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

[Erik Hesselink]

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 at 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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