Question

I have defined a type called node along with a list of nodes.

type node = {name: string; description: string}
nodes = [] : list(node)

I have created a function called createNewNode() which creates a new node, assigns it to selectedNode, and adds it to the array nodes.

line 19: createNewNode() =
line 20:   selectedNode = {name="" remoteFSRoot=""} : node
line 21:   nodes = [nodes | selectedNode]
  ...

When I compile this I get the following error:

Error
File "node.opa", line 21, characters 10-32, (21:10-21:32 | 592-614)
Expression has type { hd: list(node); tl: node } / 'c.a but is coerced into
list('a).
Types { name: string; description: string } and
{ hd: 'a; tl: list('a) } / { nil } are not compatible
Hint:
  One of the sum types may be missing the following cases of the
  other:
  { nil }
  { hd tl }.

What does this compile message mean and how do I fix it?

Answer 1

I think you simply inversed nodes and selectedNode at Line 21 :

nodes = [selectedNode | nodes]