[Haskell-cafe] Ensuring all values of an ADT are explicitly handled OR finding all occurrences of type X in my app

[Eric Seidel]

> 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.