Scala foldLeft on Maps

How do you use Map.foldLeft? According to the docs it looks like

foldLeft [B] (z: B)(op: (B, (A, B)) ⇒ B) : B

But I'm having difficulty:

Map("first"->1,"second"->2).foldLeft(0)((a,(k,v)开发者_如何学编程) => a+v )

error: not a legal formal parameter

The error points to the open bracket in front of k.

If you want to use the (a, (k, v)) syntax, you need to advise the compiler to use pattern matching.

Map("first"->1, "second"->2).foldLeft(0){ case (a, (k, v)) => a+v }

Note that a case statement requires curly braces.

I think, you can't do the pattern match on tuples as you expect:

Map("first"->1,"second"->2).foldLeft(0)((a, t) => a + t._2)

Actually, using values and sum is simpler.

Map("first"->1,"second"->2).values.sum

The trick is to use a partial function as the code block, in other words you add a case statement that matches on the arguments:

Map("first" -> 1, "second" -> 2).foldLeft(0) { case (a, (k, v)) => a + v }

This is not really an answer to your question but I found it useful when starting out with folds, so I'll say it anyway! Note that the /: method "alias" for foldLeft can be clearer for two reasons:

xs.foldLeft(y) { (yy, x) => /* ... */ }

(y /: xs) { (yy, x) => /* ... */ }

Note that in the second line:

• it's more clear that the value y is being folded into the collection xs
• you can easily remember the ordering of the Tuple2 argument is the same as the ordering of the method "call"