functional programming - Haskell: What is immutable data?

Question

in most articles about Haskell you'll find a statement like "Data in Haskell is immutable". I don't quite understand why. For example:

let a = 123
let a = 456

in the main method works. I just changed the data of a from 123 to 456. What am I missing? It's probably a stupid mistake in my train of thought :/

Have a good day!

Answer 1

Actually, a hasn't changed. Try this in ghci to see:

> a = 123
> action = print a
> a = 456
> action
123

Compare with a language that has mutable variables, e.g. python:

>>> a = 123
>>> def action(): print a
... 
>>> a = 456
>>> action()
456