[Haskell-cafe] Redundant/overlapping patterns?

[Viktor Dukhovni]

On Fri, Jul 21, 2023 at 10:17:33PM +0200, Andreas Källberg wrote:

> I think unreachable would be instead of “overlapping”, rather than
> instead of “redundant”, since those refer to different things.
> Redundant patterns are when the constructor is already known, while
> overlapping is when the case unreachable.

It is a bit more general than "constructor known", since GHC is
also able to rule out reachability even when the constructor is
not fully known:

    λ> f x = case (x, x) of { (True, True) -> 0; (False, False) -> 1; (_, _) -> 2 }

    <interactive>:5:64: warning: [GHC-53633] [-Woverlapping-patterns]
        Pattern match is redundant
        In a case alternative: (_, _) -> ...

Whatever `x` is, the first two patterns are sufficient, though it could
be either.  Regardless of how it is that the compiler figures out that
no value of the scrutinee can reach a pattern, once that is known, the
pattern is unreachable, and so redundant in context.  That context is
the "shape" of the scrutinee plus any earlier patterns.

The fact that static knowledge of the scrutinee is also taken into
account can make the error message slightly surprising if one is just
focusing on the list of patterns.  But 'redundant' is still morally
true.

-- 
    Viktor.