Does an unused let binding have any effect in Haskell?

Question

I just realised that it is actually legal to write this:

let _ = sum [1..100]
in  "Hello"

The let-binding appears to do absolutely nothing.

But now I'm wondering about the exact semantics here. It is possible to write a program which contains a _ binding, and yet deleting that binding would visibly alter the meaning of said program?

Basically, I'm wondering whether it's safe to automatically delete such bindings. As far as I can tell, the value of this binding cannot possibly affect anything. However, it seems hypothetically possible that it's type might affect something else. Can anybody construct an example?

Answer 1

Here is one example. With the _ binding, the output is 8.0, but without it, it is 8. (Admittedly, this isn't a very big difference, but I'm sure this could be used as a basis for something more substantial.)

main :: IO ()
main = let x = 5
           _ = asTypeOf x 6.0
       in print $ x + 3