1 }:(Int=>Int))
compose({ _+1 }:(Int=>Int), { _-1 }:(Int=>Int))

Or combine some of these ideas into this, which is the shortest version I could find:

compose((_:Int)+1,(_:Int)-1)(10)

It's also worth mentioning that there already is a compose method on Function1:

({ x: Int => x+1 } compose { y: Int =>  y-1 })(10)

Upvotes: 5

Marth
Marth

Reputation: 24812

You need to add parenthesis around x: Int/y:Int :

scala> compose((x: Int) => x+1, (y: Int) => y-1)(10)
res34: Int = 10

scala> def main(args: Array[String]): Unit = {
     |     compose((x: Int) => x+1, (y: Int) => y-1)(10)
     | }
main: (args: Array[String])Unit

Upvotes: 5

Related Questions