Comment on the commoning up of CoPats in Match

diff --git a/compiler/deSugar/Match.lhs b/compiler/deSugar/Match.lhs
index 2d64633..d793343 100644 (file)
--- a/compiler/deSugar/Match.lhs
+++ b/compiler/deSugar/Match.lhs
@@ -745,6 +745,10 @@ sameGroup (PgN l1)   (PgN l2)   = True             -- Needs conditionals
 sameGroup (PgNpK l1) (PgNpK l2) = l1==l2       -- Order is significant
                                                -- See Note [Order of n+k]
 sameGroup (PgCo        t1)  (PgCo t2)  = t1 `coreEqType` t2
+       -- CoPats are in the same goup only if the type of the
+       -- enclosed pattern is the same. The patterns outside the CoPat
+       -- always have the same type, so this boils down to saying that
+       -- the two coercions are identical.
 sameGroup _          _          = False
  
 patGroup :: Pat Id -> PatGroup