scala anonymous functions question

Question

I am trying to filter a map from strings to ints in scala and I am running into a weird issue.

If I put the following into REPL:

scala> val myMap = Map("a" -> 1, "b" -> 2, "c" -> 3)
myMap: scala.collection.immutable.Map[java.lang.String,Int] = 
| Map(a -> 1, b -> 2, c -> 3)

It's ok so far, and this works...

scala> myMap.filter(_._2 > 1)
res9: scala.collection.immutable.Map[java.lang.String,Int] = Map(b -> 2, c -> 3)

but this fails...

scala> myMap.filter((k:java.lang.String, v:Int) => v > 1)
<console>:9: error: type mismatch;
 found   : (java.lang.String, Int) => Boolean
 required: ((java.lang.String, Int)) => Boolean
   myMap.filter((k:java.lang.String, v:Int) => v > 1)

My question is what is going on with the error message and the extra pair of parentheses? If I try putting in an extra set of parentheses I get error: not a legal formal parameter.

Kipton Barros · Accepted Answer · 2011-08-22 22:57:55Z

16

myMap.filter expects a function of type Tuple2[String, Int] => Boolean, which is equivalent to ((String, Int)) => Boolean. You're passing it a function of type (String, Int) => Boolean; that is, a function that takes two arguments rather than a single Tuple2.

Two ways to get it to work:

 myMap.filter { case (k, v) => v > 1 }

and

 myMap.filter(Function.tupled((k, v) => v > 1))

The first works by pattern matching, and the second by converting the function (String, Int) => Boolean to ((String, Int)) => Boolean.

By the way, there has been discussion about unifying tuples and function argument lists. Maybe in a future version of Scala, all functions will take a single parameter (which might be a tuple).

edited Aug 22, 2011 at 22:57

answered Aug 22, 2011 at 22:21

Kipton Barros

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7 Comments

andyczerwonka Over a year ago

Agreed. Function.tupled is explicit while the pattern match seems more like a trick.

chuck taylor Over a year ago

Sorry if this is a dumb question, but why does the pattern matching with case work with curly braces {} but not parentheses?

Kipton Barros Over a year ago

Not a dumb question at all. I don't know a better answer than "that's what the Scala Language Spec says". You might be interested in this discussion of the difference between () and {}: stackoverflow.com/questions/4386127/…

Daniel C. Sobral Over a year ago

@Kipton Is Function.tupled deprecated? It had been deprecated, but that had been reversed because Function.tupled is friendlier to type inference.

Kipton Barros Over a year ago

@Daniel, it's deprecated according to the source code linked from these nightly docs: scala-lang.org/archives/downloads/distrib/files/nightly/docs/…

|

Diego Sevilla · Accepted Answer · 2011-08-22 22:17:29Z

6

filter accepts a function that accepts only one parameter. In your expression, the expression accepts two parameters. However, the element happen to be a pair, so you may think you can give two parameters. The correct way of putting it would be something like:

myMap.filter (p => p._2 > 1)

That is, I receive a pair, p, and its second element must be greater than 1.

answered Aug 22, 2011 at 22:17

Diego Sevilla

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