suchThat usages in ScalaCheck

I'm learning scala by reading the code from ScalaCheck and find many combinators are suffixed by suchThat. However, in many cases suchThat doesn't look quite necessary. I'm wondering why they are designed in this way. Here's some excerpted from GitHub:

Example 1.

/** Picks a random value from a list */
def oneOf[T](xs: Seq[T]): Gen[T] =
    choose(0, xs.size-1).map(xs(_)).suchThat(xs.contains)

map picks an element from xs and therefore xs.contains seems redundant.

Example 2.

def containerOfN[C[_],T](n: Int, g: Gen[T])
    (implicit evb: Buildable[T,C], evt: C[T] => Traversable[T]): Gen[C[T]] =
    sequence[C,T](Traversable.fill(n)(g)) suchThat { c =>
      c.size == n && c.forall(g.sieveCopy)
    }

Why is c.size == n needed, given that a successful sequence would return in the length of n?

Example 3.

/** Generates a string of alpha characters */
def alphaStr: Gen[String] =
    listOf(alphaChar).map(_.mkString).suchThat(_.forall(_.isLetter))

Generated strings would consist of list of alphaChar which is chosen from uppercase and lowercase letters. Why do we need a suchThat to check isLetter again?

Solution

See https://github.com/rickynils/scalacheck/commit/2d92eb61a89a8ec2e16d0af8599f9baafc9b5418

Basically, this is needed for the ScalaCheck test case simplification to know what bounds the generated values originally had.