I feel like there's no good technical solution here. For example, this is potentially dangerous in the face of a changing data type, but presumably wouldn't fall under the definition of a "default branch":

data T = A | B -- Later, C could be added which might need special handling

case ... of
[] -> ...

(A : xs) -> ...

(_ : xs) -> ...

On the other hand, this would presumably be a "default branch" but I challenge you to replace the wildcard pattern with an exhaustive list of pattern matches:

case ... of
0 -> ...
1 -> ...
_ -> ...

Of course this is a special case, but there are many data types in the wild with a large number of constructors: think of generated enumerations, for example.

I'd say education is the best option here. I thought wildcards were convenient, until I realized their effect on maintainability in the face of future data type changes.

Erik

On 31 January 2017 at 18:31, Eric Seidel <eric@seidel.io> wrote:

> Note that
>
> case <foo>
> | [] -> ...
> | (_ : xs) -> ...
>
> also contains a wildcard-pattern. So emitting a warning for every use
> of wildcard patterns would likely lead to a lot of pain.

Yes, good point, that would be too restrictive. When I said wildcard-pattern I was thinking specifically of a top-level wildcard, so your example would be accepted, but e.g.

case ... of
[] -> ...
_ -> ...

would be rejected.

> You'd instead want to warn about "default branch", e.g.
>
> case <foo>
> | [] -> ...
> | (1 : xs) -> ...
> | (_ : xs) -> ...
>
> here the wildcard pattern does correspond to a "default branch" and
> might hence deserve a warning.

This sounds promising, but how would you define “default branch”? Seems like it could be an involved definition, which could make the warning unpredictable for users.

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