datatype constructor as syntax

I have a rather esoteric question about Standard ML. I hope I'm not being too annoying asking it in a forum specific to MLton.

A datatype declaration introduces a new sum type and one or more constructors:

	datatype mytype = foo | bar of int;

On the one hand, the constructors smell syntactically like regular old functions:

	- bar;
	val it = fn : int -> mytype

	- bar 3;
	val it = bar 3 : mytype

On the other, they somehow become syntax. With the above defined:

	- val bar = 23;
	stdIn:24.5-24.8 Error: data constructor bar used without argument in pattern

That is, I can not shadow the old constructor version of bar with a new variable binding. This implies that with respect to constructors, SML's grammar is not context-free. I managed to figure out that at least one reason might be because constructors are used in pattern matching:

	- val i = bar 3;
	val i = bar 3 : mytype

	- val (bar j) = i;
	val j = 3 : int

My question: Is this the reason? I guess this features is cool, but one unnerving thing about it is that this kind of pattern matching is inherently non-exhaustive.

Thanks much,

-thant

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