monta - 3 months ago 16

Scala Question

If there a way in Scala to define the inv function below in one line?

`// Function to invert a decision function such as even/odd/positive/...`

def inv(f: Int => Boolean):(Int => Boolean) = {

def g(a:Int):Boolean = {

!f(a)

}

g

}

// Test

def even(x:Int):Boolean = (x % 2 == 0)

val odd = inv(even)

println("odd(99) is %s".format(odd(99)))

----

odd(99) is true

Tried below with !f or !f(a) as below but got errors. Not sure what exactly is wrong. If explanation can be provided, it would be appreciated.

`def inv(f: Int => Boolean):(Int => Boolean) = !f`

----

error: value unary_! is not a member of Int => Boolean

def inv(f: a:Int => b:Boolean):(Int => Boolean) = !f(a)

----

error: ')' expected but ':' found.

def inv(f: a:Int => b:Boolean):(Int => Boolean) = !f(a)

^

Answer

You can write

```
def inv(f: Int => Boolean):(Int => Boolean) = a => !f(a)
```

What's wrong with `!f`

: `f`

isn't a `Boolean`

.

What's wrong with `def inv(f: a:Int => b:Boolean)`

: When the parser looks at this definition, it knows that `f:`

is going to be followed by a type. `a`

can be a type, but it can't be followed by `:`

in this case (`a:Int => b:Boolean`

is not a type).