Explicit polymorphism in OCaml

explicit_polymorphic_function.ml

type 'value expression =
  | Const of 'value
  | IfThenElse of bool expression * 'value expression * 'value expression

let rec eval : 'value. 'value expression -> 'value = function
  | Const value -> value
  | IfThenElse (condition, then_expr, else_expr) ->
      if eval condition
      then eval then_expr
      else eval else_expr

let test_expr =
  IfThenElse (
    IfThenElse (Const false, Const false, Const true),
    Const "ok",
    Const "error"
  )

let () =
  print_endline (eval test_expr)