I am trying to understand the code found here. I am an OCaml person and not a Haskeller.

The author defines the following algebraic data type and some relevant functions — the details of which do not really matter.

data Node k v
  = Node { nKey :: k
         , nValue :: v
         , nSibling :: Node k v
         , nChild :: Node k v }
  | Head { nSibling :: Node k v
         , nChild :: Node k v }
  | Nil

nodeLTKey :: forall k v. Ord k => Node k v -> k -> Bool
nodeEQKey :: forall k v. Eq k => Node k v -> k -> Bool
sibling :: forall k v. Node k v -> Node k v

Later in the insert function around line 113, they define a helper function of type Node k v -> (Node k v, Int). I have extracted the part I am confused about below.

go Nil = (Nil, wins)
go n
  | nodeLTKey sN k = (n {nSibling = fst $ go sN}, -1)
  | nodeEQKey sN k = (n {nSibling = sN {nValue = v}}, -1) 
  -- how can they update the inner record?
  where
    sN = sibling n
    -- How can the compiler be sure that sN is a Node and not Head / Nil?

The sibling function returns a Node k v and only one of the possible variants of a Node k v actually has a field called nValue. I don’t understand how the second guard can be sure that sN will have an nValue field — i.e that sN will be of the Node case and not Head / Nil.