diff options
Diffstat (limited to 'compiler/GHC/Hs/Extension.hs')
| -rw-r--r-- | compiler/GHC/Hs/Extension.hs | 33 |
1 files changed, 14 insertions, 19 deletions
diff --git a/compiler/GHC/Hs/Extension.hs b/compiler/GHC/Hs/Extension.hs index 45753eaf47..b24bdf19b8 100644 --- a/compiler/GHC/Hs/Extension.hs +++ b/compiler/GHC/Hs/Extension.hs @@ -181,32 +181,27 @@ following: type instance XXHsDecl (GhcPass _) = NoExtCon data HsDecl p = ... - | XHsDecl (XXHsDecl p) + | XHsDecl !(XXHsDecl p) -This means that any function that wishes to consume an HsDecl will need to -have a case for XHsDecl. This might look like this: +The field of type `XXHsDecl p` is strict for a good reason: it allows the +pattern-match coverage checker to conclude that any matches against XHsDecl +are unreachable whenever `p ~ GhcPass _`. To see why this is the case, consider +the following function which consumes an HsDecl: ex :: HsDecl GhcPs -> HsDecl GhcRn ... ex (XHsDecl nec) = noExtCon nec -Ideally, we wouldn't need a case for XHsDecl at all (it /is/ supposed to be -an unused extension constructor, after all). There is a way to achieve this -on GHC 8.8 or later: make the field of XHsDecl strict: - - data HsDecl p - = ... - | XHsDecl !(XXHsDecl p) - -If this is done, GHC's pattern-match coverage checker is clever enough to -figure out that the XHsDecl case of `ex` is unreachable, so it can simply be -omitted. (See Note [Extensions to GADTs Meet Their Match] in Check for more on -how this works.) +Because `p` equals GhcPs (i.e., GhcPass 'Parsed), XHsDecl's field has the type +NoExtCon. But since (1) the field is strict and (2) NoExtCon is an empty data +type, there is no possible way to reach the right-hand side of the XHsDecl +case. As a result, the coverage checker concludes that the XHsDecl case is +inaccessible, so it can be removed. +(See Note [Strict argument type constraints] in GHC.HsToCore.PmCheck.Oracle for +more on how this works.) -When GHC drops support for bootstrapping with GHC 8.6 and earlier, we can make -the strict field changes described above and delete gobs of code involving -`noExtCon`. Until then, it is necessary to use, so be aware of it when writing -code that consumes unused extension constructors. +Bottom line: if you add a TTG extension constructor that uses NoExtCon, make +sure that any uses of it as a field are strict. -} -- | Used as a data type index for the hsSyn AST; also serves |