diff options
| -rw-r--r-- | compiler/GHC/Builtin/primops.txt.pp | 22 |
1 files changed, 17 insertions, 5 deletions
diff --git a/compiler/GHC/Builtin/primops.txt.pp b/compiler/GHC/Builtin/primops.txt.pp index 66d9aace80..126bc0f612 100644 --- a/compiler/GHC/Builtin/primops.txt.pp +++ b/compiler/GHC/Builtin/primops.txt.pp @@ -2528,11 +2528,23 @@ primop WriteMutVarOp "writeMutVar#" GenPrimOp primop AtomicModifyMutVar2Op "atomicModifyMutVar2#" GenPrimOp MutVar# s a -> (a -> c) -> State# s -> (# State# s, a, c #) { Modify the contents of a 'MutVar#', returning the previous - contents and the result of applying the given function to the - previous contents. Note that this isn't strictly - speaking the correct type for this function; it should really be - @'MutVar#' s a -> (a -> (a,b)) -> 'State#' s -> (# 'State#' s, a, (a, b) #)@, - but we don't know about pairs here. } + contents @x :: a@ and the result of applying the given function to the + previous contents @f x :: c@. + + The @data@ type @c@ (not a @newtype@!) must be a record whose first field + is of lifted type @a :: Type@ and is not unpacked. For example, product + types @c ~ Solo a@ or @c ~ (a, b)@ work well. If the record type is both + monomorphic and strict in its first field, it's recommended to mark the + latter @{-# NOUNPACK #-}@ explicitly. + + Under the hood 'atomicModifyMutVar2#' atomically replaces a pointer to an + old @x :: a@ with a pointer to a selector thunk @fst r@, where + @fst@ is a selector for the first field of the record and @r@ is a + function application thunk @r = f x@. + + @atomicModifyIORef2Native@ from @atomic-modify-general@ package makes an + effort to reflect restrictions on @c@ faithfully, providing a + well-typed high-level wrapper.} with out_of_line = True has_side_effects = True |