In this passage, after the seq call, :sprint reveals x and y to be evaluated:

let x = 1 + 2 :: Int

let y = x + 1

:sprint x

:sprint y

seq y ()

:sprint x

:sprint y

The above is a verbatim quote of Marlow's Concurrency book, chapter 2. 

In the below, however, :sprint shows them unevaluated both before and after the seq call.

let x = 1 + 2

let y = x + 1

:sprint x

:sprint y

seq y ()

:sprint x

:sprint y

Why?