Implement Or Patterns (Proposal 0522)wip/or-pats

This commit introduces a language extension, `OrPatterns`, as described in proposal 0522.

It extends the syntax by the production
`pat -> (one of pat1, ..., patk)`.
The or-pattern `pat` succeeds iff one of the patterns `pat1`, ..., `patk` succeed, in this order.

Currently, or-patterns cannot bind variables. They are still of great use as they discourage the use of wildcard patterns in favour of writing out all "default" cases explicitly:

```
isIrrefutableHsPat pat = case pat of
  ...
  (one of WildPat{}, VarPat{}, LazyPat{})
      = True
  (one of PArrPat{}, ConPatIn{}, LitPat{}, NPat{}, NPlusKPat{}, ListPat{})
      = False
```

This makes code safer where data types are extended now and then - just like GHC's `Pat` in the example when adding the new `OrPat` constructor. This would be catched by `-fwarn-incomplete-patterns`, but not when a wildcard pattern was used.

- Update submodule haddock.

2 files changed, 7 insertions, 1 deletions

diff --git a/compiler/Language/Haskell/Syntax/Extension.hs b/compiler/Language/Haskell/Syntax/Extension.hs
index 9ad16c0cd7..ce1ac48647 100644
--- a/compiler/Language/Haskell/Syntax/Extension.hs
+++ b/compiler/Language/Haskell/Syntax/Extension.hs
@@ -66,7 +66,7 @@ See also Note [IsPass] and Note [NoGhcTc] in GHC.Hs.Extension.
 -}
 
 -- | A placeholder type for TTG extension points that are not currently
--- unused to represent any particular value.
+-- used to represent any particular value.
 --
 -- This should not be confused with 'DataConCantHappen', which are found in unused
 -- extension /constructors/ and therefore should never be inhabited. In
@@ -591,6 +591,7 @@ type family XBangPat     x
 type family XListPat     x
 type family XTuplePat    x
 type family XSumPat      x
+type family XOrPat       x
 type family XConPat      x
 type family XViewPat     x
 type family XSplicePat   x
diff --git a/compiler/Language/Haskell/Syntax/Pat.hs b/compiler/Language/Haskell/Syntax/Pat.hs
index 66b9708bfe..80edb216a5 100644
--- a/compiler/Language/Haskell/Syntax/Pat.hs
+++ b/compiler/Language/Haskell/Syntax/Pat.hs
@@ -52,6 +52,7 @@ import Data.Ord
 import Data.Int
 import Data.Function
 import qualified Data.List
+import qualified Data.List.NonEmpty as NEL
 
 type LPat p = XRec p (Pat p)
 
@@ -137,6 +138,10 @@ data Pat p
     --            'GHC.Parser.Annotation.AnnOpen' @'('@ or @'(#'@,
     --            'GHC.Parser.Annotation.AnnClose' @')'@ or  @'#)'@
 
+  | OrPat       (XOrPat p)
+                (NEL.NonEmpty (LPat p))
+    -- ^ Or Pattern
+
   | SumPat      (XSumPat p)        -- after typechecker, types of the alternative
                 (LPat p)           -- Sum sub-pattern
                 ConTag             -- Alternative (one-based)