functional programming - OCaml - Error while adding a parametrized function into a module -

i extend given definition:

module type statechart = sig     type 'a t     val : 'a t -> 'a     val create : 'a -> 'a t end   module concretestatechart : statechart = struct     type 'a t = { state : 'a } (* internal configuration , current state expressed 'a parameter *)     let m = m.state;;   (* obtaining internal value *)      let create x = { state = x };; end 

into 2 modules kind of transaction of type k performed. tried define following code, but:

module type statechart = sig     type 'a t     type k     val : 'a t -> 'a     val create : 'a -> 'a t     val transaction : 'a t -> k -> 'a t end   module concretestatechart : statechart = struct     type 'a t = { state : 'a } (* internal configuration , current state expressed 'a parameter *)     type k = 'a option     let m = m.state;;   (* obtaining internal value *)      let create x = { state = x };;     let transaction x y = (* stupid statechart changes state dependingly option parameter *)         match y         | z -> { state = z}         | none -> x;;    end 

it seems k definition doesn't remember 'a binding performed in 'a t:

type k = 'a option;;

error: unbound type parameter 'a

how define such transaction laterly implemented in concretestatechart module?

the type variables appear in right hand side of data type definition must quantified: k should defined type parameter:

type 'a k = 'a option 

what want express, k must option of content type of t, cannot expressed in level of data type definitions, in uses of them @ transaction:

val transaction : 'a t -> 'a k -> 'a t 

, means content types of t , k must same.