With dependently-typed features it is more useful (see Trac #2431). For example, consider these two candidate definitions of absurd:
{{{#!hs data a :==: b where Refl :: a :==: a
absurd :: True :~: False -> a absurd x = error "absurd" -- (A) absurd x = case x of {} -- (B) }}}
We much prefer (B). Why? Because GHC can figure out that `(True :~: False)` is an empty type. So (B) has no partiality and GHC should be able to compile with [http://downloads.haskell.org/~ghc/master/users-guide /using-warnings.html#ghc-flag--Wincomplete-patterns -Wincomplete-
#11390: GHC does not warn about redundant patterns -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: warnings Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): I thought that was what the documentation for `EmptyCase` is alluding to: patterns]. (Though the pattern match checking is not yet clever enough to do that.) On the other hand (A) looks dangerous, and GHC doesn’t check to make sure that, in fact, the function can never get called. I wasn't sure whether the "Though the pattern match checking is not yet clever enough to do that." statement had changed in 8, I'll edit the code to enable `LambdaCase` as well. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11390#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler