Why does lazy evaluation seem to work differently for a Num than an Int in Haskell?

Question

In GHCI if I do..

let x = 1 + 2

then

:sprint x

I get

x = _

then if I do

x

3

then..

:sprint x

I still get

x = _

However if I do

let x = 1 + 2 :: Int

Then the second :sprint x gives me

3

Just wondering why that is?

Answer 1

Look at the type of 1 + 2: it's Num a => a. That means it can be any numeric type, specified by whoever uses x. So if you evaluate it once, GHCI's defaulting rules mean it defaults to Int, and then you evaluate x :: Int, which is indeed 3 :: Int. However, that can't be saved as the value for x, because someone might later try to use it as x :: Double, for example, and the answer would be different.