MatchError for PartialFunctions composition with orElse

Question

While writing a Specs2 specification for an Actor I got a somewhat puzzling MatchError for composition of several partial functions.

A minimal example:

val testPf1 = PartialFunction[Any, Boolean]{ case 2 ⇒ true }
val testPf2 = PartialFunction[Any, Boolean]{ case 1 ⇒ true }
val testPf = testPf1 orElse testPf2
testPf.isDefinedAt(1)
testPf.isDefinedAt(2)
testPf(1)
testPf(2)

leads to the output:

testPf1: PartialFunction[Any,Boolean] = <function1>
testPf2: PartialFunction[Any,Boolean] = <function1>
testPf: PartialFunction[Any,Boolean] = <function1>
res0: Boolean = true
res1: Boolean = true
scala.MatchError: 1 (of class java.lang.Integer)
    at com.dasgip.controller.common.informationmodel.programming.parametersequence.A$A161$A$A161$$anonfun$testPf1$1.apply(PFTest.sc0.tmp:33)
    at com.dasgip.controller.common.informationmodel.programming.parametersequence.A$A161$A$A161$$anonfun$testPf1$1.apply(PFTest.sc0.tmp:33)
    at scala.PartialFunction$$anonfun$apply$1.applyOrElse(PFTest.sc0.tmp:243)
    at scala.PartialFunction$OrElse.apply(PFTest.sc0.tmp:163)
    at #worksheet#.#worksheet#(PFTest.sc0.tmp:36)

That totally confused me. If for a given input isDefinedAt on the composition of two partial functions returns true, I would expect that I can also apply it to the same input.

Answer 1

Hence, I learned that changing the first two lines to:

val testPf1: PartialFunction[Any, Boolean] = { case 2 ⇒ true }
val testPf2: PartialFunction[Any, Boolean] = { case 1 ⇒ true }

makes the composition work as expected.

The reason for the MatchError was that with

PartialFunction[Any, Boolean]{ case 2 => true } 

I actually seem to be calling PartialFunction.apply, which converts a Function1 to a PartialFunction.

So that the statement expands to

 PartialFunction.apply[Any, Boolean](_ match { case 2 => true })

and then converted to

{ case x => f(x) }

which, of course, will always return true for isDefined and throw a MatchError on input not matched by f.