What is `Cons` in Data.FixedList source code?

Question

In the source code for Data.FixedList, I found the following definition:

data FixedList f =>
  Cons f a = (:.) {
    head :: a,
    tail :: (f a)
  }  deriving (Eq, Ord)

As someone very new to Haskell, it's hard to figure out what's going on here. I understand syntax such as data TypeName = TypeName { a :: Int, b :: Int} deriving (Show) or data TypeName = TypeA | TypeB, but the code above is over my head. Any documentation / or walk-through would be much appreciated!

score 7 · Accepted Answer · edited May 23 '17 at 11:58

First of all the FixedList f => enforces the constraint that f is a FixedList when using the constructor. I'm not sure why it's used here. It is generally advised against.

In Haskell, you can make infix data constructors with symbols starting with :. In the library, the constructor is put in brackets (:.) so it can be used with record syntax. Without record syntax it would look like this:

data Cons f a = a :. f a

Pattern matching is very similar to lists. Here's a simple function using it:

mapHead :: FixedList f => (a -> a) -> Cons f a -> Cons f a
mapHead f (a :. as) = f a :. as

Here's a definition without using an infix constructor.

data Cons f a = Cons
  { head :: a
  , tail :: f a
  }

score 4 · Answer 2 · answered Jul 18 '16 at 00:44

This is mostly fancy formatting of things you've seen before.

data FixedList f => Cons f a = (:.) { head :: a
                                    , tail :: (f a)
                                    } deriving (Eq, Ord)

FixedList f is just a typeclass constraint on the Cons f a datatype. (:.) is an infix data constructor.

What is `Cons` in Data.FixedList source code?

2 Answers2