Empty partial function in Scala

Question

It seems to me like the { case ... => ... } syntax for partial functions require at least one case:

scala> val pf: PartialFunction[String, String] = { case "a" => "b" } 
pf: PartialFunction[String,String] = <function1>

scala> val pf: PartialFunction[String, String] = { }                
<console>:5: error: type mismatch;
 found   : Unit
 required: PartialFunction[String,String]
       val pf: PartialFunction[String, String] = { }
                                                 ^

So, what's the best way to define an "empty" partial function? Is there a better way than "manually" overriding isDefinedAt and apply?

There's now an empty partial function defined on the `PartialFunction` companion object. https://issues.scala-lang.org/browse/SI-4927 — Kipton Barros, Aug 28 '11 at 16:31

score 60 · Accepted Answer · answered Aug 26 '11 at 03:03

60

Map is a PartialFunction so you can do:

val undefined: PartialFunction[Any, Nothing] = Map.empty

answered Aug 26 '11 at 03:03

mpilquist

3,855
21
22

3

actually there is also the heavily under-documented `FSM.NullFunction` (which does the same thing). I’m adding docs now. – Roland Kuhn Nov 28 '12 at 14:20
39

Since 2.10 `PartialFunction.empty` is available – Andrejs Aug 31 '13 at 09:01

score 51 · Answer 2 · answered Aug 31 '13 at 08:51

51

Since Scala 2.10 you can use:

val emptyPf = PartialFunction.empty[String, String]

answered Aug 31 '13 at 08:51

Andrejs

26,885
12
107
96

missingfaktor · Answer 3 · 2011-08-25T11:10:42.313

8

scala> def pfEmpty[A, B] = new PartialFunction[A, B] {
     |   def apply(a: A): B = sys.error("Not supported")
     |   def isDefinedAt(a: A) = false
     | }
pfEmpty: [A, B]=> java.lang.Object with PartialFunction[A,B]

scala> val f = pfEmpty[String, String]
f: java.lang.Object with PartialFunction[String,String] = <function1>

scala> f.lift
res26: (String) => Option[String] = <function1>

scala> res26("Hola")
res27: Option[String] = None

As @didierd said in the comments, due to argument variances, a single instance can cover all possible argument types.

scala> object Undefined extends PartialFunction[Any, Nothing] {
     |   def isDefinedAt(a: Any) = false
     |   def apply(a: Any): Nothing = sys.error("undefined")
     | }
defined module Undefined

scala> val f: PartialFunction[String, String] = Undefined
f: PartialFunction[String,String] = <function1>

scala> f.lift apply "Hola"
res29: Option[String] = None

edited Aug 25 '11 at 11:10

answered Aug 25 '11 at 10:50

missingfaktor

90,905
62
285
365

He says: a better way than "manually" overriding `isDefinedAt` and `apply` ;) – Nicolas Aug 25 '11 at 10:52
2

@Nicolas: Yes, define a function that does it for you. (`pfEmpty` here) ;) – missingfaktor Aug 25 '11 at 10:54
1

@Nicolas: To be honest, I missed that line when I read the question. Thanks for pointing it out! I know my question doesn't satisfy the constraint specified in the question, but anyway I am leaving it here with a hope that it might be useful for someone. – missingfaktor Aug 25 '11 at 10:58
3

Given arguments variances, a single instance could do it all: object Undefined[Any, Nothing] {... your impl ...} – Didier Dupont Aug 25 '11 at 11:00
1

There are two question marks in my post. @missingfaktor gives a great answer to the first one, and implicitly says *"not that I know of"* for the second. :-) +1! – aioobe Aug 25 '11 at 11:06
2

Shouldn't apply raise a scala.MatchError exception? That's what applying a PF that's not defined for the input usually does. – Hristo Deshev Aug 25 '11 at 11:10
@Hristo: Doesn't matter in this case, I _guess_. (Since `isDefinedAt` is always `false`.) – missingfaktor Aug 25 '11 at 12:18
1

Mix of the various solutions: val undefined : PartialFunction[Any, Nothing] = {case _ if false => sys.error("undefined")} – Didier Dupont Aug 25 '11 at 13:43
@didierd: That's good enough to post as a separate answer. :-) – missingfaktor Aug 25 '11 at 13:57

score 6 · Answer 4 · answered Aug 25 '11 at 14:01

6

Stealing from everyone, a possible mix of it all :

val undefined : PartialFunction[Any, Nothing] = {case _ if false =>
  sys.error("undefined")
}

answered Aug 25 '11 at 14:01

Didier Dupont

29,398
7
71
90

what good will the `sys.error` do? It will never get executed. – aioobe Aug 25 '11 at 14:11
@aioobe: It has a type `Nothing`. – missingfaktor Aug 25 '11 at 14:13
A more self-contained expression with type Nothing, which could replace the sys.error call: undefined(undefined). – Didier Dupont Aug 25 '11 at 15:57

Daniel C. Sobral · Answer 5 · 2011-08-25T12:20:05.220

5

Shortest one I can think of:

{ case _ if false => "" }

edited Aug 25 '11 at 12:20

answered Aug 25 '11 at 12:03

Daniel C. Sobral

295,120
86
501
681

Don't you need a `""` after the `=>` for it to be a valid `PartialFunction[String, String]`. If so, it's a bit unfortunate since it's bound to the result type of `""`, no? (Seems like an alternative would be to do `=> throw new RuntimeException()`, but that's perhaps even uglier.) What's your throughts? – aioobe Aug 25 '11 at 12:13
@aioobe You do. I didn't pay attention to the type of the PF. – Daniel C. Sobral Aug 25 '11 at 12:20
Would changing `""` to `null` (or anything else?) make it applicable for all possible partial functions (not just the ones returning `String`)? – aioobe Aug 25 '11 at 14:12
@aioobe No. To make it generally applicable, you have to use `error` or something similar, as they return `Nothing`. `Nothing` is the only subtype of everything. – Daniel C. Sobral Aug 25 '11 at 15:05

Nicolas · Answer 6 · 2011-08-25T10:59:34.693

4

A solution (which is more a hack) is to ensure that the case is never true: { case x if x != x => sys.error("unexpected match") }

Simple curiosity, why do you need such a function?

edited Aug 25 '11 at 10:59

answered Aug 25 '11 at 10:54

Nicolas

24,509
5
60
66

I have an API that accepts a partial function, and I want to provide it with a partial function that's not defined for any argument. – aioobe Aug 25 '11 at 11:07

score 3 · Answer 7 · answered Aug 30 '11 at 09:45

3

It may be interesting to know that it is planned to add an empty member to the scala library and to see how it is implemented: https://github.com/scala/scala/commit/6043a4a7ed5de0be2ca48e2e65504f56965259dc

answered Aug 30 '11 at 09:45

Nicolas

24,509
5
60
66

It's an old question and an old answer. The current implementation can be found there: https://github.com/scala/scala/blob/master/src/library/scala/PartialFunction.scala#L246 (l.246-259) – Nicolas Sep 30 '12 at 08:42

Empty partial function in Scala

7 Answers7