scala - Instantiating Case Class with Trait -

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how can instantiate tree per below trait , case classes?

sealed trait tree[+a]  case class leaf[a](value: a) extends tree[a] case class branch[a](left: tree[a], right: tree[a]) extends tree[a]

source: functional programming in scala

example: how code following tree of type string?

           "top"           /     \   "middle-left"    "middle-right"        /          \   "bottom-left"   "bottom-right"

with class hierarchy have given wouldn't able create resembling example tree want, because branch can accept left , right sub-trees, not value (the text "top").

if want branch nodes have value, modify class hierarchy follows:

sealed trait tree[+a] case class leaf[a](value: a) extends tree[a] case class branch[a](value: a, left: option[tree[a]] = none, right: option[tree[a]] = none) extends tree[a]

note option-al nature of sub-trees, default of none, allowing missing left or right sub-trees without resorting nulls.

your example tree generated follows:

val tree = branch("top",                   some(branch("middle-left", some(leaf("bottom-left")))),                   some(branch("middle-right", right = some(leaf("bottom-right")))))

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