Re: datatype constructor as syntax

On Wed, Mar 19, 2014 at 7:37 PM, Phil Clayton <[email protected]> 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.

The issue is that the static semantics specified in the Definition
allows a 'val rec' (and, therefore, a 'fun') declaration to override
the constructor status of an identifier, but the dynamic semantics of
a 'val rec' (and, therefore, a 'fun') declaration does not override
the constructor status, and hence binding attempts to perform a
pattern match that necessarily fails with a Bind exception.

It seems that most SML implementations resolve this inconsistency by
adopting a dynamic semantics where a 'val rec' declaration does
override the constructor status.

MLton, on the other hand, resolves this inconsistency by adopting a
static semantics where a 'val rec' declaration is not allowed to
override the constructor status of an identifier.  This was adopted in
2004 (https://github.com/MLton/mlton/commit/110d6b6) as part of
Stephen's implementation of a proper front end.  I looked at the mail
archive from the time of the commit, but couldn't find any discussion
as to why the choice was to change the front end, rather than
documenting a deviation from the Definition with respect to the
dynamic semantics.

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