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Compiler feature - useful or not?
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Date: -- (:)
From: Martin Jambon <martin.jambon@e...>
Subject: Re: [Caml-list] Compiler feature - useful or not?
On Tue, 13 Nov 2007, Yaron Minsky wrote:

> There's a simple trick that Steven Weeks introduced to us and that we now
> use at Jane Street for this kind of thing.
>
> You write down a signature:
>
> module type Abs_int : sig
> type t
> val to_int : t -> int
> val of_int : int <- t
> end
>
> And then you write concrete module Int that implements this signature.  You
> can then write:
>
> module Row : Abs_int = Int
> module Col : Abs_int = Int
>
> You can now use Row.t and Col.t as abstract types.  The boilerplate is
> written once, but can be used over and over.  I've personally seen more
> use-cases for this with strings than with ints (to separate out different
> kinds of identifiers)

That's the best solution I've seen so far.

Otherwise there's still the following:
   http://martin.jambon.free.fr/ocaml.html#opaque

which in theory doesn't require new module or type declarations, but it 
adds type parameters, which can be confusing.


Martin


> y
>
> On Nov 13, 2007 6:41 PM, Edgar Friendly <thelema314@gmail.com> wrote:
>
>> 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)
>> 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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>>
>

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