haskell - What does let 5 = 10 do? Is it not an assignment operation?

Question

If I say let 5 = 10, why does 5 + 1 return 6 instead of 11?

Answer 1

When you say

let 5 = 10

it's not a redefinition of 5, it's a pattern matching, the same which occurs when you say

foo 5 = undefined
 ... foo 10 ...

The pattern simply fails if it's ever matched.

In let-expressions the match is lazy. This means the match is only being done when a variable bound by it is evaluated. This allows us to write things like

 let foo = undefined in 10

In your expression, no variable is bound, so the pattern is never matched.

Arguably such patterns with no variables make no sense in let-bindings and should be detected by the compiler, but the language doesn't forbid them.