CODEWITHSUNDEEP
scalafunction
CyberPlayerOne
CyberPlayerOne

Reputation: 3180

How to call this Scala function when "error: Unit does not take parameters"

When I define this function, I can call it without a problem:

scala> val c = (_: String) + "sd"
c: String => String = <function1>

scala> c("1")
res13: String = 1sd

However if I let the above function print the result without returning it, I have the following error:

scala> val b = print((_: String) + "sd")
<function1>b: Unit = ()

scala> b("1")
<console>:26: error: Unit does not take parameters
       b("1")
        ^

I know the system method print returns Unit type, but shouldn't the function b be a function that can be called with an argument as I defined? Why the above b function can not be called with b("1")?

How to understand and solve the problem? Thank you.

Upvotes: 1

Views: 361

Answers (2)

Mario Galic
Mario Galic

Reputation: 48430

Indeed, as you have already discovered

print((_: String) + "sd")

expands to

print((x: String) => x + "sd")

instead of the desired

(x: String) => print(x + "sd")

because _ binds to the innermost expression delimiter () or {}.

Upvotes: 2

jwvh
jwvh

Reputation: 51271

Consider the following sequence.

An anonymous function from String to String.

scala> (_: String) + "sd"
res0: String => String = $$Lambda$1156/0x00000008406c1040@6b8bdcc6

Print the String representation of the anonymous function.

scala> println((_: String) + "sd")
$line10.$read$$iw$$iw$$$Lambda$1158/0x00000008406c3040@31f5b923

Print the String representation of the anonymous function. Save the result in variable b.

scala> val b = println((_: String) + "sd")
$line11.$read$$iw$$iw$$$Lambda$1159/0x00000008406a7040@173cfb01
b: Unit = ()

The print() and println() passed parameter is call-by-reference, not call-by-name, so it is fully evaluated at the call site before being passed to print(). In this case print() receives a function, it doesn't become part of a larger function.

Upvotes: 1

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