How to specify a function type in the parameters of a Kotlin lambda?

So I was trying to register some functions in a map, the code can be resumed like this:

private val things = mutableMapOf<String, Any>()

fun put(name: String, value: Any) {
    things[name] = value
}

fun main() {
    put("foo") { a: String, b: (() -> String) ->
        println(a + b())
    }
}

(Code on Kotlin Playground)

But the kotlin parser complains about the type of c and I get for line 8 and 9:

Unresolved reference: a
Unexpected tokens (use ';' to separate expressions on the same line)
Unresolved reference: a
Unresolved reference: b

It seems like a bug but I’m not sure.

Of course, this could be worked around my specifying the type of value in register to (String, () -> String) -> Unit, but this is the API and I have no access to it.

This could also be worked around by using a Function0<String>, but why wouldn’t it work with the more "kotlin" syntax ?

>Solution :

Removing the parentheses around the function type worked for me:

put("foo") { a: String, b: () -> String ->
    println(a + b())
}

I do agree this is weird. You can put parentheses around String, but not () -> String. This also works with anonymous function literals:

put("foo", fun(a: String, b: (() -> String)) {
    
})

The syntax in the language spec does say that type can be a parenthesizedType:

type:
[typeModifiers] (functionType | parenthesizedType | nullableType | typeReference | definitelyNonNullableType)

Perhaps there is some quirk in the parser that causes the parsing the fail…