Updating a variable

"I wrote a simple piece of code to update the value of x, but it hangs."

x = 0
x = x + 1
x

x = 0
x = x + 1
main = print x

In Haskell, values are always immutable. Writing a = b does not mean "set the value of a to the current value of b". It means "bind the name a to the expression b. So you have asked Haskell to make x refer to the value x + 1.

Haskell tries to fulfill your request. When you ask for x on the third line, it looks up its value, which is x + 1. It then looks up the value of x in x + 1, which again, is x + 1, to obtain (x + 1) + 1. This continues indefinitely, which is why the program hangs.

Last update: January 12, 2023
Created: January 7, 2023

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