Scala - trouble with type inference in lambda expression

Question

Attempting to compile the function below in scala 2.11.0

def dafuq(canvas: Array[Array[Boolean]]): Array[Array[Boolean]] = {
        for (r <- canvas.reverse) yield r.zipWithIndex.map((c: Boolean, i: Int) => c) 
}

yields

Solution.scala:6: error: type mismatch;
   found   : (Boolean, Int) => Boolean
   required: ((Boolean, Int)) => ?
        for (r <- canvas.reverse) yield r.zipWithIndex.map((c: Boolean, i: Int) => c) 
                                                                                ^

The function is bogus but it illustrates a problem that I have encountered. I'm quite new to Scala so this could be a rookies mistake but I cannot find any solution or explanation to this problem. Do you know what causes the above behaviour?

Also is return type hinting possible for lambda expressions?

Dimitri · Accepted Answer · 2014-11-24 23:40:17Z

3

I ran into this compile error many times too, but it's not a type inference issue. You supplied an anonymous function with 2 arguments, but the compiler expected a function with a single tuple argument. 2 solutions:

def dafuq(canvas: Array[Array[Boolean]]): Array[Array[Boolean]] = {
        for (r <- canvas.reverse) yield r.zipWithIndex.map(ci => ci._1)
}

or

def dafuq(canvas: Array[Array[Boolean]]): Array[Array[Boolean]] = {
        for (r <- canvas.reverse) yield r.zipWithIndex.map { case (c, i) => c }
}

In the first solution ci is inferred to a pair (Boolean, Int). In the second solution we use a pattern match expression.

Also is return type hinting possible for lambda expressions?

I don't know, but you can add type annotations on any expression. For example: case (c, i) => c: Boolean.

edited Nov 24, 2014 at 23:40

answered Nov 24, 2014 at 23:12

Dimitri

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2 Comments

Chris Martin Over a year ago

The second snippet uses a pattern match expression to define a function. It isn't a PartialFunction; it's a Function1 (and it isn't partial, as it's defined over its entire domain). SLS section 8.5 specifies that a pattern match expression desugars to either FunctionN or PartialFunction depending on the context. In this case, since map expects a Function1, that's what it is.

Chris Martin Over a year ago

I only know this because someone else has corrected me on stackoverflow before, so... keep passing it along :)

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