Question

I want to represent a graph in Haskell in the following manner:

For each node I want to store it’s value and a list of adjacent nodes. The problem which I’m having difficulties with is that I want the adjacent nodes to be stored as references to other nodes.

For example, I want node ny to be stored as („NY“ (l p)) where l and p are adjacent nodes,and not as („NY“ („London“ „Paris“)).
I tried something like this :

data Node a = Node { value :: a
                   , neighbors :: [Node a]
                   }deriving (Show)

let n1 = Node {value=1, neighbors=[n2]}
let n2 = Node {value=1, neighbors=[n1 n3]}
let n3 = Node {value=1, neighbors=[n2]}

But I get en error in let. What am I doing wrong ?

Answer 1

Two problems:

1. let is an expression form and at top level the compiler is expecting a declaration form.

2. You need a single nest of bindings; by using three lets, you’ve split the definitions into three separate scopes.

The following code compiles, and when I ask for n1, I get an infinite string printout as expected:

module Letnest 
where
data Node a = Node { value :: a
                   , neighbors :: [Node a]
                   } deriving (Show)

n1 = Node {value=1, neighbors=[n2]}
n2 = Node {value=1, neighbors=[n1, n3]}
n3 = Node {value=1, neighbors=[n2]}