Apostrophe in identifiers in Haskell

Question

I found this code snipped on the internet:

digits 0 = [0] digits n = digits' n []   where digits' 0 ds = ds         digits' n ds = let (q,r) = quotRem n 10                        in digits' q (r:ds)  sumOfDigits = sum . digits 

Can someone quickly explain what the " ' " sign ( digits n = digits' n [] ) after the recursive function call is for? I've seen some other code examples in Haskell (tutorials), but im not understandig this one. A quick explanation is appreciated.

Answer 1

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.

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You can run this in GHCi, by the way,

Prelude> :{  Prelude| let x  = 1 Prelude|     x' = x * 2 Prelude| in x' Prelude| :} 2 

Answer 2

It's just another character allowed in identifiers. Think of it as another letter.