[E1016] RECURSIVE TYPE ALIAS

A type alias refers to itself, directly or through other aliases of the same module.

-- RECURSIVE TYPE ALIAS ---------------------------- src/Tree.sky:3:12 [E1016]

3 | type alias Node =
  |            ^^^^ this type alias refers to itself

The type alias `Node` refers to itself. A type alias is only another name for
its body, so a recursive one never ends. Make it a custom type instead, which
gives the recursion a name to stop at: `type Node = Node { … }`, and wrap and
unwrap the record with the `Node` constructor. See
docs/migration/v0.27.md#recursive-type-alias

Added in v0.27.0.

What it covers

A cycle cannot cross modules: that would need an import cycle, which [E1010] already refuses.

Why it is an error

An alias is expanded wherever it is used, never named. A recursive alias has no finite expansion. Before v0.27.0 the checker either printed a confusing record vs Node type mismatch or accepted the declaration, and the Go it emitted was an invalid recursive type that go build refused. sky check passed and sky build failed, which breaks "if it compiles, it works". Elm refuses the same declaration for the same reason.

When a module has an [E1016], the type checker does not run on it, so this is the only error you see for that module until you fix it.

How to fix it

Make the alias a custom type with one constructor. The constructor is the name the recursion stops at:

-- before
type alias Node =
    { value : Int, next : Maybe Node }


-- after
type Node
    = Node { value : Int, next : Maybe Node }


value : Node -> Int
value (Node n) =
    n.value

Build a value with the constructor (Node { value = 1, next = Nothing }) and read it by matching on it, as value does above.