Page 18 - Region Shift Function about functional-swift HOT 3 CLOSED

Yes, that's a mistake. We'll fix this in the next update.

from functional-swift.

When you update the book for this bug, don't just update the code. Please update the text to make it clear that it is the point that is being shifted, not the region. The current text is wrong in its description of how the function works (and the bug is a direct consequence of the incorrect explanation). I bought this book because I am not familiar with FP, so the explanations need to be clear and correct.

(I had to write several tests in the swift playground to solve this problem. I hope you did the same thing.)
let px1y1 = Position(x:1, y:1)
let px5y5 = Position(x:5, y:5)
let cr2x0y0 = circle(2.0)
cr2x0y0(px1y1) // true.
cr2x0y0(px5y5) // false.

let cr2x4y4 = shift(Position(x:4, y:4), cr2x0y0)
cr2x4y4(px1y1) // false
cr2x4y4(px5y5) // true (but the code in the book returns false)

from functional-swift.

Ah, I see that there is an explanation, later in the text. Unfortunately, when I can't understand a text book, I try to understand each sentence before going on to the next, so didn't see the correct explanation that followed the incorrect one.

from functional-swift.