无法推断通用参数“ T” /无法显式专门化通用函数

Question

I have a function with this signature written in a Swift project in Xcode 7.3.1:

func DLog<T>(@autoclosure object: () -> T, _ file: String = #file, _ function: String = #function, _ line: Int = #line) {
}

The compiler complains about Generic parameter 'T' could not be inferred for this call:

DLog({ var text = "Returning output list\n"; for outline in outlines { text = text + outline.debugDescription + "\n"; }; return text; })

When I try to provide the type it complains Cannot explicitly specialize a generic function :

DLog<String>({ var text = "Returning output list\n"; for outline in outlines { text = text + outline.debugDescription + "\n"; }; return text; })

I have tried few more approaches but nothing satisfied the compiler. I have also failed to find a hint how to solve this case.

How to build the text inside the () => T argument and pass it to the function properly?

Answer 1

Compilation fails because of @autoclosure attribute. When you pass some expression to function that takes @autoclosure the compiler creates a closure with no parameters that returns result of that expression. So, when you pass { var text = "Returning output list\\n"; for outline in outlines { text = text + outline.debugDescription + "\\n"; }; return text; } { var text = "Returning output list\\n"; for outline in outlines { text = text + outline.debugDescription + "\\n"; }; return text; } { var text = "Returning output list\\n"; for outline in outlines { text = text + outline.debugDescription + "\\n"; }; return text; } then compiler creates a closure returning a closure returning string.

To fix this, you can either add () to the end of expression:

DLog({ () -> String in var text = "Returning output list\n"; for outline in outlines { text = text + outline.debugDescription + "\n"; }; return text; }())

or simplify expression to simple method call, eg

DLog(outlines.reduce("Returning output list\n") { $0 + $1.debugDescription + "\n"; })

Answer 2

I don't know why it is causing it, but compiling it in swift 3 make this error message appear:

Unable to infer complex closure return type; add explicit type to disambiguate

So I tried to explicitly add the return type for the closure:

DLog(
    { () -> String in 
        var text = "Returning output list\n" 
        for outline in outlines { 
            text = text + outline.debugDescription + "\n"; 
        } 
        return text 
    }
)

and it worked.