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.

While I also appreciate the possibility of playing tricks like these, I 
don't think it's a good idea to rebind an identifier as fundamental as 
NONE in real code, no matter how easy or hard it is.

/Andreas


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