Question

I am working through Ninety-Nine Scala Problems to learn more Scala. I am on P12 and coded the below solution for the problem.

def decode(l : List[Tuple2[Int,Symbol]]) : List[Symbol] 
            = l foldLeft(List[Symbol]()) { (symbols:List[Symbol], e:Tuple2[Int, Symbol]) => symbols ::: List(e._2) }

And I am getting the below compiler error.

 error: type mismatch;
 found   : (List[Symbol], (Int, Symbol)) => List[Symbol]
 required: Int
                        = l foldLeft(List[Symbol]()) { (symbols:List[Symbol], e:
Tuple2[Int, Symbol]) => symbols ::: List(e._2) }

What is causing the compiler error?

Scala version: Scala code runner version 2.10.0-M3 — Copyright 2002-2011, LAMP/EPFL.

Answer 1

It looks like it’s your use of the infix foldLeft call, you just need to change it to:

def decode(l : List[Tuple2[Int,Symbol]]) : List[Symbol] 
        = l.foldLeft(List[Symbol]()) { (symbols:List[Symbol], e:Tuple2[Int, Symbol]) => symbols ::: List(e._2) }

Note the “l.foldLeft” rather than “l foldLeft”, I suspect the compiler can’t quite determine what is a parameter to what.