Re: datatype constructor as syntax

On Jan 1, 2014, at 23:34 , Thant Tessman <[email protected]> wrote:
>> I don't understand what you mean by
>>   ... this kind of pattern matching is inherently non-exhaustive.
> 
> To reiterate the example:
> 
> datatype mytype = foo | bar of int;
> 
> On the one hand, this is exhaustive:
> 
>  case i of
>    foo => "foo"
>  | bar j => "bar";
> 
> But the use of constructors in mere variable declaration patterns without the case can't be exhaustive:
> 
>  val (bar j) = i;

It can be exhaustive for other datatypes:

  datatype time = TIME of real
  val TIME t = f()

> On top of that, when I enter this interactively in SML/NJ, there is no warning. However, I just tried it with MLton, and it does indeed produce a warning:

It is mandatory that inexhaustive bindings produce a warning, _except_ on the toplevel (try “let val bar j = i in j end” for contrast). This presumably is to make deconstructing known values in the REPL a less annoying experience (which MLton doesn’t have). Clearly, though, the whole REPL focus of the Definition is an anachronism.

/Andreas

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