Standard ML: Naming datatypes of function arguments possible?

I'm new to ML and with to have a function that receives a special pre-defined datatype, and able to reference to its entire argument datatype, rather its components.

Here's a stupid example:

datatype frame = Frame of string list * string list 

(* Type: fn : string * frame -> frame *)
val lookup_variable_value_in_frame = 
  fn (string(var), Frame(variables, values)) =>     
    ... 
    Frame(variables, values) 
    ... ;

1) I want to return the given frame. Must I build another Frame ?

2) I wish to pass the given frame to another function, must I provide a new Frame(variables, values) again ?

I wish I could write somthing like this:

val looku开发者_如何学运维p_variable_value_in_frame = 
     fn (string(var), frame : Frame(variables, values)) =>  ... 

then I'll be able to use the frame or its components .

Thank you.

Your datatype has already has a name, which is frame. You don't have to build another frame for returning or passing to another function. The first option is using explicit type annotation:

(* Type: fn : string * frame -> frame *)
val lookup_variable_value_in_frame = 
  fn (var: string, f: frame) =>     
    ... 
    f 
    ... ;

This option is not common, it should used only when you need types less generic than they are inferred by the type checker. Another option is using as keyword to make another binding to the value:

val lookup_variable_value_in_frame = 
     fn (var, f as Frame(variables, values)) =>  
         ...(* using f, variables or values here *)

Note that there is no such thing like string(var) in SML, either use var or var: string for explicit type annotation.