Why does shouldHaveSingleElement and other Collection "shoulds" have predicate instead of block for assertions?

What I do not really get is: when does your library want predicates (T)->Boolean and when does it require a function block that itself can do multiple assertions using matchers and should constructs?

I.e.:

list.shouldHaveSingleElement {
   it.name == "Bob"
}

vs. (not the case)

list.shouldHaveSingleElement {
  it.name shouldBe "Bob"
}

I’ve stumbled across both ways, but the simple predicate one is used for collections - and I do not like it at all, because the error messages don’t reveal any reason for failing the match whereas the latter one could state that something was expected to be “Bob” but is “Alice” for example.

Is there a reason behind this that I cannot see?

[sksamuel]

The std lib has the method .single() to extract only a single element from a list, and throw if the list does not have size 1.

[sksamuel]

I think the difference here, is that they are single predicates. So each predicate is executed in turn on the next element of the list.

With inspectors, everything in the block is executed every each element, and the counts then compared to the requirements.