_ must follow method

Question

Simplified, from a more complex program:

scala> type T = (String) => String
defined type alias T

scala> def f(s: String) = s + " (parsed)" 
f: (s: String)java.lang.String

scala> f _
res0: (String) => java.lang.String = <function1>

scala> def g(func: T) = func _    
<console>:6: error: _ must follow method; cannot follow (String) => String
       def g(func: T) = func _
                    ^

I don't really understand why this doesn't work. What is the difference between a method and something in the form of (Type1, Type2 ...) => Type, and what's the right way of getting the function partial from something like that?

Just as case to be studied, consider: `val f = (x:Int,y:Int) => x+y`, now try `f _` and `def m = f _`. They all working, and gives `() => (Int, Int) => Int`. — pedrofurla, Mar 14 '11 at 23:18
possible duplicate of [Difference between method and function in Scala](http://stackoverflow.com/questions/2529184/difference-between-method-and-function-in-scala) — Daniel C. Sobral, Mar 15 '11 at 13:51

score 9 · Answer 1 · edited May 23 '17 at 12:24

9

In Scala there is a difference between methods and functions. Methods always belong to an object, but functions are objects. A method m can be converted into a function using m _

See Difference between method and function in Scala

edited May 23 '17 at 12:24

Community

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answered Mar 14 '11 at 21:01

Esko Luontola

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score 6 · Accepted Answer · answered Mar 14 '11 at 21:35

scala> def g(func: String => String) = func(_)
g: (func: (String) => String)(String) => String

Parenthesis make all the difference. This is one of the tricky things about the binding of _; it can be used to lift a method to a closure, and it can be used for partial application, but the two usages are not the same!

score 4 · Answer 3 · answered Mar 14 '11 at 21:34

4

is this what you're trying to do?

scala> def g(func: T) = func
g: (func: (String) => String)(String) => String

scala> g(f)("test")
res8: String = test (parsed)

answered Mar 14 '11 at 21:34

brandon

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score 3 · Answer 4 · answered Mar 14 '11 at 20:55

3

I think you're looking for this: http://jim-mcbeath.blogspot.com/2009/05/scala-functions-vs-methods.html

answered Mar 14 '11 at 20:55

Antonin Brettsnajdr

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_ must follow method

4 Answers4