Re: Warn about variable patterns which match only a single data constructor

* Matthew Fluet:

> On Sun, Jan 29, 2012 at 10:12 AM, Florian Weimer <[email protected]> wrote:
>> I'm working on patch which adds another pattern match warning,
>> intended to cover this case:
>>
>> datatype t = A | B
>>
>> fun i A = 0
>>  | i C = 1
>>
>> As it stands, this produces many false positives.  I guess the warning
>> should fire only if the variable pattern matches a single,
>> parameter-less data constructor.  Any ideas how to implement that?
>
> I'm not quite sure that I follow the situation that is being warned
> about.  Is it that the last match matched only one constructor or that
> the whole match only (explicitly) matched one constructor?

The last match (that is, ideally, any variable in a pattern).

> It might be worth noting that the "warnUnused true" warning would warn
> about the unused variable "C" in the above.  That seems to usually
> cover the situations where you meant to use a constructor in a
> pattern, but mistakenly used a variable.

Correct.  I'm actually worried about this:

datatype t = A | C
fun i A = 0
  | i B = i B

Which might have started as this snippet:

datatype t = A | B
fun i A = 0
  | i B = i B

Exhaustiveness and redundancy checks under source code edits are a pet
peeve of mine, and I would like to get some actual data.

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