Re: [Caml-list] Compiler feature - useful or not?

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am Dienstag, den 13.11.2007, 17:41 -0600 schrieb Edgar Friendly:
> When one writes
> 
> type row = int
> type col = int
> 
> This allows one to use the type names "row" and "col" as synonyms of
> int.  But it doesn't prevent one from using a value of type row in the
> place of a value of type col.  OCaml allows us to enforce row as
> distinct from int two ways:
> 
> 1) Variants:
> type row = Row of int
> type col = Col of int
> 
> Downside: unnecessary boxing and tagging
> conversion from row -> int: (fun r -> match r with Row i -> i)

Note that you can write much shorter:

(fun Row i -> i), or 

let some_function (Row i) = ...

If the small runtime penalty is not essential, this is probably the best
way to do it.

Gerd


> conversion from int -> row: (fun i -> Row i)
> 
> 2)  Functors:
> module type RowCol =
> sig
>   type row
>   val int_of_row : row -> int
>   val row_of_int : int -> row
>   type col
>   val int_of_col : col -> int
>   val col_of_int : int -> col
> end
> 
> module Main = functor (RC: RowCol) -> struct
>  (* REST OF PROGRAM HERE *)
> end
> 
> Any code using rows and cols could be written to take a module as a
> parameter, and because of the abstraction granted when doing so, type
> safety is ensured.
> 
> Downside: functor overhead, misuse of functors, need to write
> boilerplate conversion functions
> conversion from row -> int, int -> row: provided by RowCol boilerplate
> 
> IS THE FOLLOWING POSSIBLE:
> Modify the type system such that one can declare
> 
> type row = new int
> type col = new int
> 
> Row and col would thus become distinct from int, and require explicit
> casting/coercion (2 :> row).  There would be no runtime overhead for use
> of these types, only bookkeeping overhead at compilation.
> 
> Downside: compiler changes (hopefully not too extensive)
> conversion from row -> int: (fun r -> (r :> int)) (* might need (r : row
> :> int) if it's not already inferred *)
> conversion from int -> row: (fun i -> (i :> row))
> 
> Thoughts?  Do any of you use Variants or Functors to do this now?  Do
> you find this style of typing useful?
> 
> E.
> 
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