Seemingly spurious "does not take arguments" error with case class constructor

Question

I have a case class that has a few parameters for its constructor, and I define an accompanying class object that defines an alternative constructor that takes a different set of arguments, like so: ``` case class MyClass (c: Char, mc: List[MyClass]) object MyClass { def apply(c: Char, mc: MyClass): MyClass = { MyClass(c, List(mc)) } } ``` I want to use the original case class constructor in a `foldRight`: ``` object SomeStuff { def problem (s: String) { assert(!s.isEmpty) val zero = MyClass('a', Nil) val mc2 = "Pillsy Pillsy Pillsy" foldRight(zero) { MyClass(_, List(_)) } } } ``` When I do this, I get an error message from the compiler: "MyClass does not take parameters." If I comment out the section `val mc2 = ...` this problem goes away, but `MyClass` is clearly taking parameters in the definition of `zero`. I feel like I must be missing something really basic, but I have no idea what it is. I've tried a couple workarounds, like defining a helper method or a function value to use a…

Accepted Answer

First, `foldRight` takes two arguments, so you can't use operator notation (in its current form), but you can:

```
("foo" foldRight zero) { ... }
```

or alternatively,

```
("foo" :\ zero) { ... }
```

Second, `{ MyClass(_, List(_)) }` will desugar into

```
{ x => MyClass(x, y => List(y)) }
```

so, you should name arguments and use `new` keyword to ensure that constructor is called instead of `apply` method:

```
"Pillsy Pillsy Pillsy".foldRight(zero) { (c, mc) =>
  new MyClass(c, List(mc))
}
```

or use supplemental `apply` method if you want to go with underscores:

```
"Pillsy Pillsy Pillsy".foldRight(zero) { MyClass(_, _) }
```

Seemingly spurious "does not take arguments" error with case class constructor

Solution

Problem