Apostrophe in identifiers in Haskell
The apostrophe is just part of the name. It is a naming convention (idiom) adopted in Haskell.
The convention in Haskell is that, like in math, the apostrophe on a variable name represents a variable that is somehow related, or similar, to a prior variable.
An example:
let x = 1
x' = x * 2
in x'
x' is related to x, and we indicate that with the apostrophe.
You can run this in GHCi, by the way,
Prelude> :{
Prelude| let x = 1
Prelude| x' = x * 2
Prelude| in x'
Prelude| :}
2
It's just another character allowed in identifiers. Think of it as another letter.