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Some exploration of the problem Dan Bodart found and discussed in http://dan.bodar.com/2013/12/04/wat-scala/
package gotchas
object NoArgMethods {
val l = List(1, 2, 3)
val correct = l.toSet // has type Set[Int]
val wrong = l.toSet() // has type Boolean
// toSet() is converted to toSet.apply()
// (no surprise there)
// what is surprising is that toSet.apply() compiles!
// apply on toSet is a unary method, what happens when its applied with 0 args?
// looks like the compiler is creating a call with unit
val s = l.toSet
// fully expanded
// -------------------
val s1 = l.toSet.apply() // why does this compile?
//val s2 = s.apply() // fails with 'not enough arguments'
val s3 = l.toSet apply () // can see that this might be ambiguous; see below
//val s4 = s apply () // fails with 'not enough arguments'
// partially expanded
// -------------------
val s5 = l.toSet.apply () // can see that this might be ambiguous; see below
//val s6 = s.apply () // fails with 'not enough arguments'
//val s7 = l.toSet.apply // fails with 'Missing arguments'
//val s8 = s.apply // fails with 'Missing arguments'
// ^ Good. The wouldn't expect the compiler to ever infer a missing arg
val s9 = l.toSet apply() // why does this compile?
//val s10 = s apply() // fails with 'not enough arguments'
//val s11 = l.toSet apply // fails with 'Missing arguments'
//val s12 = s apply // fails with 'Missing arguments'
// ^ Good. The wouldn't expect the compiler to ever infer a missing arg
// The problem never occurs when empty apply call is made against a val/var of the correct type, it only occurs
// when made against an intermediate object. Why?
// I can sort of see the ambiguity where you're calling foo(Int) like
// foo ()
// is that meant to be interpreted as:
// foo() (called with no args)
// or
// foo(()) (called with one arg of unit type)
// After speaking to Dan B, the problem becomes deeper - the inferred type of the intermediate Set is Set[Any],
// so we're asking the set if unit is a member
val sa: Set[Any] = l.toSet // remove the type annotation and the below line fails to compile (as the previous example of the same shape)
val sa1 = sa()
}
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