CODEWITHSUNDEEP
haskellcoding-styleidentifier
kiltek
kiltek

Reputation: 3211

Apostrophe in identifiers in Haskell

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.

Upvotes: 25

Views: 8979

Answers (2)

augustss
augustss

Reputation: 23014

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

Upvotes: 11

Don Stewart
Don Stewart

Reputation: 137987

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

Upvotes: 42

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