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| author | parsonsmatt <parsonsmatt@gmail.com> | 2021-11-11 16:14:15 -0700 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2022-04-30 16:51:12 -0400 |
| commit | 0cdef80783b95ea2d2a49c5a9ee4ec06cde3958e (patch) | |
| tree | e091d1c7ae2824fa5200ad9329dad1a4995d8618 | |
| parent | 5630dde68185f96da026a4e0c722fe6631633299 (diff) | |
| download | haskell-0cdef80783b95ea2d2a49c5a9ee4ec06cde3958e.tar.gz | |
Add a note about instance visibility across component boundaries
In principle, the *visible* instances are
* all instances defined in a prior top-level declaration group
(see docs on `newDeclarationGroup`), or
* all instances defined in any module transitively imported by the
module being compiled
However, actually searching all modules transitively below the one being
compiled is unreasonably expensive, so `reifyInstances` will report only the
instance for modules that GHC has had some cause to visit during this
compilation. This is a shortcoming: `reifyInstances` might fail to report
instances for a type that is otherwise unusued, or instances defined in a
different component. You can work around this shortcoming by explicitly importing the modules
whose instances you want to be visible. GHC issue #20529
has some discussion around this.
Fixes #20529
| -rw-r--r-- | libraries/template-haskell/Language/Haskell/TH/Syntax.hs | 19 |
1 files changed, 16 insertions, 3 deletions
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs index fdf2e6bf39..684771eb86 100644 --- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs +++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs @@ -639,7 +639,8 @@ $(do newDeclarationGroup :: Q [Dec] newDeclarationGroup = pure [] -{- | @reifyInstances nm tys@ returns a list of visible instances of @nm tys@. That is, +{- | @reifyInstances nm tys@ returns a list of all visible instances (see below for "visible") +of @nm tys@. That is, if @nm@ is the name of a type class, then all instances of this class at the types @tys@ are returned. Alternatively, if @nm@ is the name of a data family or type family, all instances of this family at the types @tys@ are returned. @@ -657,8 +658,20 @@ instance heads which unify with @nm tys@, they need not actually be satisfiable. There is one edge case: @reifyInstances ''Typeable tys@ currently always produces an empty list (no matter what @tys@ are given). -An instance is visible if it is imported or defined in a prior top-level -declaration group. See the documentation for 'newDeclarationGroup' for more details. +In principle, the *visible* instances are +* all instances defined in a prior top-level declaration group + (see docs on @newDeclarationGroup@), or +* all instances defined in any module transitively imported by the + module being compiled + +However, actually searching all modules transitively below the one being +compiled is unreasonably expensive, so @reifyInstances@ will report only the +instance for modules that GHC has had some cause to visit during this +compilation. This is a shortcoming: @reifyInstances@ might fail to report +instances for a type that is otherwise unusued, or instances defined in a +different component. You can work around this shortcoming by explicitly importing the modules +whose instances you want to be visible. GHC issue <https://gitlab.haskell.org/ghc/ghc/-/issues/20529#note_388980 #20529> +has some discussion around this. -} reifyInstances :: Name -> [Type] -> Q [InstanceDec] |