Haskell beginner - defining and using a type

Question

I am trying to do the equivalent of this correctly working code :

len   :: [a] -> Int 
len [] = 0 
len (x:xs) = 1 + len xs 

But for the type MyList that I am defining as :

data MyList a = Nil | Cons a (MyList a)

Here is my attempt :

mylen :: (MyList a) -> Int
mylen Nil = 0
mylen (Cons a (MyList a)) = 1 + mylen (MyList a)

But I get these errors:

Conflicting definitions for 'a'
Not in scope: data constructor 'MyList'

I can't figure out how to get it to work.

Answer 1

Your (reasonable) definition of MyList :

data MyList a = Nil | Cons a (MyList a)

...states that there are two constructors.

1. Nil (taking no arguments), and
2. Cons , which takes two arguments: the first of type a , the second of type MyList a

So to pattern match, you have to write something like Cons item rest , eg

mylen :: (MyList a) -> Int
mylen Nil = 0
mylen (Cons item rest) = 1 + mylen rest

But since you never use item , it is customary to replace it with _ .

Notice how a , Int and MyList a are types. On the other hand, item , 0 and 1 , rest , and even Nil and Cons item rest are all instances of those types.

Answer 2

  mylen :: MyList a -> Int
  mylen Nil = 0
  mylen (Cons _ next) = 1 + mylen next