[Ur] (minor) hidden table constraints

[Adam Chlipala]

On 10/17/2013 09:26 AM, Adam Chlipala wrote:
> With (1), you can do (in Template.urs):
>     con product_constraints :: {{Unit}}
>     constraint [Pkey] ~ product_constraints
>     val product : sql_table [Id = int, Caption = string, Slogan = 
> string, Logo = string, PageId = int]
>                             ([Pkey = [Id]] ++ product_constraints)
>
> It might be worth extending the [table] syntactic sugar to do 
> something like the above automatically; maybe I'll try it later.

OK, I've now pushed that extension, and it seems to work well!  Here's 
an example, demonstrating a kind of subtyping over the constraints of 
tables.  That is, a table may be considered to implement a signature 
item that only declares a subset (possibly empty) of the full constraint 
set.

functor F(M : sig
               table t : { A : int, B : int }
                 PRIMARY KEY A
           end) = struct
     open M

     fun getByA a = oneRow1 (SELECT * FROM t WHERE t.A = {[a]})
end

table u : { A : int, B : int }
   PRIMARY KEY A,
   CONSTRAINT B UNIQUE B

open F(struct
            val t = u
        end)