Re: datatype constructor as syntax

On Wed, Mar 19, 2014 at 7:46 PM, Phil Clayton <[email protected]> wrote:
> 19/03/14 23:37, Phil Clayton wrote:
>> 19/03/14 19:16, Matthew Fluet wrote:
>>> On Tue, Dec 31, 2013 at 3:47 AM, Florian Weimer <[email protected]> wrote:
>>>> * Henry Cejtin:
>>>>
>>>>> Sorry,  I  thought  I  was  being  clever,  but  if  you look at Andreas
>>>>> Rossberg's Defects in the Revised Definition of Standard ML
>>>>>       http://www.ps.uni-saarland.de/Publications/documents/Rossberg_DefectsSML.pdf
>>>>> you will see that he points out that rule 126 of the  dynamic  semantics
>>>>> implies  that  this  should really give you a bind exception at runtime.
>>>>> This was fixed (meaning you get the in MLton way back in 2001 to give an
>>>>> error at compile time.
>>>>
>>>> Curiously, the fun-based approach
>>>>
>>>>     datatype mytype = foo | bar of int
>>>>     fun bar _ = ()
>>>>     val bar = 23
>>>>
>>>> still runs without raising an exception when compiled with MLton.
>>>
>>> That seems to be an oversight.
>>
>> Perhaps I missed the point here but I'm hoping that it's intentional
>> that MLton allows the status of a constructor id to be overwritten by a
>> fun binding.  I am relying on code as follows to create a value NONE:
>>
>>     local
>>       fun NONE () = <expression>
>>     in
>>       val NONE = NONE ()
>>     end
>>
>> Note that this does not create an unreferenced id, which avoids spurious
>> warnings from such checks, for example, using Poly/ML with
>>     PolyML.Compiler.reportUnreferencedIds := true;
>>
>> If this isn't allowed, how should a value called NONE be created?
>
> Responding to myself:
>
>    local
>      structure S : sig val NONE : int option end = struct open Option end
>      open S
>    in
>      val NONE = <expression>
>    end
>
> Yuk!  Is there anything nicer?

Ah, that is clever.  The signature exactly specifies the
constructor/non-constructor status of the identifier.

You could adopt a slightly more generic approach like:

local
  structure S : sig type t val NONE : t end = struct datatype t = NONE end
  open S
in
  val NONE = <expression>
end

which should work whether or not NONE is an in-scope constructor and,
if it is in scope, without needing to know its type.

The disadvantage of this approach is that one cannot use a "structure"
declaration within a "let" expression.

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