F# - Expected to be option but has a type -

i trying build binary tree in f# when tried test code, met problem above.

here code:

type treenode<'a> = { key: int; val: 'a } type tree<'a> =  { lt: tree<'a> option; treenode: treenode<'a>; rt: tree<'a> option; }  //insert node according binary tree operation let rec insert (node: treenode<'a>) (tree: tree<'a> option) =     match tree     | none -> {lt = none; rt = none; treenode = node }     | t when node.key < t.treenode.key -> insert node t.lt     | t when node.key > t.treenode.key -> insert node t.rt  let t = seq { in 1 .. 10 -> { key = i; val = } }|> seq.fold (fun -> insert a) none 

your insert function takes option<tree<'t>> returns tree<'t>. when performing fold, need keep state of same type - if want use none represent empty tree, state needs optional type.

the way fix wrap result of insert in some:

let tree =    seq { in 1 .. 10 -> { key = i; val = } }   |> seq.fold (fun -> some(insert a)) none