OCaml Binding a variable with nested types

Question

I am new to OCaml and am trying to jump into a large OCaml project. While tracing through the types that make up other types I was trying to think how I would use these, so I broke it down to a small example that I think is pretty close to how it is in the larger project.

If I have the record type expression made of three fields:

type expression = {e_id:int; e_node:exp_node; e_loc:location}
and exp_node =
  | Const of int
  | Lval of lval
  | SizeofStr of string
and lval =
  | Var of varinfo
  | Mem of expression
and location = {x:int; y:int}
and varinfo = {vname:string; vorigname:string}

I can bind a variable of this type if the e_node field is an integer:

let exp_const = {e_id=10;
                 e_node= Const 10;
                 e_loc={x=10; y=10}}

Now if I want that e_node field to be of the Lval type which is the record type lval I can not figure out how to do it. I tried:

let exp_lval_var =
  {e_id=11;
   e_node= {vname="int_val"; vorigname="int_val1"};
   e_loc={x=10; y=20}}

But it says that This expression has type varinfo but an expression was expected of type exp_node. But if you follow the types, it is!?

Am I not defining types correctly here? Or am I not using the types correctly? I couldn't really find many more complex examples of using OCaml types. Any suggestions here would be appreciated.

Jeffrey Scofield Jeffrey Scofield · Accepted Answer · 2017-03-24T21:09:09

Just like you wrote Const 10 (as opposed to just 10), you need to write Lval (Var { ... }) to get the right kind of value for your e_node field.

let exp_lval_var =
  {e_id=11;
   e_node= Lval (Var {vname="int_val"; vorigname="int_val1"});
   e_loc={x=10; y=20}}

To follow the types for the first example:

10 is of type int
Const 10 is of type exp_node

For the second example

{vname="int_val"; vorigname="int_val1"} is of type varinfo
Var {vname="int_val"; vorigname="int_val1"} is of type lval
Lval (Var {vname="int_val"; vorigname="int_val1"}) is of type exp_node

Value constructors like Const, Var, and Lval are not just labels for documentation in OCaml. They must appear as part of the expression to make a value of their type.

OCaml Binding a variable with nested types

1 Answers